Correctness in analytical chemistry. Quantitative analysis. Chemical methods of analysis. Analytical chemistry

analysis method name the principles underlying the analysis of matter, that is, the type and nature of the energy that causes perturbation of the chemical particles of matter.

The analysis is based on the dependence between the recorded analytical signal on the presence or concentration of the analyte.

Analytical signal is a fixed and measurable property of an object.

In analytical chemistry, analysis methods are classified according to the nature of the property being determined and according to the method of recording the analytical signal:

1.chemical

2.physical

3.Physical and chemical

Physico-chemical methods are called instrumental or measuring, as they require the use of instruments, measuring instruments.

Consider a complete classification of chemical methods of analysis.

Chemical methods of analysis- based on the measurement of the energy of a chemical reaction.

During the reaction, the parameters associated with the consumption of starting materials or the formation of reaction products change. These changes can either be observed directly (precipitate, gas, color) or measured such as reagent consumption, product mass, reaction time, etc.

By goals methods of chemical analysis are divided into two groups:

I. Qualitative analysis- consists in the detection of individual elements (or ions) that make up the analyzed substance.

Qualitative analysis methods are classified:

1. cation analysis

2. anion analysis

3. analysis of complex mixtures.

II.Quantitative analysis- consists in determining the quantitative content of individual constituent parts complex substance.

Quantitative chemical methods classify:

1. Gravimetric(weight) method of analysis is based on the isolation of the analyte in its pure form and its weighing.

Gravimetric methods according to the method of obtaining the reaction product are divided into:

a) chemogravimetric methods are based on measuring the mass of the product of a chemical reaction;

b) electrogravimetric methods are based on measuring the mass of the product of an electrochemical reaction;

c) thermogravimetric methods are based on measuring the mass of a substance formed during thermal exposure.

2. Volumetric methods of analysis are based on measuring the volume of a reagent consumed for interaction with a substance.

Volumetric methods, depending on the state of aggregation of the reagent, are divided into:

a) gas volumetric methods, which are based on the selective absorption of the determined component of the gas mixture and the measurement of the volume of the mixture before and after absorption;

b) liquid volumetric (titrimetric or volumetric) methods are based on measuring the volume of a liquid reagent consumed for interaction with the analyte.

Depending on the type of chemical reaction, methods of volumetric analysis are distinguished:

Protolithometry is a method based on the course of a neutralization reaction;

redoxometry - a method based on the occurrence of redox reactions;

complexometry - a method based on the course of the reaction of complexation;

· precipitation methods - methods based on the reactions of precipitation formation.

3. Kinetic methods of analysis are based on determining the dependence of the rate of a chemical reaction on the concentration of reactants.

Lecture No. 2. Stages of the analytical process

The solution of the analytical problem is carried out by performing the analysis of the substance. According to IUPAC terminology analysis [‡] called the procedure for obtaining experimentally data on the chemical composition of a substance.

Regardless of the chosen method, each analysis consists of the following stages:

1) sampling (sampling);

2) sample preparation (sample preparation);

3) measurement (definition);

4) processing and evaluation of measurement results.

Fig1. Schematic representation of the analytical process.

Sample selection

Conducting chemical analysis begins with the selection and preparation of samples for analysis. It should be noted that all stages of the analysis are interconnected. Thus, a carefully measured analytical signal does not provide correct information about the content of the analyte, if the selection or preparation of the sample for analysis is not carried out correctly. Sampling error often determines the overall accuracy of the component determination and makes it meaningless to use high-precision methods. In turn, sampling and sample preparation depend not only on the nature of the analyzed object, but also on the method of measuring the analytical signal. The methods and procedure for sampling and its preparation are so important in chemical analysis that they are usually prescribed by the State Standard (GOST).

Consider the basic rules for sampling:

The result can only be correct if the sample is sufficiently representative, that is, accurately reflects the composition of the material from which it was selected. The more material is selected for the sample, the more representative it is. However, a very large sample is difficult to handle and increases analysis time and cost. Thus, it is necessary to take a sample so that it is representative and not very large.

· The optimal mass of the sample is due to the heterogeneity of the analyzed object, the size of the particles from which the heterogeneity begins, and the requirements for the accuracy of the analysis.

· Lot homogeneity must be ensured to ensure representativeness of the sample. If it is not possible to form a homogeneous batch, then stratification of the batch into homogeneous parts should be used.

· When sampling, the state of aggregation of the object is taken into account.

· The condition for the uniformity of sampling methods must be met: random sampling, periodic, staggered, multi-stage sampling, blind sampling, systematic sampling.

· One of the factors that should be taken into account when choosing a sampling method is the possibility of changing the composition of the object and the content of the determined component over time. For example, a variable composition of water in a river, a change in the concentration of components in food products, etc.

V.F. Yustratov, G.N. Mikileva, I.A. Mochalova

ANALYTICAL CHEMISTRY

Quantitative chemical analysis

Tutorial

For university students

2nd edition, revised and enlarged

higher vocational education for intercollegiate use

as a textbook in analytical chemistry for students studying in the areas of training 552400 "Food Technology", 655600 "Production of food from plant materials",

655900 "Technology of raw materials, products of animal origin"

and 655700 "Technology of food products

special purpose and Catering»

Kemerovo 2005

UDC 543.062 (07)

V.F. Yustratov, G.N. Mikileva, I.A. Mochalova

Edited by V.F. Yustratova

Reviewers:

V.A. Nevostruev, head Department of Analytical Chemistry

Kemerovo state university, Dr. chem. sciences, professor;

A.I. Gerasimov, Associate Professor, Department of Chemistry and Technology

inorganic substances of the Kuzbass State Technical

University, Ph.D. chem. Sciences

Kemerovo Technological Institute

Food Industry

Yustratova V.F., Mikileva G.N., Mochalova I.A.

Yu90 Analytical chemistry. Quantitative chemical analysis: Proc. allowance. - 2nd ed., revised. and additional - / V.F. Yustratov, G.N. Mikileva, I.A. Mochalova; Ed. V.F. Yustratova; Kemerovo Technological Institute of Food Industry - Kemerovo, 2005. - 160 p.

ISBN 5-89289-312-X

The basic concepts and sections of analytical chemistry are outlined. All stages of quantitative chemical analysis from sampling to obtaining results and methods for their processing are considered in detail. The manual includes a chapter on instrumental methods of analysis, as the most promising. The use of each of the described methods in the technochemical control of the food industry is indicated.

The textbook is compiled in accordance with state educational standards in the areas of "Food Technology", "Food Production from Vegetable Raw Materials and Products of Animal Origin", "Technology of Food Products for Special Purposes and Public Catering". Contains guidelines students on lecture notes and work with the textbook.

Designed for students of all forms of learning.

UDC 543.062 (07)

BBC 24.4 i 7

ISBN 5-89289-312-X

© V.F. Yustratov, G.N. Mikileva, I.A. Mochalova, 1994

© V.F. Yustratov, G.N. Mikileva, I.A. Mochalova, 2005, addition

© KemTIPP, 1994

FOREWORD

The textbook is intended for students of technological specialties of universities of the food profile. Second edition, revised and enlarged. When processing the material, the advice and comments of the head of the Department of Analytical Chemistry of the Voronezh State Technological Academy, Honored Worker of Science and Technology of the Russian Federation, Doctor of Chemical Sciences, Professor Ya.I. Korenman. The authors express their deep gratitude to him.

Over the past ten years since the publication of the first edition, new textbooks on analytical chemistry have appeared, but none of them fully complies with the State educational standards in the areas of "Technology of food products", "Production of food products from vegetable raw materials", "Technology of raw materials and products of animal origin", "Technology of food products for special purposes and public catering".

In the manual, the material is presented in such a way that the student sees the "task of analytical chemistry" as a whole: from sampling to obtaining analysis results, methods of processing them and analytical metrology. A brief history of the development of analytical chemistry, its role in food production is given; the basic concepts of qualitative and quantitative chemical analyzes, ways of expressing the composition of solutions and preparing solutions, formulas for calculating the results of analysis are given; theory of methods of titrimetric analysis: neutralization (acid-base titration), redoximetry (redox titration), complexometry, precipitation and gravimetry. The application of each of them in the food industry is indicated. When considering titrimetric methods of analysis, a structural-logical scheme is proposed that simplifies their study.

When presenting the material, the modern nomenclature of chemical compounds, modern generally accepted concepts and ideas are taken into account, new scientific data are used to argue the conclusions.

The manual additionally includes a chapter on instrumental methods of analysis, as the most promising, and shows current trends in the development of analytical chemistry.

According to the form of presentation, the text of the manual is adapted for students of I-II courses, who still lack the skills of independent work with educational literature.

Sections 1, 2, 5 were written by V.F. Yustratova, sections 3, 6, 8, 9 - G.N. Mikileva, section 7 - I.A. Mochalova, section 4 - G.N. Mikileva and I.A. Mochalova.

ANALYTICAL CHEMISTRY AS A SCIENCE

Analytical chemistry is one of the branches of chemistry. If we give the most complete definition of analytical chemistry as a science, then we can use the definition proposed by Academician I.P. Alimarin.

"Analytical chemistry is a science that develops the theoretical foundations of analysis chemical composition substances, developing methods for the identification and detection, determination and separation of chemical elements, their compounds, as well as methods for establishing the chemical structure of compounds.

This definition is quite voluminous and difficult to remember. In high school textbooks, more concise definitions are given, the meaning of which is as follows.

Analytical chemistryis the science of methods for determining the chemical composition and structure of substances (systems).

1.1. From the history of the development of analytical chemistry

Analytical chemistry is a very ancient science.

As soon as goods and materials appeared in society, the most important of which were gold and silver, it became necessary to check their quality. Cupellation, the test by fire, was the first widely used technique for the analysis of these metals. This quantitative technique involves weighing the analyte before and after heating. The mention of this operation is found in tablets from Babylon dated 1375-1350. BC.

Scales have been known to mankind since before the times of ancient civilization. Weights found for scales date back to 2600 BC.

According to the generally accepted point of view, the Renaissance can be considered the starting point, when individual analytical techniques took shape in scientific methods.

But the term "analysis" in the modern sense of the word was introduced by the English chemist Robert Boyle (1627-1691). He first used the term in 1654.

Fast development analytical chemistry began at the end of the 17th century. in connection with the emergence of manufactories, the rapid growth of their number. This gave rise to a variety of problems that could only be solved using analytical methods. The need for metals, in particular iron, increased greatly, which contributed to the development of the analytical chemistry of minerals.

Chemical analysis was elevated to the status of a separate branch of science - analytical chemistry - by the Swedish scientist Thornburn Bergman (1735-1784). Bergman's work can be considered the first textbook of analytical chemistry, which provides a systematic overview of the processes used in analytical chemistry, grouped according to the nature of the analyzed substances.

The first well-known book devoted entirely to analytical chemistry is The Complete Chemical Assay Office, written by Johann Goetling (1753-1809) and published in 1790 in Jena.

A huge number of reagents used for qualitative analysis is systematized by Heinrich Rose (1795-1864) in his book "A Guide to Analytical Chemistry". Separate chapters of this book are devoted to some elements and known reactions of these elements. Thus, Rose in 1824 was the first to describe the reactions of individual elements and gave a scheme systematic analysis, which has survived in its main features to the present day (for a systematic analysis, see Section 1.6.3).

In 1862, the first issue of the "Journal of Analytical Chemistry" was published - a journal devoted exclusively to analytical chemistry, which is published to this day. The magazine was founded by Fresenius and published in Germany.

The foundations of weight (gravimetric) analysis - the oldest and most logical method of quantitative analysis - were laid by T. Bergman.

Methods of volumetric analysis began to be widely included in analytical practice only in 1860. Description of these methods appeared in textbooks. By this time, devices (devices) for titration had been developed and a theoretical substantiation of these methods was given.

The main discoveries that made it possible to make a theoretical substantiation of volumetric methods of analysis include the law of conservation of the mass of matter, discovered by M.V. Lomonosov (1711-1765), a periodic law discovered by D.I. Mendeleev (1834-1907), the theory of electrolytic dissociation developed by S. Arrhenius (1859-1927).

The foundations of volumetric methods of analysis were laid over almost two centuries, and their development is closely connected with the demands of practice, first of all, the problems of bleaching fabrics and the production of potash.

Many years have been spent on the development of convenient, accurate instruments, the development of operations for grading volumetric glassware, manipulations when working with precision glassware, and methods for fixing the end of titration.

It is not surprising that even in 1829 Berzelius (1779-1848) believed that volumetric methods of analysis could only be used for approximate estimates.

For the first time now generally accepted terms in chemistry "pipette"(Fig. 1) (from the French pipe - pipe, pipette - tubes) and "burette"(Fig. 2) (from the French burette - bottle) are found in the publication of J.L. Gay-Lussac (1778-1850), published in 1824. Here he also described the titration operation in the form it is done now.

Rice. 1. Pipettes Fig. 2. Burettes

The year 1859 turned out to be significant for analytical chemistry. It was in this year that G. Kirchhoff (1824-1887) and R. Bunsen (1811-1899) developed spectral analysis and turned it into a practical method of analytical chemistry. Spectral analysis was the first of the instrumental methods of analysis, which marked the beginning of their rapid development. See section 8 for more details on these analysis methods.

At the end of the 19th century, in 1894, the German physical chemist V.F. Ostwald published a book on the theoretical foundations of analytical chemistry, the fundamental theory of which was the theory of electrolytic dissociation, on which chemical methods of analysis are still based.

Started in the 20th century (1903) was marked by the discovery of the Russian botanist and biochemist M.S. The color of the phenomenon of chromatography, which was the basis for the development of various variants of the chromatographic method, the development of which continues to this day.

In the twentieth century analytical chemistry developed quite successfully. There was a development of both chemical and instrumental methods of analysis. The development of instrumental methods was due to the creation of unique devices that allow recording the individual properties of the analyzed components.

Russian scientists have made a great contribution to the development of analytical chemistry. First of all, the names of N.A. Tananaeva, I.P. Alimarina, A.K. Babko, Yu.A. Zolotov and many others.

The development of analytical chemistry has always taken into account two factors: the developing industry has formed a problem that needs to be solved, on the one hand; on the other hand, the discoveries of science adapted to the solution of problems of analytical chemistry.

This trend continues to this day. Computers and lasers are widely used in analysis, new methods of analysis are emerging, automation and mathematization are being introduced, methods and means of local non-destructive, remote, continuous analysis are being created.

1.2. General problems of analytical chemistry

General tasks of analytical chemistry:

1. Development of the theory of chemical and physico-chemical methods of analysis, scientific substantiation, development and improvement of techniques and research methods.

2. Development of methods for separating substances and methods for concentrating microimpurities.

3. Improvement and development of methods for the analysis of natural substances, the environment, technical materials, etc.

4. Ensuring chemical-analytical control in the process of conducting various research projects in the field of chemistry and related fields of science, industry and technology.

5. Maintenance of chemical-technological and physical-chemical production processes at a given optimal level based on systematic chemical-analytical control of all parts of industrial production.

6. Creation of methods for automatic control of technological processes, combined with control systems based on the use of electronic computing, recording, signaling, blocking and control machines, instruments and devices.

It can be seen from the foregoing that the possibilities of analytical chemistry are wide. This allows it to be used to solve a wide variety of practical problems, including in the food industry.

1.3. The role of analytical chemistry in the food industry

Methods of analytical chemistry allow solving the following problems in the food industry:

1. Determine the quality of raw materials.

2. Control the process of food production at all its stages.

3. Control the quality of products.

4. Analyze production waste for the purpose of their disposal (further use).

5. Determine in raw materials and food products substances that are toxic (harmful) to the human body.

1.4. Analysis method

Analytical chemistry studies methods of analysis, various aspects of their development and application. According to the recommendations of the authoritative international chemical organization IUPAC *, the method of analysis is the principles underlying the analysis of a substance, i.e. the type and nature of the energy that causes perturbation of the chemical particles of matter. The principle of analysis is in turn determined by the phenomena of nature on which chemical or physical processes are based.

In the educational literature on chemistry, the definition of the method of analysis, as a rule, is not given. But since it is important enough, it must be formulated. In our opinion, the most acceptable definition is the following:

The method of analysis is the sum of the rules and techniques for performing analysis, which make it possible to determine the chemical composition and structure of substances (systems).

1.5. Classification of analysis methods

In analytical chemistry, there are several types of classification of methods of analysis.

1.5.1. Classification based on the chemical and physical properties of the analyzed substances (systems)

Within this classification, following groups analysis methods:

1. Chemical methods of analysis.

This group of methods of analysis includes those in which the results of the analysis are based on a chemical reaction occurring between substances. At the end of the reaction, the volume of one of the participants in the reaction or the mass of one of the reaction products is recorded. Then the results of the analysis are calculated.

2. Physical methods of analysis.

Physical methods of analysis are based on the measurement of the physical properties of the analyzed substances. Most widely, these methods fix optical, magnetic, electrical, and thermal properties.

3. Physical and chemical methods of analysis.

They are based on the measurement of some physical property (parameter) of the analyzed system, which changes under the influence of a chemical reaction occurring in it.

* IUPAC - International Union of Pure and Applied Chemistry. Scientific institutions of many countries are members of this organization. Russian Academy Nauk (as the successor of the Academy of Sciences of the USSR) has been a member of it since 1930.

In modern chemistry, physical and physico-chemical methods of analysis are called instrumental analysis methods. “Instrumental” means that this method of analysis can be carried out only with the use of an “instrument” - a device capable of recording and evaluating physical properties (see Section 8 for details).

4. Separation methods.

When analyzing complex mixtures (and this is the majority natural objects and food products) it may be necessary to separate the analyte from the interfering components.

Sometimes in the analyzed solution of the determined component is much less than can be determined by the chosen method of analysis. In this case, before determining such components, it is necessary to preconcentrate them.

concentration- this is an operation, after which the concentration of the determined component can increase from n to 10 n times.

Separation and concentration operations are often combined. At the stage of concentration in the analyzed system, some property can clearly manifest itself, the fixation of which will allow us to solve the problem of the amount of the analyte in the mixture. The method of analysis may begin with a separation operation, sometimes it also includes concentration.

1.5.2. Classification based on the mass of a substance or volume

solution taken for analysis

A classification demonstrating the possibilities of modern methods of analysis is presented in Table. 1. It is based on the mass of substances or volume of solution taken for analysis.

Table 1

Classification of methods of analysis depending on the mass of the substance

or volume of solution taken for analysis

1.6. Qualitative Analysis

The analysis of a substance can be carried out in order to establish its qualitative or quantitative composition. Accordingly, a distinction is made between qualitative and quantitative analysis.

The task of qualitative analysis is to establish the chemical composition of the analyzed object.

Analyzed object can be an individual substance (simple or very complex, such as bread), as well as a mixture of substances. As part of an object, its various components may be of interest. It is possible to determine which ions, elements, molecules, phases, groups of atoms the analyzed object consists of. In food, ions are most often determined, simple or complex substances that are either useful (Ca 2+, NaCl, fat, protein, etc.) or harmful to the human body (Cu 2+ , Pb 2+ , pesticides, etc. ). This can be done in two ways: identification and discovery.

Identification- establishing the identity (identity) of the chemical compound under study with a known substance (standard) by comparing their physical and chemical properties .

For this, certain properties of the given reference compounds are preliminarily studied, the presence of which is assumed in the analyzed object. For example, chemical reactions are carried out with cations or anions (these ions are standards) in the study of inorganic substances, or the physical constants of reference organic substances are measured. Then perform the same tests with the test compound and compare the results.

Detection- checking the presence in the analyzed object of certain main components, impurities, etc. .

Qualitative chemical analysis is mostly based on the transformation of the analyte into some new compound with characteristic properties: a color, a certain physical state, a crystalline or amorphous structure, a specific smell, etc. These characteristic properties are called analytical features.

A chemical reaction, during which analytical signs appear, is called high-quality analytical reaction.

Substances used in analytical reactions are called reagents or reagents.

Qualitative analytical reactions and, accordingly, the reagents used in them, depending on the field of application, are divided into group (general), characteristic and specific.

Group reactions allow you to isolate from a complex mixture of substances under the influence of a group reagent whole groups of ions that have the same analytical feature. For example, ammonium carbonate (NH 4) 2 CO 3 belongs to group reagents, since with Ca 2+, Sr 2+, Ba 2+ ions it forms white carbonates insoluble in water.

characteristic called such reactions in which reagents interacting with one or a small number of ions participate. The analytical feature in these reactions, most often, is expressed in a characteristic color. For example, dimethylglyoxime is a characteristic reagent for the Ni 2+ ion (pink precipitate) and for the Fe 2+ ion (water-soluble red compound).

The most important in qualitative analysis are specific reactions. specific a reaction to a given ion is such a reaction that makes it possible to detect it under experimental conditions in a mixture with other ions. Such a reaction is, for example, an ion detection reaction, proceeding under the action of alkali when heated:

Ammonia released can be identified by a specific, easily recognizable odor and other properties.

1.6.1. Reagent brands

Depending on the specific area of ​​application of reagents, a number of requirements are imposed on them. One of them is the requirement for the amount of impurities.

The amount of impurities in chemical reagents is regulated by special technical documentation: state standards(GOST), technical conditions (TU), etc. The composition of impurities can be different, and it is usually indicated on the manufacturer's label of the reagent.

Chemical reagents are classified according to the degree of purity. Depending on the mass fraction of impurities, the reagent is assigned a brand. Some brands of reagents are presented in Table. 2.

table 2

Reagent brands

Usually, in the practice of chemical analysis, reagents are used that meet the qualification "analytical grade" and "chemically pure". The purity of the reagents is indicated on the label of the original packaging of the reagent. Some industries introduce their own additional purity qualifications for reagents.

1.6.2. Methods for Performing Analytical Reactions

Analytical reactions can be performed "wet" and "dry" ways. When performing a reaction "wet" by the interaction of the analyte and the corresponding reagents occurs in solution. For its implementation, the test substance must be previously dissolved. The solvent is usually water or, if the substance is insoluble in water, another solvent. Wet reactions occur between simple or complex ions, therefore, when applied, it is these ions that are detected.

"Dry" method of performing reactions means that the test substance and reagents are taken in the solid state and the reaction between them is carried out by heating them to a high temperature.

Examples of reactions performed by the "dry" way are the reactions of coloring the flame with salts of certain metals, the formation of colored pearls (glasses) of sodium tetraborate (borax) or sodium and ammonium hydrogen phosphate when fusing them with salts of certain metals, as well as fusing the solid under study with "fluxes", for example: mixtures of solid Na 2 CO 3 and K 2 CO 3, or Na 2 CO 3 and KNO 3.

The reactions carried out by the "dry" way also include the reaction that occurs when the test solid is triturated with some solid reagent, as a result of which the mixture acquires a color.

1.6.3. Systematic analysis

A qualitative analysis of an object can be carried out in two ways. different methods.

Systematic analysis - this is a method of conducting qualitative analysis according to the scheme, when the sequence of operations for adding reagents is strictly defined.

1.6.4. Fractional Analysis

An analysis method based on the use of reactions that can be used to detect the desired ions in any sequence in individual portions of the initial solution, i.e. without resorting to certain scheme ion detection, called fractional analysis.

1.7. Quantitative Analysis

The task of quantitative analysis is to determine the content (mass or concentration) of a particular component in the analyzed object.

Important concepts of quantitative analysis are the concepts of "determined substance" and "working substance".

1.7.1. Substance being identified. working substance

A chemical element, ion, simple or complex substance, the content of which is determined in a given sample of the analyzed product, is commonly called "identifiable substance" (O.V.).

The substance with which this determination is carried out is called working substance (RV).

1.7.2. Ways of expressing the composition of a solution used in analytical chemistry

1. The most convenient way to express the composition of a solution is the concentration . Concentration is a physical quantity (dimensional or dimensionless) that determines the quantitative composition of a solution, mixture or melt. When considering the quantitative composition of a solution, most often, they mean the ratio of the amount of solute to the volume of the solution.

The most common is the molar concentration of equivalents. Its symbol, written, for example, for sulfuric acid is C eq (H 2 SO 4), the unit of measurement is mol / dm 3.

There are other designations for this concentration in the literature. For example, C (1 / 2H 2 SO 4). The fraction in front of the sulfuric acid formula indicates which part of the molecule (or ion) is equivalent. It is called the equivalence factor, denoted by f equiv. For H 2 SO 4 f equiv = 1/2. The equivalence factor is calculated based on the stoichiometry of the reaction. The number showing how many equivalents are contained in the molecule is called the equivalence number and is denoted by Z*. f equiv \u003d 1 / Z *, therefore, the molar concentration of equivalents is also denoted in this way: C (1 / Z * H 2 SO 4).

2. In the conditions of analytical laboratories, when it takes a long time to perform a series of single analyzes using one calculation formula, a correction factor, or correction K, is often used.

Most often, the correction refers to the working substance. The coefficient shows how many times the concentration of the prepared solution of the working substance differs from the concentration expressed in rounded numbers (0.1; 0.2; 0.5; 0.01; 0.02; 0.05), one of which may be in calculation formula:

K is written as numbers with four decimal places. From the record: K \u003d 1.2100 to C eq (HCl) \u003d 0.0200 mol / dm 3 it follows that C eq (HCl) \u003d 0.0200 mol / dm 3 is the standard molar concentration of HCl equivalents, then the true is calculated by formula:

3. Titer is the mass of the substance contained in 1 cm 3 of the volume of the solution.

Titer most often refers to a solution of the working substance.

The unit of titer is g/cm 3 , the titer is calculated to the sixth decimal place. Knowing the titer of the working substance, it is possible to calculate the molar concentration of the equivalents of its solution.

(4)

4. The titer of the working substance according to the analyte- this is the mass of the substance to be determined, equivalent to the mass of the working substance contained in 1 cm 3 of the solution.

5. The mass fraction of the solute is equal to the ratio of the mass of the solute A to the mass of the solution:

6. Volume fraction solute is equal to the ratio of the volume of solute A to the total volume of the solution:

Mass and volume fractions are dimensionless quantities. But most often the expressions for calculating the mass and volume fraction are written in the form:

; (9)

. (10)

In this case, the unit for w and j is a percentage.

Attention should be paid to the following circumstances:

1. When performing an analysis, the concentration of the working substance must be accurate and expressed as a number containing four decimal places if the concentration is molar equivalents; or a number containing six decimal places if it is a caption.

2. In all calculation formulas adopted in analytical chemistry, the unit of volume is cm 3. Since the glassware used in the analysis for measuring volumes allows you to measure the volume with an accuracy of 0.01 cm 3, it is with this accuracy that the numbers expressing the volumes of the solutions of analytes and working substances involved in the analysis should be recorded.

1.7.3. Methods for preparing solutions

Before proceeding with the preparation of the solution, the following questions should be answered.

1. For what purpose is the solution prepared (for use as an RV, to create a certain pH value of the medium, etc.)?

2. In what form is it most appropriate to express the concentration of the solution (in the form of molar concentration of equivalents, mass fraction, titer, etc.)?

3. With what accuracy, i.e. up to which decimal place should the number expressing the selected concentration be determined?

4. What volume of solution should be prepared?

5. Based on the nature of the substance (liquid or solid, standard or non-standard), which method of preparing the solution should be used?

The solution can be prepared in the following ways:

1. Accurate hitch.

If a substance from which to prepare the solution, is standard, i.e. meets certain (listed below) requirements, then the solution can be prepared by an accurate sample. This means that the sample weight is calculated and measured on an analytical balance with an accuracy of four decimal places.

The requirements for standard substances are as follows:

a) the substance must have a crystalline structure and correspond to a certain chemical formula;

c) the substance must be stable during storage in solid form and in solution;

d) a large one is desirable molar mass substance equivalent.

2. From the fix channel.

A variation of the method of preparing a solution for an accurate sample is the method of preparing a solution from fixanal. The role of an accurate sample is performed by the exact amount of the substance in the glass ampoule. It should be borne in mind that the substance in the ampoule can be standard (see paragraph 1) and non-standard. This circumstance affects the methods and duration of storage of solutions of non-standard substances prepared from fixanals.

FIXANAL(standard-titer, norm-dose) is a sealed ampoule, in which it is in dry form or in the form of a solution of 0.1000, 0.0500 or another number of moles of substance equivalents.

To prepare the required solution, the ampoule is broken over a funnel equipped with a special punching device (strike). Its contents are quantitatively transferred into a volumetric flask of the required capacity and the volume is adjusted with distilled water to the ring mark.

A solution prepared by an accurate sample or from fixanal is called titrated, standard or standard solution I, because its concentration after preparation is accurate. Write it as a number with four decimal places if it is a molar concentration of equivalents, and with six decimal places if it is a title.

3. By approximate weight.

If the substance from which the solution is to be prepared does not meet the requirements for standard substances, and there is no suitable fixanal, then the solution is prepared by an approximate weight.

Calculate the mass of the substance that must be taken to prepare the solution, taking into account its concentration and volume. This mass is weighed on technical scales with an accuracy of the second decimal place, dissolved in a volumetric flask. Get a solution with an approximate concentration.

4. By diluting a more concentrated solution.

If a substance is produced by the industry in the form of a concentrated solution (it is clear that it is non-standard), then its solution with a lower concentration can only be prepared by diluting the concentrated solution. When preparing a solution in this way, it should be remembered that the mass of the solute must be the same both in the volume of the prepared solution and in the part of the concentrated solution taken for dilution. Knowing the concentration and volume of the solution to be prepared, calculate the volume of the concentrated solution to be measured, taking into account its mass fraction and density. Measure the volume with a graduated cylinder, pour into a volumetric flask, dilute to the mark with distilled water, and mix. The solution prepared in this way has an approximate concentration.

The exact concentration of solutions prepared by an approximate sample and by diluting a concentrated solution is established by carrying out a gravimetric or titrimetric analysis, therefore, solutions prepared by these methods, after their exact concentrations are determined, are called solutions with a fixed titer, standardized solutions or standard solutions II.

1.7.4. Formulas used to calculate the mass of a substance needed to prepare a solution

If a solution with a given molar concentration of equivalents or titer is prepared from dry substance A, then the calculation of the mass of the substance that must be taken to prepare the solution is carried out according to the following formulas:

; (11)

. (12)

Note. The unit of measurement of volume is cm 3.

The calculation of the mass of a substance is carried out with such accuracy, which is determined by the method of preparation of the solution.

Calculation formulas, used in the preparation of solutions by the dilution method, are determined by the type of concentration to be obtained and the type of concentration to be diluted.

1.7.5. Scheme of Analysis

The main requirement for analysis is that the results obtained correspond to the true content of the components. The results of the analysis will satisfy this requirement only if all the analysis operations are performed correctly, in a certain sequence.

1. The first step in any analytical determination is sampling for analysis. As a rule, an average sample is taken.

Average sample- this is a part of the analyzed object, small in comparison with its entire mass, the average composition and properties of which are identical (the same) in all respects to its average composition.

Sampling methods various kinds products (raw materials, semi-finished products, finished products of different industries) are very different from each other. When sampling, they are guided by the rules described in detail in the technical manuals, GOSTs and special instructions dedicated to the analysis of this type of product.

Depending on the type of product and type of analysis, the sample can be taken in the form of a certain volume or a certain mass.

Sampling- this is a very responsible and important preparatory operation of the analysis. An incorrectly selected sample can completely distort the results, in which case it is generally meaningless to perform further analysis operations.

2. Sample preparation for analysis. A sample taken for analysis is not always prepared in some special way. For example, when determining the moisture content of flour, bread and bakery products using the arbitration method, a certain sample of each product is weighed and placed in an oven. Most often, the analysis is subjected to solutions obtained by appropriate processing of the sample. In this case, the task of sample preparation for analysis is reduced to the following. The sample is subjected to such processing, in which the amount of the analyzed component is preserved, and it completely goes into solution. In this case, it may be necessary to eliminate foreign substances that may be in the analyzed sample along with the component to be determined.

Sample preparation for analysis, as well as sampling, are described in the regulatory and technical documentation, according to which raw materials, semi-finished products and finished products are analyzed. Of the chemical operations that are included in the procedure for preparing a sample for analysis, we can name one that is often used in the preparation of samples of raw materials, semi-finished products, finished products in the food industry - this is the ashing operation.

Ashing is the process of converting a product (material) into ash. A sample is prepared by ashing when determining, for example, metal ions. The sample is burned under certain conditions. The remaining ash is dissolved in a suitable solvent. A solution is obtained, which is subjected to analysis.

3. Obtaining analytical data. During the analysis, the prepared sample is affected by a reagent substance or some kind of energy. This leads to the appearance of analytical signals (color change, the appearance of new radiation, etc.). The appeared signal can be: a) registered; b) consider the moment when it is necessary to measure a certain parameter in the analyzed system, for example, the volume of the working substance.

4. Processing of analytical data.

A) The obtained primary analytical data is used to calculate the results of the analysis.

There are different ways to convert analytical data into analysis results.

1. Calculation method. This method is used very often, for example, in quantitative chemical analysis. After completing the analysis, the volume of the working substance spent on the reaction with the analyte is obtained. Then this volume is substituted into the appropriate formula and the result of the analysis is calculated - the mass or concentration of the analyte.

2. Method of calibration (calibration) graph.

3. Method of comparison.

4. Method of additions.

5. Differential method.

These methods of processing analytical data are used in instrumental methods of analysis, during the study of which it will be possible to get to know them in detail.

B) The obtained results of the analysis must be processed according to the rules of mathematical statistics, about which we are talking in section 1.8.

5. Determining the socio-economic significance of the analysis result. This stage is final. Having completed the analysis and received the result, it is necessary to establish a correspondence between the quality of the product and the requirements of the regulatory documentation for it.

1.7.6. Method and technique of analysis

In order to be able to move from the theory of any method of analytical chemistry to a specific method of performing an analysis, it is important to distinguish between the concepts of "method of analysis" and "method of analysis".

When it comes to the method of analysis, this means that the rules are considered, following which one can obtain analytical data and interpret them (see section 1.4).

Analysis Method- this is a detailed description of all operations for performing the analysis, including taking and preparing samples (indicating the concentrations of all test solutions).

In the practical application of each method of analysis, many methods of analysis are developed. They differ in the nature of the analyzed objects, the method of taking and preparing samples, the conditions for carrying out individual analysis operations, etc.

For example, in a laboratory workshop on quantitative analysis, among others, laboratory works"Permanganometric determination of Fe 2+ in Mohr's salt solution", "Iodometric determination of Cu 2+", "Dichromatometric determination of Fe 2+". The methods for their implementation are completely different, but they are based on the same method of analysis "Redoximetry".

1.7.7. Analytical characteristics of analysis methods

In order for methods or methods of analysis to be compared or evaluated with each other, which plays an important role in their choice, each method and method has its own analytical and metrological characteristics. The analytical characteristics include the following: sensitivity coefficient (detection limit), selectivity, duration, performance.

Limit of detection(C min., p) is the smallest content, at which, using this method, it is possible to detect the presence of the determined component with a given confidence probability. Confidence probability - P is the proportion of cases in which the arithmetic mean of the result at given number definitions will be within certain limits.

In analytical chemistry, as a rule, a confidence level of P = 0.95 (95%) is used.

In other words, P is the probability of occurrence random error. It shows how many experiments out of 100 give results that are considered correct within the specified accuracy of the analysis. With P \u003d 0.95 - 95 out of 100.

Selectivity of the analysis characterizes the possibility of determining this component in the presence of foreign substances.

Versatility- the ability to detect many components from one sample at the same time.

Analysis duration- the time spent on its implementation.

Analysis performance- the number of parallel samples that can be analyzed per unit of time.

1.7.8. Metrological characteristics of analysis methods

Evaluating the methods or techniques of analysis from the point of view of the science of measurements - metrology - the following characteristics are noted: the interval of determined contents, correctness (accuracy), reproducibility, convergence.

Interval of determined contents- this is the area provided by this technique, in which the values ​​​​of the determined quantities of components are located. At the same time, it is also customary to note lower limit of determined contents(C n) - the smallest value of the determined content, limiting the range of determined contents.

Correctness (accuracy) of analysis- is the proximity of the obtained results to the true value of the determined value.

Reproducibility and convergence of results analysis are determined by the scatter of repeated analysis results and are determined by the presence of random errors.

Convergence characterizes the dispersion of results under fixed conditions of the experiment, and reproducibility- under changing conditions of the experiment.

All analytical and metrological characteristics of the method or method of analysis are reported in their instructions.

Metrological characteristics are obtained by processing the results obtained in a series of repeated analyzes. Formulas for their calculation are given in section 1.8.2. They are similar to formulas used for static processing of analysis results.

1.8. Errors (errors) in the analysis

No matter how carefully one or another quantitative determination is carried out, the result obtained, as a rule, differs somewhat from the actual content of the determined component, i.e. the result of the analysis is always obtained with some inaccuracy - an error.

Measurement errors are classified as systematic (certain), random (uncertain) and gross or misses.

Systematic errors- these are errors that are constant in value or vary according to a certain law. They can be methodical, depending on the specifics of the method of analysis used. They may depend on the instruments and reagents used, on incorrect or insufficiently careful performance of analytical operations, on the individual characteristics of the person performing the analysis. Systematic errors are difficult to notice, as they are constant and appear during repeated determinations. To avoid errors of this kind, it is necessary to eliminate their source or introduce an appropriate correction into the measurement result.

Random errors are called errors that are indefinite in magnitude and sign, in the appearance of each of which no regularity is observed.

Random errors occur in any measurement, including any analytical determination, no matter how carefully it is carried out. Their presence has the effect of repeated definitions of one or another component in a given sample, performed by the same method, usually give slightly different results.

Unlike systematic errors, random errors cannot be taken into account or eliminated by introducing any corrections. However, they can be significantly reduced by increasing the number of parallel determinations. The influence of random errors on the result of the analysis can be theoretically taken into account by processing the results obtained in a series of parallel determinations of this component using the methods of mathematical statistics.

Availability gross errors or misses manifests itself in the fact that among relatively close results, one or several values ​​are observed that stand out noticeably in magnitude from the general series. If the difference is so large that we can talk about a gross error, then this measurement is immediately discarded. However, in most cases it is impossible to immediately recognize that other result as incorrect only on the basis of “jumping out” from the general series, and therefore it is necessary to carry out additional research.

There are options when it makes no sense to conduct additional studies, and at the same time it is undesirable to use incorrect data to calculate the overall result of the analysis. In this case, the presence of gross errors or misses is determined according to the criteria of mathematical statistics.

Several such criteria are known. The simplest of these is the Q-test.

1.8.1. Determining the presence of gross errors (misses)

In chemical analysis, the content of a component in a sample is determined, as a rule, by a small number of parallel determinations (n ​​£ 3). To calculate the errors of definitions in this case, they use the methods of mathematical statistics developed for a small number of definitions. The results of this small number of determinations are considered as randomly selected - sampling- from all conceivable results of the general population under the given conditions.

For small samples with the number of measurements n<10 определение грубых погрешностей можно оценивать при помощи range of variation by Q-criterion. To do this, make the ratio:

, (13)

where X 1 - suspiciously distinguished result of the analysis;

X 2 - the result of a single definition, closest in value to X 1 ;

R - range of variation - the difference between the largest and smallest values ​​of a series of measurements, i.e. R = X max. - X min.

The calculated value of Q is compared with the tabular value of Q (p, f). The presence of a gross error is proved if Q > Q(p, f).

The result, recognized as a gross error, is excluded from further consideration.

The Q-criterion is not the only indicator whose value can be used to judge the presence of a gross error, but it is calculated faster than others, because. allows you to immediately eliminate gross errors without performing other calculations.

The other two criteria are more accurate, but require a full calculation of the error, i.e. the presence of a gross error can be said only by performing a complete mathematical processing of the analysis results.

Gross errors can also be identified:

A) standard deviation. The result X i is recognized as a gross error and discarded if

. (14)

B) Accuracy of direct measurement. The result X i is discarded if

. (15)

About quantities indicated by signs , see section 1.8.2.

1.8.2. Statistical processing of analysis results

Statistical processing of the results has two main tasks.

The first task is to present the result of the definitions in a compact form.

The second task is to evaluate the reliability of the obtained results, i.e. the degree of their correspondence to the true content of the determined component in the sample. This problem is solved by calculating the reproducibility and accuracy of the analysis using the formulas below.

As already noted, reproducibility characterizes the scatter of repeated analysis results and is determined by the presence of random errors. The reproducibility of the analysis is evaluated by the values ​​of standard deviation, relative standard deviation, variance.

The overall scatter characteristic of the data is determined by the value of the standard deviation S.

Sometimes, when assessing the reproducibility of an assay, the relative standard deviation Sr is determined.

The standard deviation has the same unit as the mean, or true value m of the quantity being determined.

The method or technique of analysis is the better reproducible, the lower the absolute (S) and relative (Sr) deviation values ​​for them.

The scatter of the analysis data about the mean is calculated as the variance S 2 .

(18)

In the presented formulas: Xi - individual value of the quantity obtained during the analysis; - arithmetic mean of the results obtained for all measurements; n is the number of measurements; i = 1…n.

The correctness or accuracy of the analysis is characterized by the confidence interval of the average value of p, f. This is the area within which, in the absence of systematic errors, the true value of the measured quantity is found with a confidence probability P.

, (19)

where p, f - confidence interval, i.e. confidence limits within which the value of the determined quantity X may lie.

In this formula, t p, f is the Student's coefficient; f is the number of degrees of freedom; f = n - 1; P is the confidence level (see 1.7.7); t p, f - given tabular.

Standard deviation of the arithmetic mean. (20)

The confidence interval is calculated either as an absolute error in the same units in which the result of the analysis is expressed, or as a relative error DX o (in %):

Therefore, the result of the analysis can be represented as:

. (23)

The processing of the results of the analysis is greatly simplified if the true content (m) of the determined component is known when performing analyzes (control samples, or standard samples). Calculate the absolute (DX) and relative (DX o, %) errors.

DX \u003d X - m (24)

1.8.3. Comparison of two average results of the analysis performed

different methods

In practice, there are situations when an object needs to be analyzed by different methods, in different laboratories, by different analysts. In these cases, average results differ from each other. Both results characterize some approximation to the true value of the desired quantity. In order to find out whether both results can be trusted, it is determined whether the difference between them is statistically significant, i.e. "too big. The average values ​​of the desired value are considered compatible if they belong to the same general population. This can be solved, for example, by the Fisher criterion (F-criterion).

where are the dispersions calculated for different series of analyses.

F ex - is always greater than one, because it is equal to the ratio of the larger variance to the smaller one. The calculated value of F ex is compared with the table value of F table. (confidence probability P and the number of degrees of freedom f for experimental and tabular values ​​should be the same).

When comparing F ex and F table options are possible.

A) F ex > F tab. The discrepancy between the variances is significant and the considered samples differ in reproducibility.

B) If F ex is significantly less than F table, then the difference in reproducibility is random and both variances are approximate estimates of the same general population variance for both samples.

If the difference between the variances is not significant, you can find out if there is a statistically significant difference in the average results of the analysis obtained different ways. To do this, use the Student's coefficient t p, f. Calculate the weighted average standard deviation and t ex.

; (27)

where are the average results of the compared samples;

n 1 , n 2 - the number of measurements in the first and second samples.

Compare t ex with t table with the number of degrees of freedom f = n 1 +n 2 -2.

If at the same time t ex > t table, then the discrepancy between is significant, the samples do not belong to the same general population and the true values ​​in each sample are different. If t ex< t табл, можно все данные рассматривать как единую выборочную совокупность для (n 1 +n 2) результатов.

TEST QUESTIONS

1. What does analytical chemistry study?

2. What is the analysis method?

3. What groups of methods of analysis are considered by analytical chemistry?

4. What methods can be used to perform qualitative analysis?

5. What are analytical features? What can they be?

6. What is a reagent?

7. What reagents are needed to perform a systematic analysis?

8. What is fractional analysis? What reagents are needed for its implementation?

9. What do the letters “chemically pure”, “ch.d.a.” mean? on the chemical label?

10. What is the task of quantitative analysis?

11.What is the working substance?

12. In what ways can a working substance solution be prepared?

13. What is a standard substance?

14. What do the terms “standard solution I”, “standard solution II” mean?

15. What is the titer and titer of the working substance according to the analyte?

16. How is the molar concentration of equivalents briefly indicated?

Analytical chemistry is a section that allows you to control the production and quality of products in various sectors of the economy. Exploration of natural resources is based on the results of these studies. Methods of analytical chemistry are used to control the degree of environmental pollution.

Practical significance

Analysis is the main option for determining the chemical composition of feed, fertilizers, soils, agricultural products, which is important for the normal functioning of the agro-industrial sector.

Qualitative and quantitative chemistry are indispensable in biotechnology and medical diagnostics. The efficiency and effectiveness of many scientific fields depends on the degree of equipment of research laboratories.

Theoretical basis

Analytical chemistry is a science that allows you to determine the composition and chemical structure of matter. Its methods help answer questions related not only to the constituent parts of a substance, but also to their quantitative ratio. With their help, you can understand in what form a particular component is in the substance under study. In some cases, they can be used to determine the spatial arrangement of composite components.

When thinking over methods, information is often borrowed from related fields of science, it is adapted to a specific area of ​​research. What questions does analytical chemistry solve? Methods of analysis make it possible to develop theoretical foundations, establish the boundaries of their use, evaluate metrological and other characteristics, and create methods for analyzing various objects. They are constantly updated, modernized, becoming more versatile and efficient.

When talking about the method of analysis, they assume the principle that is put in the expression of the quantitative relationship between the property being determined and the composition. Selected methods of conducting, including the identification and elimination of interference, devices for practical activities and options for processing the measurements taken.

Functions of Analytical Chemistry

There are three main areas of knowledge:

• decision general issues analysis;
• creation analytical methods;
• working out specific tasks.

Modern analytical chemistry is a combination of qualitative and quantitative analysis. The first section deals with the issue of the components included in the analyzed object. The second gives information about the quantitative content of one or more parts of the substance.

Classification of methods

They are divided into the following groups: sampling, decomposition of samples, separation of components, identification and determination of them. There are also hybrid methods that combine separation and definition.

Methods of determination are of the greatest importance. They are divided according to the nature of the analyzed property and the variant of registration of a certain signal. Problems in analytical chemistry often involve the calculation of certain components based on chemical reactions. To carry out such calculations, a solid mathematical foundation is required.

Among the main requirements that apply to the methods of analytical chemistry, we highlight:

• correctness and excellent reproducibility of the results obtained;
• low limit of determination of specific components;
• express;
• selectivity;
• simplicity;
• experiment automation.

When choosing an analysis method, it is important to clearly know the purpose and objectives of the study, to evaluate the main advantages and disadvantages of the available methods.

The chemical method of analytical chemistry is based on the qualitative reactions characteristic of certain compounds.

Analytical signal

After sampling and sample preparation are completed, the chemical analysis stage is carried out. It is associated with the detection of components in a mixture, the determination of its quantitative content.

Analytical chemistry is a science in which there are many methods, one of them is the signal. An analytical signal is the average of several measurements of a physical quantity at the last stage of analysis, which is functionally related to the content of the desired component. If it is necessary to detect a certain element, they use an analytical signal: sediment, color, line in the spectrum. Determining the amount of the component is associated with the mass of the deposit, the intensity of the spectral lines, and the magnitude of the current.

Methods of masking, concentration, separation

Masking is the inhibition or complete suppression of a chemical reaction in the presence of those substances that can change its speed or direction. There are two types of masking: equilibrium (thermodynamic) and non-equilibrium (kinetic). For the first case, conditions are created under which the reaction constant decreases so much that the process proceeds insignificantly. The concentration of the masked component will be insufficient for reliable fixation of the analytical signal. Kinetic masking is based on the growth of the difference between the velocities of the analyte and the masked substance with a constant reagent.

Carrying out concentration and separation is due to certain factors:

• there are components in the sample that interfere with the determination;
• the concentration of the analyte does not exceed the lower limit of detection;
• the detected components are unevenly distributed in the sample;
• the sample is radioactive or toxic.

Separation is the process by which the components present in the original mixture can be separated from each other.

Concentration is an operation due to which the ratio of the number of small elements to the number of macrocomponents increases.

Precipitation is suitable for separating several. Use it in combination with methods of determination designed to obtain an analytical signal from solid samples. The division is based on the different solubility of substances used in aqueous solutions.

Extraction

The Department of Analytical Chemistry involves laboratory research related to extraction. By it is meant the physicochemical process of the distribution of a substance between immiscible liquids. Extraction is also called the process of mass transfer during chemical reactions. Such research methods are suitable for extracting, concentrating macro- and microcomponents, as well as for group and individual isolation in the analysis of various natural and industrial objects. These techniques are simple and fast to perform, guarantee excellent concentration and separation efficiency, and are fully compatible with a variety of detection methods. Thanks to extraction, it is possible to consider the state of the component in solution at different conditions, as well as to identify its physico-chemical characteristics.

Sorption

It is used for concentration and separation of substances. Sorption technologies provide good selectivity of mixture separation. This is the process of absorption of vapors, liquids, gases by sorbents (solid-based absorbers).

Carburation and electrowinning

What else does analytical chemistry do? The textbook contains information about the method of electrodischarge, in which a concentrated or separated substance is deposited on solid electrodes in the form of a simple substance or as part of a compound.

Electrolysis is based on the precipitation of a specific substance with the help of electric current. The most common option is cathodic deposition of low-activity metals. The material for the electrode can be platinum, carbon, copper, silver, tungsten.

electrophoresis

It is based on differences in the speeds of movement of particles of different charges in an electric field with a change in tension, particle size. Currently, two forms of electrophoresis are distinguished in analytical chemistry: simple (frontal) and on a carrier (zone). The first option is suitable for a small volume of solution that contains the components to be separated. It is placed in a tube where there are solutions. Analytical chemistry explains all the processes that occur at the cathode and anode. In zone electrophoresis, the movement of particles is carried out in a stabilizing medium that keeps them in place after the current is turned off.

The carburizing method consists in the restoration of constituent parts on metals that have a significant negative potential. In such a case, two processes occur at once: cathodic (with the release of the component) and anode (the cementing metal dissolves).

Evaporation

Distillation relies on the varying volatility of chemicals. There is a transition from a liquid form to a gaseous state, then it condenses, again turning into a liquid phase.

With simple distillation, a single-stage separation process proceeds, followed by concentration of the substance. In the case of evaporation, those substances that are present in volatile form are removed. For example, among them there may be macro- and micro-components. Sublimation (sublimation) involves the transfer of a substance from a solid phase to a gas, bypassing the liquid form. A similar technique is used in cases where the substances to be separated are poorly soluble in water or melt poorly.

Conclusion

In analytical chemistry, there are many ways to isolate one substance from a mixture, to identify its presence in the sample under study. Chromatography is one of the most used analytical methods. It allows you to identify liquid, gaseous, solid substances that have molecular weight from 1 to 106 a. e. m. Thanks to chromatography, it is possible to obtain complete information about the properties and structure of organic substances of various classes. The method is based on the distribution of components between the mobile and stationary phases. Stationary is a solid substance (sorbent) or a liquid film that is deposited on a solid substance.

The mobile phase is a gas or liquid that flows through the stationary part. Thanks to this technology, it is possible to identify individual components, carry out the quantitative composition of the mixture, and separate it into components.

In addition to chromatography, gravimetric, titrimetric, and kinetic methods are used in qualitative and quantitative analysis. All of them are based on the physical and chemical properties of substances, allow the researcher to detect certain compounds in the sample, and to calculate their quantitative content. Analytical chemistry can rightfully be considered one of the most important branches of science.

Depending on the task, there are 3 groups of methods of analytical chemistry:

• 1) detection methods allow you to determine which elements or substances (analytes) are present in the sample. They are used for qualitative analysis;
• 2) methods of determination allow to establish the quantitative content of analytes in the sample and are used for quantitative analysis;
• 3) separation methods make it possible to isolate the analyte and separate the interfering components. They are used in qualitative and quantitative analysis. There are various methods of quantitative analysis: chemical, physicochemical, physical, etc.

Chemical methods are based on the use of chemical reactions (neutralization, redox, complexation and precipitation) in which the analyte enters. In this case, a qualitative analytical signal is a visual external effect of the reaction - a change in the color of the solution, the formation or dissolution of a precipitate, the release of a gaseous product. In quantitative determinations, the volume of the evolved gaseous product, the mass of the formed precipitate, and the volume of a reagent solution with a precisely known concentration, spent on interaction with the analyte, are used as an analytical signal.

Physical methods do not use chemical reactions, but measure any physical properties (optical, electrical, magnetic, thermal, etc.) of the analyte, which are a function of its composition.

Physico-chemical methods use a change in the physical properties of the analyzed system as a result of chemical reactions. Physicochemical methods also include chromatographic methods of analysis based on the processes of sorption-desorption of a substance on a solid or liquid sorbent under dynamic conditions, and electrochemical methods (potentiometry, voltammetry, conductometry).

Physical and physico-chemical methods are often combined under common name instrumental methods of analysis, since analytical instruments and apparatuses are used for analysis that record physical properties or their change. When carrying out a quantitative analysis, an analytical signal is measured - a physical quantity associated with the quantitative composition of the sample. If quantitative analysis is carried out using chemical methods, then the determination is always based on a chemical reaction.

There are 3 groups of quantitative analysis methods:

• - Gas analysis
• - Titrimetric analysis
• - Gravimetric analysis

The most important among the chemical methods of quantitative analysis are gravimetric and titrimetric methods, which are called classical methods of analysis. These methods are standard for evaluating the correctness of a definition. Their main field of application is the precision determination of large and medium quantities of substances.

Classical methods of analysis are widely used in the chemical industry to control the progress of the technological process, the quality of raw materials and finished products, industrial waste. Based on these methods, pharmaceutical analysis is also carried out - determining the quality of drugs and medicines, which are produced by chemical and pharmaceutical enterprises.

MOSCOW AUTOMOTIVE AND ROAD INSTITUTE (STATE TECHNICAL UNIVERSITY)

Department of Chemistry

I approve the head. department professor

I.M. Papisov "___" ____________ 2007

A.A. LITMANOVICH, O.E. LITMANOVYCH

ANALYTICAL CHEMISTRY Part 1: Qualitative Chemical Analysis

Toolkit

for students of the second year of the specialty "Engineering environmental protection"

MOSCOW 2007

Litmanovich A.A., Litmanovich O.E. Analytical Chemistry: Part 1: Qualitative Chemical Analysis: Methodological Guide / MADI

(GTU) - M., 2007. 32 p.

The main chemical laws of the qualitative analysis of inorganic compounds and their applicability for determining the composition of environmental objects are considered. The manual is intended for students of the specialty "Environmental Engineering".

© Moscow Automobile and Road Institute (state Technical University), 2008

CHAPTER 1. SUBJECT AND OBJECTIVES OF ANALYTICAL CHEMISTRY. ANALYTICAL REACTIONS

1.1. Subject and tasks of analytical chemistry

Analytical chemistry- the science of methods for studying the composition of substances. With the help of these methods, it is established which chemical elements, in what form and in what quantity are contained in the object under study. In analytical chemistry, two large sections are distinguished - qualitative and quantitative analysis. The tasks set by analytical chemistry are solved with the help of chemical and instrumental methods (physical, physicochemical).

AT chemical methods analysis the element to be determined is converted into a compound having such properties, with the help of which it is possible to establish the presence of this element or to measure its amount. One of the main ways to measure the amount of a formed compound is to determine the mass of a substance by weighing on an analytical balance - a gravimetric method of analysis. Methods of quantitative chemical analysis and instrumental methods of analysis will be discussed in part 2 methodological manual in analytical chemistry.

An urgent direction in the development of modern analytical chemistry is the development of methods for analyzing environmental objects, waste and waste water, gas emissions from industrial enterprises and road transport. Analytical control makes it possible to detect the excess content of especially harmful components in discharges and emissions, and helps to identify sources of environmental pollution.

Chemical analysis is based on the fundamental laws of general and inorganic chemistry with which you are already familiar. Theoretical basis chemical analysis include: knowledge of the properties of aqueous solutions; acid-base equilibria in aqueous

solutions; redox equilibria and properties of substances; patterns of complexation reactions; conditions for the formation and dissolution of the solid phase (precipitates).

1.2. analytical reactions. Conditions and methods for their implementation

Qualitative chemical analysis is carried out using analytical reactions accompanied by noticeable external changes: for example, gas evolution, color change, formation or dissolution of a precipitate, in some cases - the appearance of a specific odor.

Basic requirements for analytical reactions:

1) High sensitivity, characterized by the value of the detection limit (Cmin) - the lowest concentration of the component in the solution sample, at which this analysis technique allows you to confidently detect this component. The absolute minimum value of the mass of a substance that can be detected by analytical reactions is from 50 to 0.001 μg (1 μg = 10–6 g).

2) Selectivity- characterized by the ability of the reagent to react with as few components (elements) as possible. In practice, they try to detect ions under conditions under which the selective reaction becomes specific, i.e. allows you to detect this ion in the presence of other ions. As examples of specific reactions(of which there are few) are as follows.

a) The interaction of ammonium salts with an excess of alkali when heated:

NH4Cl + NaOH → NH3 + NaCl + H2O . (one)

The released ammonia is easy to recognize by its characteristic odor (“ammonia”) or by a change in the color of a wet indicator paper brought to the neck of the test tube. Reaction

allows you to detect the presence of ammonium ions NH4 + in the analyzed solution.

b) Interaction of ferrous salts with potassium hexacyanoferrate (III) K3 with the formation of a precipitate of blue color(Turnbull blue, or Prussian blue). Reaction (well familiar to you on the topic "Corrosion of metals" in the course

These reactions make it possible to detect Fe2+ and Fe3+ ions in the analyzed solution.

Specific reactions are convenient in that the presence of unknown ions can be determined fractional method- in separate samples of the analyzed solution containing other ions.

3) The speed of the reaction ( high speed) and ease of implementation.

The high reaction rate ensures the achievement of thermodynamic equilibrium in the system in a short time (practically with the rate of mixing of components in reactions in solution).

When performing analytical reactions, it is necessary to remember what determines the shift in the equilibrium of the reaction in the right direction and its flow to a large depth of transformation. For reactions occurring in aqueous solutions of electrolytes, the shift in thermodynamic equilibrium is affected by the concentration of ions of the same name, the pH of the medium, and the temperature. In particular, temperature depends the value of equilibrium constants - constants

dissociation for weak electrolytes and solubility products (PR) for sparingly soluble salts, bases

These factors determine the depth of the reaction, the yield of the product and the accuracy of the determination of the analyte (or the very possibility of detecting a certain ion at a small amount and concentration of the analyte).

The sensitivity of some reactions increases in an aqueous organic solution, for example, when acetone or ethanol is added to an aqueous solution. For example, in an aqueous ethanol solution, the solubility of CaSO4 is much lower than in an aqueous solution (the SP value is lower), which makes it possible to unambiguously detect the presence of Ca2+ ions in the analyzed solution at much lower concentrations than in an aqueous solution, and also to most completely free the solution from these ions (precipitation with H2 SO4 ) to continue the analysis of the solution.

In qualitative chemical analysis, a rational sequence is developed in the separation and detection of ions - a systematic course (scheme) of analysis. In this case, ions are separated from the mixture in groups, based on their equal relation to the action of certain group reagents.

One portion of the analyzed solution is used, from which groups of ions are sequentially isolated in the form of precipitation and solutions, in which individual ions are then detected . The use of group reagents makes it possible to decompose the complex problem of qualitative analysis into a number of simpler ones. The ratio of ions to the action of certain

group reagents is the basis analytical classification of ions.

1.3. preliminary analysis an aqueous solution containing a mixture of salts, by color, smell, pH value

The presence of a color in a clear solution proposed for analysis may indicate the presence of one or several ions at once (Table 1). The intensity of the color depends on the concentration of the ion in the sample, and the color itself can change if

metal cations form more stable complex ions than complex cations with H2O molecules as ligands, for which the color of the solution is indicated in Table. one .

		Table 1
Mortar color	Possible cations	Possible
Turquoise	Cu2+
	Cr3+
	Ni2+	MnO4 2-
	Fe3+ (due to hydrolysis)	CrO4 2- , Cr2 O7 2-
	Co2+	MnO4-

pH measurement of the proposed solution ( if the solution is prepared in water, and not in a solution of alkali or acid) also

gives additional		information about			possible composition
						table 2
Own-				Possible		Possible
ny pH water-
solution
	Hydrolysis			Na+ , K+ , Ba2+ ,		SO3 2- , S2- , CO3 2- ,
	educated			Ca2+		CH3COO-
				metals s-		(corresponding
	basis			electronic		acids are weak
	weak acid			families)		electrolytes)
	Hydrolysis			NH4+		Cl-, SO4 2- , NO3 - , Br-
	educated					(corresponding
				practically
	acid
				metals		electrolytes)
	basis
	Hydrolysis			Al3+ , Fe3+
	grounds

Aqueous solutions of some salts may have specific odors depending on the pH of the solution due to the formation of unstable (decomposing) or volatile compounds. By adding NaOH solutions to the sample solution or

strong acid (HCl, H2 SO4 ), you can gently smell the solution (Table 3).

Table 3

solution pH			Corresponding ion
after adding
			in solution
	Ammonia		NH4+
	(smell of ammonia)
		unpleasant	SO3 2-
	smell (SO2)
	"Vinegar"	(acetic	CH3COO-
	acid CH3COOH)
	(hydrogen sulfide H2S)

The reason for the smell (see Table 3) is the well-known property of reactions in electrolyte solutions - the displacement of weak acids or bases (often aqueous solutions of gaseous substances) from their salts by strong acids and bases, respectively.

CHAPTER 2. QUALITATIVE CHEMICAL ANALYSIS OF CATIONS

2.1. Acid-base method for classifying cations by analytical groups

The simplest and least “harmful” acid-base (basic) method of qualitative analysis is based on the ratio of cations to acids and bases. The classification of cations is carried out according to the following criteria:

a) solubility of chlorides, sulfates and hydroxides; b) basic or amphoteric character of hydroxides;

c) the ability to form stable complex compounds with ammonia (NH3) - ammoniates (i.e. amino complexes).

All cations are divided into six analytical groups using 4 reagents: 2M HCl solution, 1M H2SO4 solution, 2M NaOH solution and concentrated aqueous ammonia solution

NH4 OH (15-17%) (Table 4).

Table 4 Classification of cations by analytical groups

	Group		Result
			group action
			reagent
		Ag+ , Pb2+	Precipitate: AgCl, PbCl2
	1M H2SO4	(Pb2+ ), Ca2+ ,	Precipitate (white): BaSO4,
		Ba2+	(PbSO4 ), CaSO4
		Al3+ , Cr3+ , Zn2+	Solution: [Аl(OH)4]–,
	(excess)		– , 2–
	NH4 OH (conc.)	Fe2+ ​​, Fe3+ , Mg2+ ,	Precipitate: Fe(OH)2,
		Mn2+	Fe(OH)3 , Mg(OH)2 ,
			Mn(OH)2
	NH4 OH (conc.)	Cu2+ , Ni2+ , Co2+	Mortar (painted):
			2+ , blue
			2+ , blue
			2+ , yellow (on
			the air turns blue due to
			oxidation to Co3+ )
	Is absent	NH4 + , Na+ , K+

Obviously, the above list of cations is far from complete and includes the cations most frequently encountered in practice in the analyzed samples. In addition, there are other principles of classification by analytic groups.

2.2. Intragroup analysis of cations and analytical reactions for their detection

2.2.1. First group (Ag+ , Pb2+ )

Test solution containing Ag+, Pb2+ cations

↓ + 2M HCl solution + C 2 H5 OH (to reduce the solubility of PbCl2)

If PC > PR, are formed white precipitates of a mixture of chlorides,

which are separated from the solution (the solution is not analyzed):

Ag+ + Cl– ↔ AgCl↓ and Pb2+ + 2Cl– ↔ PbCl2 ↓ (3)

Obviously, at low concentrations of precipitated cations, the concentration of Cl– anions should be relatively high

↓ To sediment part + H2 O (distilled) + boiling

Partially goes into solution	In the sediment - all AgCl and
Pb 2+ ions (equilibrium shift	partially PbCl2
(3) to the left, because PC< ПР для PbCl2 )
	↓ + NH4 OH (conc.)
Detection in solution,
	1. Dissolution of AgCl due to
separated from sediment:	complexation:
1. With KI reagent (after	AgCl↓+ 2NH4 OH(e) →
cooling):	→+ +Cl– +2H2O
Pb2+ + 2I– → PbI2 ↓ (golden
crystals) (4)
	↓+ 2M HNO3 solution
	↓ to pH<3
	2. Precipitation of AgCl due to
	decay of a complex ion:
	Cl– + 2HNO3
	→AgCl↓+ 2NH4 + + 2NO3

↓ To the 2nd part of the sediment of the mixture of chlorides + 30%