Methods qualitative analysis are divided into physical, physicochemical and chemical.
Physical and physicochemical methods of analysis are based on the measurement of any parameter of the system, which is a function of the composition. For example, spectral analysis examines the emission spectra that occur when a substance is introduced into the flame of a burner or an electric arc. By the presence in the spectrum of lines characteristic of these elements, they learn about the elementary composition of the substance.
In physicochemical methods of analysis, the elementary composition of substances is judged by one or another characteristic properties of atoms or ions used in this method... For example, in chromatography, the composition of a substance is determined by the characteristic color of the ions adsorbed in a certain order, or by the color of the compounds formed during the development of the chromatogram.
It is not always possible to establish a strict boundary between physical and physicochemical methods. Therefore, they are often combined under the general name "instrumental" methods.
Chemical methods are based on the conversion of an analyte into new compounds with specific properties. By the formation of characteristic compounds of elements, the elementary composition of substances is established. For example, Cu 2+ ions can be detected by the formation of a complex ion [Cu (NH 3) 4] 2+ azure of blue color.
Qualitative analytical reactions, according to the way they are performed, are divided into "wet" and "dry" reactions. The most important are the "wet" reactions. To carry out them, the test substance must be previously dissolved. In qualitative analysis, only those reactions are used that are accompanied by any external effects well noticeable to the observer: a change in the color of the solution; precipitation or dissolution of the precipitate; the release of gases with a characteristic odor or color, etc.
Especially often used are reactions accompanied by the formation of precipitates and a change in the color of the solution. Such reactions are called "opening" reactions, since they are used to detect the ions present in the solution. Identification reactions are also widely used, with the help of which the correctness of the "discovery" of one or another ion is checked. Finally, precipitation reactions are employed, which usually separate one group of ions from another or one ion from other ions.
Depending on the amount of the analyte, the volume of the solution and the technique for performing individual operations, chemical methods of qualitative analysis are divided into macro-, micro-, semi-micro- and ultra-microanalysis, etc.
In 1955, the section of analytical chemistry of the International Association for Pure and Applied Chemistry adopted the "Classification of Methods of Analysis" and proposed their new names (Table 1.1).
Classic macro chemical analysis requires for the analysis from 1 to 10 g of the substance or from 10 to 100 ml of the test solution. It is carried out in ordinary 10-15 ml test tubes, while beakers and 150-200 ml flasks, filter funnels and other equipment are also used. Microchemical analysis allows you to analyze from 0.001 to 10 -6 g of a substance or from 0.1 to 10 -4 ml of the test solution. According to the technique of performing, microchemical analysis is divided into microcrystalloscopic and drop methods of analysis.
The microcrystalloscopic method of analysis is carried out using a microscope. On a microscope slide, a drop of the test solution is brought into interaction with a drop of the reagent. The resulting chemical compound is determined by the shape of the crystals, and sometimes by its color or optical properties.
The drop method of analysis has been introduced into analytical practice by NA Tananaev since 1920. With this method, reactions are carried out with drops of solutions and reagents with high sensitivity. Their use, therefore, makes it possible to detect very small amounts of cations. This type of analysis can be carried out on a porcelain plate, glass and watch glasses, and on filter paper.
In semi-microanalysis, a chemist works with samples of an investigated substance weighing from 0.05 to 0.5 g and operates with volumes of solutions from 1 to 10 ml. This type of analysis partially uses the techniques of macroanalysis and microanalysis. The utensils and equipment are the same as in the macroanalysis, but of a reduced type.
Methods of micro- and semi-microchemical analysis have whole line advantages over methods of macrochemical analysis; they allow you to carry out drop analysis with less time and reagents.
Dry analysis is carried out with solids. It is divided into pyrochemical analysis and rubbing analysis.
Pyrochemical analysis - heating a test substance in a flame gas burner... Let's consider two methods of analysis: obtaining colored pearls; flame coloring reactions.
Getting colored pearls. A number of metal salts and oxides when dissolved in molten sodium-ammonium phosphate NaNH 4 HPO 4 · 4H 2 O or sodium tetraborate Na 2 B 4 O 7 · l0H 2 O form glasses (pearls). By observing their color, it is possible to establish which elements are present in the substance under study. For example, chromium compounds give emerald green pearls; cobalt compounds - intense blue pearls; manganese compounds - violet-amethyst pearls; iron compounds - yellow-brown pearls; nickel compound - red-brown pearls, etc. The procedure for producing pearls is quite simple. A platinum wire, one end of which is bent into an eyelet and the other is soldered into a glass tube, is heated in a gas burner flame and immersed in salt, for example sodium tetraborate. Some of the salt melts near the hot wire and sticks to it. The crystal wire is first held over the flame of the burner and then placed in the colorless part of the flame and a colorless pearl is obtained. The test substance is touched with a hot pearl, then it is heated in the oxidizing flame of the burner until the substance is completely dissolved and the color of the pearl is noted in the hot and cold state.
Flame coloration reactions. Volatile salts of many metals, when introduced into the non-luminous part of the flame of a gas burner, paint the flame into different colors typical for these metals (Table 1.2). The color depends on the incandescent vapors of free metals resulting from the thermal decomposition of salts when they are introduced into the burner flame.
Flame coloration reactions work well only with volatile salts (chlorides, carbonates and nitrates). Non-volatile salts (borates, silicates, phosphates) are moistened before being introduced into a flame with a concentrated hydrochloric acid to convert them into volatile chlorides.
Pyrochemical analysis techniques are used in qualitative analysis as a preliminary test in the analysis of a mixture of solids or as verification reactions.
Rubbing analysis proposed in 1898 by F.M. Flavitsky. In the trituration method, the solid to be examined is placed in a porcelain mortar and triturated with approximately equal amounts of the solid reagent. As a result of the reaction, a colored substance is usually formed, according to the color of which the presence of the determined ion is judged. For example, to open the cobalt ion, several crystals of cobalt chloride CoCl 2 are ground with crystals of ammonium thiocyanate NH 4 SCN. In this case, the mixture turns blue due to the formation of a complex salt of tetrarodano (II) ammonium cobaltate (NH 4) 2:
CoCI 2 + 4NH 4 SCN = (NH 2) 2 + 2NH 4 C1
To open the acetate anion CH 3 COO - the salt crystal is triturated with a small amount of solid sodium hydrogen sulfate or potassium hydrogen sulfate. The liberated free acetic acid is recognized by its smell:
CH 3 COONa + NaHSO 4 = Na 2 SO + CH 3 COOH
The method of F.M.Flavitsky was almost never applied in practice, and only in the 50s did P.M.
In the qualitative analysis, the reactions "dry" play an auxiliary role; they are usually used as preliminary tests and verification reactions.
Already in the course of the study, one can assume about its results, but usually these conclusions are considered as preliminary, and more reliable and solid data can be obtained only as a result of careful analysis.
Data analysis in social work is about integrating all the information gathered and bringing it into a form that is easy to explain.
Methods for analyzing social information can be conditionally divided into two large classes in accordance with the form in which this information is presented:
- qualitative methods focused on the analysis of information presented mainly in verbal form.
- quantitative methods are mathematical in nature and represent processing techniques digital information.
Qualitative analysis is a prerequisite for the application quantitative methods, it aims to identify internal structure data, that is, to clarify those categories that are used to describe the studied sphere of reality. At this stage, the final definition of the parameters (variables) necessary for an exhaustive description takes place. When there are clear descriptive categories, it is easy to move on to the simplest measurement procedure - counting. For example, if you select a group of people in need of some assistance, then you can count the number of such people in this microdistrict.
With a qualitative analysis, it becomes necessary to produce compression of information, that is, to get data in a more compact form.
The main method of information compression is coding - the process of analyzing high-quality information, which includes the selection of semantic segments text or real behavior, their categorization (naming) and reorganization.
To do this, find and mark in the text itself keywords, that is, those words and expressions that carry the main semantic load directly indicate the content of the text as a whole or its separate fragment. Different types of highlighting are used: underlining with one or two lines, color coding, markings on the margins, which can have the character of both additional icons and comments. For example, you can highlight those fragments where the client talks about himself. On the other hand, you can single out everything that concerns his health, you can divide those problems that the client is able to solve himself, and those problems for the solution of which he needs outside help.
Fragments similar in content are marked in the same way. This allows them to be easily identified and, if necessary, collected together. Then the selected fragments are searched for under different headings. Analyzing the text, you can compare its individual fragments with each other, identifying similarities and differences.
The material processed in this way becomes easily visible. The main points come to the fore, as if rising above the mass of details. It becomes possible to analyze the relationship between them, to identify their general structure and, on this basis, to put forward some explanatory hypotheses.
When several objects are studied simultaneously (at least two) and when comparison with the aim of detecting similarities and differences becomes the main method of analysis, the comparative method is used. The number of objects studied here is small (most often two or three), and each of them is studied in depth and comprehensively enough.
It is necessary to find a form of data presentation that is most convenient for analysis. The main technique here is schematization. The scheme always simplifies real relationships, coarsens the true picture. In this sense, schematization of relations is at the same time a compression of information. But it also presupposes finding a visual and easily visible form of information presentation. This purpose is served by the consolidation of data in tables or charts.
For ease of comparison, the material is tabulated. General structure the table is as follows: each cell represents the intersection of a row and a column. The table is convenient in that it can include both quantitative and qualitative data. The point of the table is that it can be glanced around. Therefore, usually the table should fit on one sheet. The pivot table used for analysis is often drawn on a large piece of paper. But a large table can always be split into several parts, that is, make several tables out of it. Most often, the row corresponds to one case, and the columns represent its various aspects (signs).
Diagrams are another technique for concise and visual presentation of information. There are different types of diagrams, but almost all of them are structural diagrams, on which conventional figures (rectangles or ovals) depict elements, and lines or arrows - links between them. For example, it is convenient to use a diagram to represent the structure of any organization. Its elements are people, or rather, positions. If the organization is large, then larger elements are selected structural elements- divisions. With the help of the diagram, it is easy to represent the hierarchy of relations (subordination system): senior positions are located on the diagram above, and junior ones are below. The lines connecting the elements indicate exactly who is reporting to whom.
Representation in the form of diagrams can also be used to identify the logical structure of events or text. In this case, at first, a semantic analysis is carried out and nodal events or components are outlined, and then they are presented in a graphical form so that the connection between them becomes as clear as possible. It is clear that schematization leads to a coarsening of the picture due to the omission of many details. However, the information is compressed, transforming it into a form that is convenient for perception and memorization.
Thus, the main techniques of qualitative analysis are coding and visual presentation of information.
Quantitative analysis includes methods of statistical description of the sample and methods of statistical inference (testing statistical hypotheses).
Quantitative (statistical) methods of analysis are widely used in scientific research in general and in social sciences in particular. Sociologists use statistical methods to process the results of mass polls of public opinion. Psychologists use the apparatus of mathematical statistics to create reliable diagnostic tools - tests.
All methods quantitative analysis it is customary to divide into two large groups. Statistical description methods are aimed at obtaining a quantitative characteristics of the data obtained in a particular study. Statistical inference methods allow you to correctly extend the results obtained in a specific study to the entire phenomenon as such, to draw conclusions of a general nature. Statistical methods make it possible to identify stable trends and build on this basis theories designed to explain them.
Science always deals with the diversity of reality, but it sees its task in discovering the order of things, some stability within the observed diversity. Statistics provide convenient techniques for such analysis.
To use statistics, two basic conditions are required:
a) it is necessary to have data about a group (sample) of people;
b) these data must be presented in a formalized (codified) form.
It is necessary to take into account the possible sampling error, since only individual respondents are taken for the study, there is no guarantee that they are typical representatives of the social group as a whole. The sampling error depends on two points: on the sample size and on the degree of variation of the trait that interests the researcher. The larger the sample, the less likely it is that individuals with extreme values of the studied variable will get into it. On the other hand, the less the degree of variation of a feature, the closer each value will generally be to the true mean. Knowing the sample size, and having obtained a measure of the dispersion of observations, it is easy to derive an indicator called standard error of the mean. It gives the interval in which the true population average should lie.
Statistical inference is the process of testing hypotheses. Moreover, initially it is always assumed that the observed differences are random, that is, the sample belongs to the same the general population... In statistics, this assumption is called zero hypothesis.
Methodology for preparing final (qualification) work, requirements for its content and design
The graduation (qualification) work completes the training of a specialist in social work at the university and shows his readiness to solve theoretical and practical problems.
The final (qualification) work should be an independent completed development, in which actual problems are analyzed social work, the content and technologies of solving these problems are revealed not only in theoretical, but also in practical terms at the local, regional levels. Any graduation (qualification) work in social work should be a kind of social project.
The graduation (qualification) work must indicate that the author has deep and comprehensive knowledge of the object and subject of research, the ability to independent scientific research using the knowledge and skills gained during the development of the basic educational program;
The final (qualification) work should contain a justification for choosing a research topic, a review of published special literature on this issue, a statement of the research results obtained, specific conclusions and proposals.
The final (qualification) work must demonstrate the level of mastery of the author's methods scientific research and scientific language, his ability to concisely, logically and reasonably present the material.
The final (qualification) work should not mechanically repeat the educational work of the graduate (term papers, abstract papers, etc.).
Conclusions, suggestions and recommendations on the problems under study, put forward by the author to the bodies, organizations, institutions and services of social protection of the population, should be specific, have practical and theoretical value, have elements of novelty.
The objectives of the thesis:
Systematization, consolidation and expansion of theoretical and practical knowledge on social work, their application in solving specific practical problems;
Development of skills of independent work;
Mastering the research methodology, generalization and logical presentation of the material.
In the thesis, the student must show:
Strong theoretical knowledge on the chosen topic, problematic presentation of theoretical material;
Ability to study and summarize general and special literature on the topic, solve practical problems, draw conclusions and proposals;
Skills of analysis and calculations, experimentation, possession of computer technology;
Ability to competently apply methods for assessing the social effectiveness of the proposed activities.
The thesis has a clear composition: an introduction, a main part, consisting of several chapters, and a conclusion.
The introduction indicates the topic and purpose of the thesis, substantiates the relevance of the research, its theoretical and practical significance, are called the main research methods. It provides a rationale for addressing this topic, its relevance at the moment, the meaning, purpose and content of the tasks set, the object and subject of the study are formulated, it is reported what the theoretical significance and practical value of the results obtained are.
Themes of graduation (qualification) work are approved by the graduating departments. The topic should correspond to the specialty, when formulating it, it is advisable to take into account the scientific directions that have developed at the department and the possibility of providing students with qualified scientific leadership. It is desirable that the topics are relevant and have novelty, theoretical and practical significance. When formulating a topic, it is necessary to take into account the presence or absence of literature and practical materials, the student's own developments on the topic ( term papers, scientific reports, etc.), the student's interest in the chosen topic, the student's ability to conduct the necessary research.
Consequently, the introduction is a rather responsible part of the thesis, since it predetermines the further disclosure of the topic and contains the necessary qualification characteristics.
Relevance of the topic, importance, significance at the present time, modernity, topicality - a prerequisite for any scientific work... Justification of relevance is the initial stage of any research that characterizes the professional training of a student in how he knows how to choose a topic, formulate, how correctly he understands it and assesses it from the point of view of modernity, its scientific or practical significance. Coverage of relevance shouldn't be verbose. It is enough to show the essence of the problem, to determine where the border lies between knowledge and ignorance about the subject of research.
From the formulation of the scientific problem and evidence that its part, which is the object of the study of this work, has not yet received sufficient development and coverage in the scientific literature, it is logical to move to the formulation of the goal of the research undertaken, and also indicate the specific tasks to be solved in accordance with this purpose. Purpose of the study- what the graduate student is striving for in his thesis, what is going to be implemented, to establish why he undertook the development of this topic. In accordance with the set goal, the student will have to formulate specific research objectives as certain stages of research that must be passed in order to achieve the set goal.
In addition to the above, mandatory element introduction is the formulation of the object and subject of research, where an object is a process or phenomenon that generates a problem situation and is selected for research, and item- what is within the boundaries of the object. The object and the subject of research are related to each other as general and particular. It is on the subject of research that the main attention of the graduate student should be directed, since it is the subject of research that determines the topic of the work indicated on the title page.
An obligatory element of the introduction of a scientific work is also an indication of research methods, which serve as a tool in obtaining factual material, being a necessary condition for achieving the goal set in such work.
The introduction also describes other elements of the scientific process. These include, in particular, an indication on which specific material the work itself was performed. It also provides a description of the main sources of information (official, scientific, literary, bibliographic), and also indicates methodological foundations conducted research.
Main part consists of several chapters, which, in turn, are divided into paragraphs. In this compositional part, the main theoretical provisions of the thesis are presented, factual material is analyzed, and statistical data are provided. Possible illustrative material can be presented here, or included in the appendix.
In the main part of the work, the student reveals the methodology and research methodology, using the following methods for this purpose: observation, comparison, analysis and synthesis, induction and deduction, theoretical modeling, ascent from the abstract to the concrete, and vice versa.
The content of the chapters of the main part should exactly correspond to the topic of the work and fully disclose it. The conclusions made by the graduate student in the research must be consistent, reasoned, scientifically grounded. At the same time, argumentation is understood as a logical process, the essence of which is that it substantiates the truth of the stated judgment with the help of other judgments, examples, arguments.
Conclusion contains conclusions on thesis. Conclusions should reflect the main content of the work, be precise and concise. They should not be replaced by mechanical summation of conclusions at the end of chapters presenting short summary, but contain something new that makes up the final results of the study. It is here that the knowledge that is new in relation to the original knowledge is contained. It is it that is submitted for discussion and assessment by the state commission and the public in the process of defending the thesis.
If the work was of practical importance, the conclusions should contain instructions on where and how they can be applied in the practice of social work. In some cases, it becomes necessary to indicate the ways to continue the research of the topic, the tasks that future researchers will have to solve in the first place. The work is completed by a list of used normative materials and a list of used literature.
Auxiliary or additional materials that clutter up the text of the main part of the work are placed in the appendix. The content of the application can be quite varied. These, for example, can be copies of original documents (Statutes, Regulations, Instructions, reports, plans, etc.), individual extracts from instructions and rules, unpublished texts, etc. In form, they can be text, tables, graphs , cards.
The appendices cannot include a bibliography of used literature, auxiliary indexes of all kinds, reference comments and notes that are not appendices to the main text, but elements of the reference and accompanying apparatus of the work, helping to use its main text.
The final qualifying work is handed over to the department in printed form. The approximate volume of work should be 2-2.5 pp. (50-60 typewritten pages). Borders of fields: on the left - 3.5 cm; on the right - 1.5 cm, above and below - 2.5 cm. Computer typing is carried out in the text version of Microsoft Word (interval 1-1.5 by multiplier, 12-14th size Times New Roman).
All pages of the work, including pages with tables and diagrams, are numbered sequentially in Arabic numerals, usually located above the middle of the text.
The title page of the thesis includes the full name of the organization in which the work was performed, the name of the department, the title of the work, the code and name of the specialty, the surname and initials of the performer, surname, initials, scientific degree (position, title) of the supervisor, city and year of writing.
The titles of chapters and paragraphs are indicated in the same sequence and in the same wording as they are given in the text of the work.
The text of the main part of the work is divided into chapters, sections, subsections, clauses, paragraphs.
The graduation work issued in accordance with the requirements must be submitted to the graduating department no later than 14 days before the defense period. The terms of pre-defense and the terms of defense of the thesis are set by the graduating department.
T. N. ORKINA
CHEMISTRY
CHEMICAL AND PHYSICO-CHEMICAL ANALYSIS
Tutorial
Orkina T.N. Chemistry. Chemical and physicochemical analysis./ SPb .: Publishing house of Polytechnic. University, 2012 .-- p.
The goals and objectives of modern analytical chemistry - chemical, physicochemical and physical methods of analysis - are stated. Detailed theoretical basis and methodologies for conducting qualitative and quantitative analyzes. The description of laboratory work on the qualitative analysis of solutions and metal alloys, as well as calculations and methods of titrimetric (volumetric) analysis are given. The fundamentals of physical and chemical analysis are considered - the construction of phase diagrams, thermal analysis of metal alloys and the construction of fusibility diagrams.
The manual is intended for students of higher educational institutions studying in various areas and specialties in the field of engineering and technology in the direction of "Materials Science", "Metallurgy" and others. The manual can be useful for students studying in any technical specialties within the discipline "Chemistry".
INTRODUCTION
Analytical chemistry Is a branch of chemistry that studies the properties and processes of transformation of substances in order to establish their chemical composition... Establishing the chemical composition of substances (chemical identification) is the answer to the question of which elements or their compounds and in what quantitative ratios are contained in the analyzed sample. Analytical chemistry develops the theoretical foundations of the chemical analysis of substances and materials, develops methods for identification, detection, separation and determination chemical elements and their compounds, as well as methods for establishing the structure of matter. The discovery, or, as they say, the discovery of the elements or ions that make up the substance under study, constitute the subject qualitative analysis. Determination of the concentration or amount of chemical substances that make up the analyzed objects is a problem quantitative analysis ... Qualitative analysis usually precedes quantitative analysis, since quantitative analysis requires knowing the qualitative composition of the analyzed sample. When the composition of the object under study is known in advance, qualitative analysis is carried out as needed.
1. METHODS OF ANALYTICAL CHEMISTRY.
To detect any component, a so-called analytical signal is usually used. BUT political signal – these are visible changes in the object of research (sediment formation, color change, etc.) or changes in the parameters of measuring instruments (deviation of the instrument arrow, change in digital readout, appearance of a line in the spectrum, etc.). Chemical reactions are used to obtain an analytical signal different types(ion exchange, complexation, redox), various processes (for example, precipitation, gas evolution), as well as various chemical, physical and biological properties of the substances themselves and the products of their reactions. Therefore, analytical chemistry has various methods to solve its problems.
Chemical methods (chemical analysis) are based on carrying out a chemical reaction between the studied sample and specially selected reagents. In chemical methods, the analytical signal resulting from a chemical reaction is observed primarily visually.
Physicochemical methods analyzes based on quantitative study of dependence compound - physical property object. An analytical signal is an electrical (potential, current strength, resistance, etc.) or any other parameter (temperature of phase transformations, hardness, density, viscosity, saturated vapor pressure, etc.) associated with a certain functional dependence with the composition and concentration of the research object. ... Physicochemical research methods are usually associated with the use of highly sensitive equipment. The advantages of these methods are their objectivity, the possibility of automation and the speed of obtaining results. An example of a physicochemical method of analysis is the potentiometric determination of the pH of a solution using potentiometer measuring instruments. This method makes it possible not only to measure, but also to continuously monitor the change in pH during the course of any processes in solutions.
IN physical methods of analysis the analytical signal is usually obtained and recorded using special equipment. Physical methods, first of all, include optical spectroscopic methods of analysis based on the ability of atoms and molecules to emit, absorb and scatter electromagnetic radiation... By registering emission, absorption or scattering electromagnetic waves analyzed sample, receive a set of signals characterizing its qualitative and quantitative composition.
There is no sharp border between all three methods, so this division is somewhat arbitrary. For example, in chemical methods, the sample is first exposed to the action of a reagent, i.e. carry out a certain chemical reaction, and only after that the physical property is observed and measured. In the analysis by physical methods, observation and measurement are performed directly with the analyzed material using special equipment, and chemical reactions, if they are carried out, play an auxiliary role. In accordance with this, in chemical methods of analysis, the main attention is paid to the correct implementation of a chemical reaction, while in physicochemical and physical methods, the main emphasis is on the appropriate instrumentation for measurement - the determination of a physical property.
2. CLASSIFICATION OF CHEMICAL AND PHYSICO-CHEMICAL METHODS.
Chemical and physicochemical methods of analysis are classified depending on the mass and volume of the analyzed samples. According to the amount of a substance or mixture of substances (sample) used for analysis, macro-, semi-micro-, submicro-, and ultramicroanalysis are distinguished. Table 1 shows the ranges of mass and volume of sample solutions recommended by the analytical chemistry department of IUPAC (abbreviation from the English International Union of Theoretical and Applied Chemistry).
Table 1
Depending on the nature of the task, the following types of analysis are distinguished.
1 . Elemental analysis - establishing the presence and content of individual elements in a given substance, i.e. finding its elemental composition.
2 . Phase Analysis - establishing the presence and content of individual phases of the material under study. For example, the carbon in steel can be in the form of graphite or in the form of iron carbides. The task of phase analysis is to find how much carbon is in the form of graphite and how much in the form of carbides.
3 . Molecular analysis (material analysis) - establishing the presence and content of molecules of various substances (compounds) in the material. For example, the amount of CO, CO 2, N 2, O 2 and other gases is determined in the atmosphere.
4 . Functional analysis - establishing the presence and content functional groups in molecules of organic compounds, for example, amino groups (-NH 2), nitro (-NO 2), hydroxyl (-OH), carboxyl (-COOH) and other groups.
Depending on the nature of the analyzed material, a distinction is made between analysis of inorganic and organic substances. The separation of the analysis of organic substances into a separate section of analytical chemistry is associated with the characteristics of organic substances. Even the first stage of analysis - transferring the sample into solution - differs significantly for organic and inorganic substances.
The main stages of any chemical analysis complex materials are as follows.
1. Taking a sample for analysis. The average composition of the sample should correspond to the average composition of the entire batch of the analyzed material.
2. Decomposition of the sample and transferring it to solution. The sample is dissolved in water or acids, fused with various substances, or other methods or chemical influences are used.
3. Carrying out a chemical reaction: X + R = P, where X is a sample component; R is a reagent; P is the reaction product.
4. Fixation or measurement of any physical parameter of the reaction product, reagent or analyte.
Let's consider in more detail two types of chemical analysis - qualitative and quantitative analysis.
3. QUALITATIVE ANALYSIS
Qualitative analysis aims at identifying components and determining qualitative composition substances or mixtures of substances. The detection or, as they say, the discovery of elements and whether ions in the composition of the investigated substance is carried out by converting them into a compound with some characteristic properties, i.e., the appearance of an analytical signal is recorded. The chemical transformations occurring in this case are called analytical response . The substance with which the discovery is carried out - reagent or reagent .
There are different techniques of qualitative analysis that require the use of different quantities test substance in accordance with Table 1. For example: in macroanalytical method take about 1 g of the substance (0.5 g for metals and alloys) and dissolve in 20-30 ml of water. The reactions are carried out in test tubes (tube analysis). In case of microanalysis substances are taken about 100 times less compared to macroanalysis (milligrams solid matter and a few tenths of milliliters of solution). To open individual parts, highly sensitive reactions are used to detect the presence of small amounts of an element or ion. The reactions are carried out either by microcrystalline or by the drop method. Microcrystalline reactions performed on a glass slide and the presence of an element is judged by the shape of the crystals formed, which are examined under a microscope. Drip reactions , accompanied by a change in the color of the solution and the formation of colored precipitates, are performed on a strip of filter paper, dropping the test solutions and reagents onto it. Sometimes drop reactions are carried out on a special “drop plate” - a porcelain plate with indentations, as well as on a watch glass or in a small porcelain crucible. Semi-microanalysis (semi-micromethod) occupies an intermediate position between macro- and microanalysis. The amount of substance required to study the composition is approximately 20-25 times less than during macroanalysis - about 50 mg of solid and 1 ml of solution. In this method, the system of macroanalysis and the discovery of ions is preserved, but all reactions are performed with small quantities of a substance, using special techniques and equipment. For example, reactions are carried out in small 1-2 ml tubes, into which solutions are introduced using pipettes. Sediments are separated only by centrifugation. Submicroanalysis and ultramicroanalysis are carried out according to special methods using microscopes of various degrees of magnification, electron microscopes and other equipment. Their consideration is beyond the scope of this manual.
In qualitative analysis, chemical reactions are carried out most often in a solution, the so-called "Wet way". But sometimes it is possible to carry out solid-phase reactions, i.e. reactions "Dry way" . The substance and the corresponding reagents are taken in solid form and heated to a high temperature for carrying out the reactions. An example of such reactions is the reaction of coloring the flame with salts of some metals. It is known that sodium salts color the flame bright yellow, potassium salts purple, copper salts green. This color can detect the presence of these elements in the test substance. The "dry path" reactions also include the formation reactions colored pearls - glassy alloys of various salts ... For example borax - Na 2 B 4 O 7 10H 2 O or pearls of double salt NaNH 4 HPO 4 4H 2 O. These methods are called pyrochemical and are widely used to determine minerals and rocks... But basically, in qualitative analysis, reactions are carried out "wet" between solutes.
Qualitative analysis methodology
The first step in any analysis is to bring the sample into solution using various solvents. When analyzing inorganic substances, water, aqueous solutions of acids, alkalis, and less often other inorganic substances are most often used as solvents. Then, the characteristic reactions of the opening of ions are carried out. Qualitative reactions of the opening of ions are chemical reactions that are accompanied by an external effect (change in the color of the solution, gas evolution, formation of a precipitate), on the basis of which it can be judged that the reaction is taking place. Most often they deal with aqueous solutions of salts, acids, bases, between which ion-exchange reactions (less often - redox) take place.
One or another analytical reaction must be performed under certain conditions, depending on the properties of the resulting compounds. If these conditions are not met, the results of the discovery of ions may turn out to be unreliable. For example, acid-soluble precipitates do not fall out of solution with an excess of acid. Therefore, the following must be observed reaction conditions.
1. The correct environment of the test solution, which is created by the addition of acid or alkali.
2. The definite temperature of the solution. For example, the reactions of precipitation formation, the solubility of which greatly increases with temperature, are carried out in the "cold". Conversely, if the reaction is extremely slow, heating is required.
(3) A sufficiently high concentration of the ion being discovered, since at low concentrations the reaction does not proceed, i.e. the reaction is insensitive.
Concept "Reaction sensitivity" quantitatively characterized by two indicators: openable minimum and limit dilution. To experimentally determine the sensitivity, the reaction is repeated many times with the test solutions, gradually decreasing the amount of the solute and the volume of the solvent. Opened minimum(Υ) - this is the smallest amount of a substance that can be opened by means of a given reaction under certain conditions of its implementation. Expressed in micrograms (1Υ - millionths of a gram, 10 -6 g). The opened minimum cannot fully characterize the sensitivity of the reaction, since the concentration of the opened ion in the solution matters. Limit dilution(1: G) characterizes the lowest concentration of a substance (ion) at which it can be opened by this reaction; where G is the mass amount of the solvent per unit mass of the substance or ion being discovered. In macroanalysis and semi-micromethod, those reactions are used, the sensitivity of which exceeds 50Υ, and the limiting dilution is 1: 1000.
When performing analytical reactions, one should take into account not only sensitivity, but also specificity of the reaction - the possibility of opening a given ion in the presence of other ions. The discovery of ions through specific reactions, carried out in separate portions of the test solution in an arbitrary sequence, is called fractional analysis ... But there are not many specific reactions. More often we have to deal with reagents that give the same or similar effect of the reaction with many ions. For example, barium chloride precipitates carbonate and sulfate ions from solution in the form of BaCO 3 and BaSO 4 precipitates. Reagents that give the same analytical signal with a limited number of ions are called selective or selective ... The smaller the number of ions opened by a given reagent, the higher the degree of selectivity of the reagent.
Sometimes foreign ions do not react with this reagent, but they reduce the sensitivity of the reaction or change the nature of the products formed. In this case, it is necessary to take into account the limiting ratio of the concentrations of the discovered and foreign ions, and also use masking agents (techniques or reagents). The interfering ion is converted into low-dissociating compounds or complex ions, its concentration in the solution decreases, and this ion no longer interferes with the opening of the analyzed ions. All of the above features and techniques are used when developing a sequence of chemical reactions in the process of analysis. If the reactions used in the analysis are nonspecific, and the interfering influence of foreign ions cannot be eliminated, then the use of the fractional method becomes impossible and resort to systematic analysis .
The systematic course of analysis is a certain sequence of reactions, designed in such a way that the discovery of each ion is made only after the discovery and removal of all ions that interfere with this discovery. In a systematic course of analysis, a complex mixture of ions is used to isolate individual groups ions, using their similar relation to the action of some reagents, called group reagent ... For example, one of the group reagents is sodium chloride, which has a similar effect on Ag +, Pb 2+, Hg 2 2+ ions. The action of sodium chloride on soluble salts containing these cations leads to the formation of precipitates insoluble in hydrochloric acid:
Ag + + Cl - = AgCl ↓
Pb 2 + Cl - = PbCl 2 ↓
Hg 2 2+ + 2Cl - = Hg 2 Cl 2 ↓
All other ions, if exposed to HCl, will go into solution, and the three cations Ag +, Pb 2+ and Hg 2 2+ will be separated from the others using the NaCl group reagent. The use of group reagents is of great convenience: difficult task breaks down into a number of simpler ones. In addition, if any group of ions is completely absent, then its group reagent will not give any precipitate with the analyzed solution. In this case, it makes no sense to carry out reactions for individual ions of this group. The result is significant savings in labor, time and reagents. From the above it follows that in the qualitative analysis the classification of ions is based on the difference in the solubility of some of the compounds they form; on the basis of this difference, the method of separating one group of ions from another is based. The main classification of cations was introduced by the outstanding Russian chemist N.A. Menshutkin (1871) and is presented in the table.
The classification of anions is based on the solubility of barium and silver salts in the corresponding acids. This classification is not strictly established, as different authors subdivide anions into a different number of groups. One of the most common options is to subdivide the anions under study into three groups, as shown in Table 3. In contrast to cations, anions in most cases do not interfere with the detection of each other; therefore, it is necessary to resort to anion separation reactions only in rare cases. More often, the detection of anions is carried out by fractional analysis, i.e. in separate portions of the test solution. In the analysis of anions, group reagents are usually used not for separating groups, but only for their detection. The absence of any group in the test solution greatly facilitates the work.
table 2
Classification of cations
| Sulfides are soluble in water | Sulfides nerast | |||
| Carbonates are water soluble | Carbonates are insoluble in water | Sulfides or (hydroxides formed when they are decomposed by water) sol. in broken. acids | Sulfides are insoluble in dilute acids | |
| I group | II group | III group | IV group | V group |
| K +, Na +, NH 4 + Mg 2+, etc. | Ba 2+, Ca 2+, Sr 2+, etc. | Al 3+, Cr 3+, Fe 3+ Fe 2+, Mn 2+, Zn 2+ Ni 2+, Co 2+, etc. | a) I subgroup (chlorides are insoluble in water) Ag + Hg 2 2+, Pb 2+, b) II subgroup (chlorides sol. in water) Hg 2+, Cu 2+, Cd 2+, Bi 3+ | Sulfides are soluble in (NH 4) 2 S 2 As 5+, As 3+ Sb 5+, Sb 3+ Sn 4+, Sn 2+, etc. |
| No group reagent | Group reagent (NH 4) 2 CO 3 | Group reagent (NH 4) 2 S | Group reagent H 2 S in the presence. HCl (for the deposition of subgroup I - HCl) | Group reagent (NH 4) 2 S 2 |
Table 3
Classification of anions
3.2. Laboratory works on the topic "Qualitative Analysis"
Lecture 3
Qualitative analysis
1. Vasiliev V.P. Analytical chemistry: In 2 vols. : Book. 1: Titrimetric and gravimetric methods of analysis: textbook. for stud. universities studying in chemical technology. specialist. - 4th ed., Stereotype. - M.: Drofa, 2004 .-- 368 p. (Pp. 33 - 35, 263, 309 - 311).
2. Lebedeva M.I. Analytical chemistry and physicochemical methods of analysis: textbook. allowance / M.I. Lebedev. - Tambov: Publishing house of Tamb. state tech. University, 2005 .-- 216 p. - http://window.edu.ru/window_catalog/files/r38085/tstu2005-134.pdf
Qualitative analysis - this is an analysis, aim which is the determination of the chemical elements, ions, substances contained in the sample.
Qualitative analysis methods
Qualitative analysis methods are different: chemical, physical, physicochemical.
Methods of qualitative analysis that make it possible to determine the content of individual elements in the analyte are called elemental analysis; functional groups - functional analysis; individual chemical compounds characterized by a certain molecular weight, – molecular analysis.
A set of various chemical, physical and physicochemical methods of separation and determination of individual structural (phase) components of heterogeneous systems that differ in properties and physical structure and bounded from each other by interfaces are called phase analysis.
Chemical methods are based on the fact that the element or ion being discovered is converted into some compound with certain properties. The chemical transformation that occurs during this is called analytical response... The substance that causes this transformation is called reagent(reagent).
Analytical reactions can classify in the following way:
1. Group reactions: the same reagent reacts with a group of ions, giving the same signal. For example, to separate a group of ions (Ag +, Pb 2 +, Hg 2 2+), they use their reaction with Cl - - ions, thus forming white precipitates (AgCl, PbCl 2, Hg 2 Cl 2).
2. Selective (selective) reactions.
For example: iodine starch reaction. First described it in 1815 german chemist F. Stromeyer... For these purposes, organic reagents are used.
For example: dimethylglyoxime + Ni 2 + → formation of a scarlet-red precipitate of nickel dimethylglyoxime.
By changing the conditions of the analytical reaction, it is possible to make nonselective reactions selective.
For example: if the reactions Ag +, Pb 2 +, Hg 2 2 + + Cl - are carried out with heating, then PbCl 2 does not precipitate, since it is readily soluble in hot water.
3. Complexation reactions are used for masking interfering ions.
For example: for the detection of Co 2 + in the presence of Fe 3 + -ions using KSCN, the reaction is carried out in the presence of F - -ions. In this case, Fe 3 + + 4F - → -, K n = 10 - 16, therefore, Fe 3 + ions are complexed and do not interfere with the determination of Co 2 + ions.
In analytical chemistry are used the following reactions:
1. Hydrolysis(by cation, by anion, by cation and anion):
Al 3 + + HOH ↔ Al (OH) 2 + + H +;
CO 3 2 - + HOH ↔ HCO 3 - + OH -;
Fe 3 + + (NH 4) 2 S + HOH → Fe (OH) 3 + ...
2. Redox reactions:
2MnSO 4 + 5K 2 S 2 O 8 + 8H 2 O 2HMnO 4 + 10KHSO 4 + 2H 2 SO 4
3. Complexation reactions:
СuSO 4 + 4NH 4 OH → SO 4 + 4H 2 O
4. Precipitation reactions:
Ba 2 + + SO 4 2 - → BaSO 4 ↓
Qualitative analysis uses only those reactions accompanied by any well noticeable external effects:
1. Formation or dissolution draft:
Hg 2 + + 2I - → HgI 2 ↓;
HgI 2 + 2KI - → K 2 HgI 4
colorless
2. Appearance, change, disappearance coloration solution (color reactions):
Mn 2 + → MnO 4 - → MnO 4 2 -
colorless purple green
3. Isolation gas:
SO 3 2 - + 2H + → SO 2 + H 2 O.
4. Reactions of education crystals strictly defined form (microcrystalloscopic reactions).
5. Coloring reactions flame.
Analytical reactions can be carried out "dry" and "wet".
Examples of reactions carried out "dry":
- flame coloring reactions (Na + - yellow; Sr 2 + - red; Ba 2 + - green; Ca 2+ - brick red, K + - violet; Li + - crimson, Tl 3 + - green, In + - blue and etc.);
- upon fusion of Na 2 B 4 O 7 and Co 2 +, Na 2 B 4 O 7 and Ni 2 +, Na 2 B 4 O 7 and Cr 3 +, “ pearls»Borax of various colors. For example, Co 2 + compounds will give an intense blue color, Cr 3 + - emerald green.
Pearl color depends from what in which cone(zone) of the flame, heating occurs - oxidizing or reducing. In the center of the flame at the base the wick temperature reaches 320 0 С - this is recovery zone, higher located oxidation zone, the temperature in the upper part reaches 1550 0 С.
Method for obtaining pearls simple. Take platinum wire one end bend in the ear and the other is soldered into a glass tube... Platinum eyelet heat up in the flame of the burner and hot dipped in salt... The adhered salt is first kept under the flame of the burner so that water does not come out too intensely, and then melted into colorless pearl (borax salt Na 2 B 4 O 7 · 7H 2 O). After that, with a still hot pearl, they touch test substance and then reintroduced into the oxidizing part of the flame, obtaining color pearl. Observe the resulting color in a cold and hot state.
Most often analytical reactions are carried out in solutions ("Wet" way). The analyzed object (an individual substance or a mixture of substances) can be located in any state of aggregation(solid, liquid, gaseous). The object for analysis is called sample, or breakdown. The same item the sample may contain in various chemical forms... For example: S 0, S 2 -, SO 4 2 -, SO 3 2 - etc. Depending on the purpose and task of the analysis, after transferring the sample to the solution, carry out elemental analysis(determination of total sulfur content) or phase analysis(determination of the sulfur content in each phase or in its individual chemical forms).
Depending on which quantities of substance carry out operations when performing an analytical reaction, distinguish:
– macroanalysis- 1 - 10 g, 10 - 100 ml;
– semi-microanalysis- 0.05 - 0.5 g, up to 10 ml;
– microanalysis- 0.001 - 10 -6 g, 0.1 - 10-4 ml;
– ultramicroanalysis- 10 -6 - 10 -9 g, 10-4 - 10 -6 ml;
– submicroanalysis- 10 -9 - 10 -12 g, 10-7 - 10 -10 ml.
Exist drop analysis method introduced into analytical practice ON THE. Tananaev (1920)... Reactions are carried out on a porcelain plate, a glass slide, but most often on a strip of filter paper.
When performing this or that analytical reaction, it is necessary to strictly observe certain conditions its course (temperature, pH of the solution, concentration) so that it proceeds fast and had enough low detection limit. For example, precipitation, the solubility of which increases with increasing temperature, must be obtained only in the cold. At the same time, some precipitation is obtained only when heated.
Very important condition- a sufficiently high concentration of the ion being discovered in the solution. The smallest amount of a substance (ion) that can be discovered with the help of this reagent in a drop of a test solution with a volume of 1 microliter (10 -6 l) is called sensitivity of reaction.
Quantitatively, the sensitivity is characterized by by the following indicators:
– open minimum (m) Is the smallest amount of a substance or ion that can be opened by a given reaction under certain conditions.
m = from before V min 10 6 mcg
m = V min 10 6 / V before μg
where from before- limiting concentration; V min- the minimum volume of the extremely diluted solution; V before- limiting dilution.
– Limiting concentration(from before) Is the ratio of the unit mass of a certain ion to the mass of the largest amount of solvent.
, [μg / ml]
– Limit dilution(V before) Is the reciprocal of the limiting concentration and shows how much of an aqueous solution (in ml) contains 1 g of the ion to be determined.
; 
– Minimum volume(V min) Is the volume of the solution containing the opening minimum of a certain ion.
, [ml]
Reaction sensitivity serving to open one and the same ion, can be very strong differ. For example, sensitivity of reaction to Cu 2+:
- if HCl is used, then m= 1 μg, complex 2-yellow-green is formed;
- if NH 3 is used, then m= 0.2 μg, a blue complex 2+ is formed;
- if K 4 is used, then m= 0.02 μg, a red-brown complex of Cu 2 is formed.
To increase the sensitivity of the reaction you can use the following tricks:
– increase the duration reactions, which is especially effective if they take part in it non-electrolytes or weak electrolytes.
– add to solution ethanol reducing the solubility of inorganic compounds if the formation of a precipitate is observed in the reaction;
– shake up aqueous reaction mixture with any immiscible with water organic liquid.
The test solution may contain not one ion, but some... Using specific reactions, you can open the corresponding ion fractional method, i.e. directly in individual portions of the test solution, regardless of the ions that combine with the given. Fractional Analysis Has Been Opened Tananaev in 1950.
Dignity fractional analysis is rapidity its implementation. It plays an important role when it is subjected to analysis. limited mix ions and compound mixes approximately known.
Disadvantage fractional method is in some cases lack of reliable specific reactions for certain ions.
Therefore, for such ions, it is necessary to develop a specific sequence of reactions for the opening of individual ions, which is systematic analysis... It consists in the fact that to the discovery of each ion start only then, when all other ions hindering its opening will be pre-opened and deleted. For example, analysis of a mixture containing Ba 2+ and Ca 2+, open with oxalate ion C 2 O 4 2-:
Ва 2+ + С 2 О 4 2- → ВаС 2 О 4 ↓ (yellow)
filtrate-Ca 2+ + C 2 O 4 2- → CaC 2 O 4 ↓ (white)
In a systematic analysis, the ions stand out from complex mixtures not one by one, but whole groups using special reagents that give the same reaction. These reagents are called group reagents (group reagents). Such reagents significantly simplify analysis.