Analytical chemistry the science of methods for studying the composition of matter. It consists of two main sections: qualitative analysis and quantitative analysis. a set of methods for establishing the qualitative chemical composition of bodies - identifying atoms, ions, molecules that make up the substance being analyzed. The most important characteristics of each qualitative analysis method are: specificity and sensitivity. Specificity characterizes the ability to detect the desired element in the presence of other elements, for example, iron in the presence of nickel, manganese, chromium, vanadium, silicon, etc. Sensitivity determines the smallest amount of an element that can be detected by this method; sensitivity is expressed for modern methods by values of the order of 1 mcg(one millionth of a gram). Quantitative analysis is a set of methods for determining the quantitative composition of bodies, i.e., the quantitative ratios in which chemical elements or individual compounds are found in the analyzed substance. The most important characteristic Each method of quantitative analysis is, along with specificity and sensitivity, accuracy. The accuracy of the analysis is expressed by the value of the relative error, which in most cases should not exceed 1-2%. Sensitivity in quantitative analysis is expressed as a percentage. Many modern methods have very high sensitivity. Thus, using the method of radioactivation analysis, it is possible to determine the presence of copper in silicon with an accuracy of 2×10 -8%. Due to some specific features in A. x. It is customary to highlight the analysis of organic substances (see below). A special place in A. x. occupies based on the entire set of methods of qualitative and quantitative, inorganic and organic analysis in their application to a particular object. Technical analysis includes analytical control of production processes, raw materials, finished products, water, air, exhaust gases, etc. There is a particularly great need for “express” methods of technical analysis, requiring 5-15 min. for a separate definition. Determining the suitability of a product for human needs is equally ancient history, as well as its production itself. Initially, such a definition was aimed at establishing the reasons for the discrepancy between the resulting properties of products and the desired or necessary ones. This applied to food products - such as bread, beer, wine, etc., for testing of which taste, smell, color were used (these testing methods, called organoleptic, are also used in modern Food Industry). Raw materials and products of ancient metallurgy - ores, metals and alloys that were used for the manufacture of production tools (copper, bronze, iron) or for decoration and exchange of goods (gold, silver), were tested for their density, mechanical properties through trial swims. A set of similar methods for testing noble alloys is still used in assay analysis. The quality of the dyes was determined ceramic products, soap, leather, fabrics, glass, medicines. In the process of such analysis, individual metals (gold, silver, copper, tin, iron), alkalis, and acids began to be distinguished. During the alchemical period of the development of chemistry (see Alchemy), characterized by the development of experimental work, the number of distinguishable metals, acids, alkalis increased, the concept of salt, sulfur as a flammable substance, etc. arose. During the same period, many instruments for chemical research were invented , weighing of the studied and used substances was applied (14-16 centuries). The main significance of the alchemical period for the future of A. x. was that purely chemical methods were discovered for distinguishing individual substances; so, in the 13th century. it was discovered that " strong vodka» ( Nitric acid) dissolves silver, but does not dissolve gold, and “aqua regia” (a mixture of nitric and hydrochloric acids) also dissolves gold. Alchemists laid the foundation for chemical definitions; Before this, substances were distinguished by their physical properties. During the period of iatrochemistry (16-17 centuries) it increased even more specific gravity chemical research methods, especially methods of “wet” qualitative research of substances transferred into solutions: for example, silver and hydrochloric acid recognized by the reaction of their formation of precipitate in a nitric acid medium; used reactions to form colored products, for example iron with tannins. The beginning of the scientific approach to chemical analysis was laid by the English scientist R. Boyle (17th century), when he, having separated chemistry from alchemy and medicine and taking the basis of chemical atomism, introduced the concept of a chemical element as a further indecomposable component various substances. According to Boyle, the subject of chemistry is the study of these elements and how they combine to form chemical compounds and mixtures. Boyle called the decomposition of substances into elements “analysis.” The whole period of alchemy and iatrochemistry was largely a period of synthetic chemistry; Many inorganic and some organic compounds were obtained. But since synthesis was closely related to analysis, the leading direction in the development of chemistry at that time was precisely analysis. New substances were obtained through a process of increasingly refined decomposition of natural products. Thus, almost until the middle of the 19th century. chemistry developed primarily as chemical chemistry; The efforts of chemists were aimed at developing methods for determining qualitatively different principles (elements) and establishing quantitative laws of their interaction. Great value in chemical analysis there was differentiation of gases that were previously considered one substance; These studies began with the Dutch scientist van Helmont (17th century), who discovered carbon dioxide. The greatest successes in these studies were achieved by J. Priestley, K.V. Scheele, A.L. Lavoisier (18th century). Experimental chemistry received a solid basis in the law of conservation of mass of substances during chemical operations established by Lavoisier (1789). True, even earlier this law was expressed in a more general form by M. V. Lomonosov (1758), and the Swedish scientist T. A. Bergman used the conservation of the mass of substances for the purposes of chemical analysis. It is Bergman who is credited with creating a systematic course of qualitative analysis, in which the substances under study, transferred to a dissolved state, are then divided into groups using precipitation reactions with reagents and further divided into even smaller groups until it is possible to determine each element separately. Bergman proposed hydrogen sulfide and alkalis as the main group reagents, which are still used today. He also systematized qualitative analysis “dry”, by heating substances, which leads to the formation of “pearls” and deposits of various colors. Further improvement of systematic qualitative analysis was carried out by French chemists L. Vauquelin and L. J. Tenard, German chemists G. Rose and K. R. Fresenius, and Russian chemist N. A. Menshutkin. In the 20-30s. 20th century Soviet chemist N. A. Tananaev, based on a significantly expanded set chemical reactions, proposed a fractional method of qualitative analysis, in which there is no need for a systematic analysis, division into groups and the use of hydrogen sulfide. Quantitative analysis was initially based on precipitation reactions of the elements being determined in the form of poorly soluble compounds, the mass of which was then weighed. This gravimetric (or gravimetric) method of analysis has also improved significantly since Bergmann's time, mainly due to improvements in balances and weighing techniques and the use of various reagents, in particular organic ones, which form the least soluble compounds. In the 1st quarter of the 19th century. French scientist J. L. Gay-Lussac proposed a volumetric method of quantitative analysis (volumetric), in which, instead of weighing, the volumes of solutions of interacting substances are measured. This method, also called the titration method or titrimetric method, is still the main method of quantitative analysis. It has expanded significantly both due to an increase in the number of chemical reactions used in it (reactions of precipitation, neutralization, complex formation, oxidation-reduction), and through the use of many indicators (substances that indicate by changes in their color the end of the reaction between interacting solutions), etc. indication means (by identifying different physical properties solutions, such as electrical conductivity or refractive index). Analysis of organic substances containing carbon and hydrogen as main elements by combustion and determination of combustion products - carbon dioxide and water - was first carried out by Lavoisier. It was further improved by J. L. Gay-Lussac and L. J. Thénard and J. Liebig. In 1911, the Austrian chemist F. Pregl developed a technique for microanalysis of organic compounds, for which a few mg original substance. Due to the complex structure of molecules of organic substances, their large sizes (polymers), pronounced isomerism, organic analysis includes not only elemental analysis - determination of relative quantities individual elements in a molecule, but also functional - determining the nature and number of individual characteristic atomic groups in a molecule. Functional analysis is based on the characteristic chemical reactions and physical properties of the compounds being studied. Almost until the middle of the 20th century. analysis of organic substances, due to its specificity, developed in its own ways, different from inorganic analysis, and in training courses was not included in A. x. Organic matter analysis was considered as part of organic chemistry. But then, as new, mainly physical, methods of analysis emerged, wide application organic reagents in inorganic analysis, both of these branches of A. ch. began to come closer and now represent a single common scientific and educational discipline. A. x. how science includes the theory of chemical reactions and chemical properties substances and as such it is in the first period of development general chemistry coincided with her. However, in the 2nd half of the 19th century, when the “wet method” took a dominant position in chemical analysis, that is, analysis in solutions, mainly aqueous, the subject of chemical analysis. began to study only those reactions that give an analytically valuable characteristic product - an insoluble or colored compound that arises during a rapid reaction. In 1894, the German scientist W. Ostwald first outlined the scientific foundations of chemistry. as a theory of chemical equilibrium of ionic reactions in aqueous solutions. This theory, supplemented by the results of all subsequent developments of ionic theory, became the basis of atomic chemistry. The work of Russian chemists M.A. Ilyinsky and L.A. Chugaev (late 19th century - early 20th century) laid the foundation for the use of organic reagents, characterized by great specificity and sensitivity, in inorganic analysis. Research has shown that each inorganic ion is characterized by a chemical reaction with organic compound, containing a certain functional grouping (the so-called functional-analytical group). Since the 20s. 20th century in chemical analysis, the role of instrumental methods began to increase, again returning analysis to the study of the physical properties of the analytes, but not those macroscopic properties with which analysis operated in the period before the creation of scientific chemistry, and atomic and molecular properties. Modern A. x. widely uses atomic and molecular emission and absorption spectra (visible, ultraviolet, infrared, x-ray, radio frequency and gamma spectra), radioactivity (natural and artificial), mass spectrometry of isotopes, electrochemical properties of ions and molecules, adsorption properties, etc. (see Colorimetry, Luminescence, Microchemical analysis, Nephelometry, Activation analysis, Spectral analysis, Photometry, Chromatography, Electron paramagnetic resonance, Electrochemical methods of analysis). The application of analytical methods based on these properties is equally successful in inorganic and organic analysis. These methods significantly deepen the possibilities of deciphering the composition and structure of chemical compounds, their qualitative and quantitative determination; they make it possible to increase the sensitivity of determination to 10 -12 - 10 -15% of an impurity, require a small amount of the analyte, and can often be used for the so-called. non-destructive testing(i.e., a substance not accompanied by destruction of the sample) can serve as the basis for automating production analysis processes. At the same time, the widespread use of these instrumental methods poses new challenges for A. kh. as a science, requires a generalization of analysis methods not only based on the theory of chemical reactions, but also on the basis physical theory structure of atoms and molecules. A. ch., which plays an important role in the progress of chemical science, is also of great importance in the control industrial processes and in agriculture. Development of A. ch. in the USSR it is closely connected with the industrialization of the country and subsequent general progress. Many universities have departments of chemical engineering that train highly qualified analytical chemists. Soviet scientists are developing the theoretical foundations of A. chemistry. and new methods for solving scientific and practical problems. With the emergence of such industries as the nuclear industry, electronics, the production of semiconductors, rare metals, and cosmochemistry, at the same time there was a need to develop new subtle and subtle methods for controlling the purity of materials, where in many cases the impurity content should not exceed one atom per 1-10 million atoms manufactured product. All these problems are successfully solved by Soviet analytical chemists. Old methods of chemical production control are also being improved. Development of A. ch. As a special branch of chemistry, the publication of special analytical journals in all industrialized countries of the world came to life. In the USSR, 2 such magazines are published - “Factory Laboratory” (since 1932) and “Journal analytical chemistry"(since 1946). There are also specialized international journals on individual sections of chemical chemistry, for example, journals on chromatography and electroanalytical chemistry. Specialists in A. x. are prepared at special departments of universities, chemical-technological technical schools and vocational schools. Lit.: Alekseev V.N., Course of qualitative chemical semi-microanalysis, 4th ed., M. 1962: his own. Quantitative Analysis, 2nd ed. , M., 1958; Lyalikov Yu.S., Physico-chemical methods analysis, 4th ed., M., 1964; Yuing G.D. Instrumental methods of chemical analysis, trans. from English, M., 1960; Lurie Yu. Yu., Handbook of Analytical Chemistry, M., 1962. Yu. A. Klyachko.
Big Soviet encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
Examines the principles and methods of determining the chemical composition of a substance. Includes qualitative analysis and quantitative analysis. Analytical chemistry arose along with inorganic chemistry earlier than other chemical sciences (until the end of the 18th century, chemistry ... ... Big Encyclopedic Dictionary
analytical chemistry- (analytics) – a science that develops a general methodology, methods and means of obtaining experimental information about chemical composition substances and developing methods for analyzing various objects. Recommendations on terminology of analytical chemistry ... ... Chemical terms
ANALYTICAL CHEMISTRY, studies the principles and methods of identifying substances and their components (qualitative analysis), as well as determining the quantitative ratio of components (atoms, molecules, phases, etc.) in a sample (quantitative analysis). Up to 1st... ... Modern encyclopedia
ANALYTICAL CHEMISTRY- ANALYTICAL CHEMISTRY, a department of chemistry that develops theoretical basics and practical methods chemical analysis (see) ... Great Medical Encyclopedia
A course in physical and colloidal chemistry, including physical and chemical methods of analysis and methods of separation and purification, plays a significant role in the training of specialists in the field of environmental engineering. The main branches of physical chemistry - chemical kinetics and chemical thermodynamics - serve as the theoretical basis for other branches of chemistry, as well as chemical technology and methods for the separation and purification of substances. Measurements of the physicochemical properties of substances form the basis of many modern instrumental (physicochemical) methods of analysis and condition monitoring environment. Since most natural objects are colloidal systems, it is necessary to study the basics of colloidal chemistry.
The dangers of environmental contamination by products containing harmful substances can be significantly reduced by careful cleaning of products. Chemical cleaning methods include treatment with reagents that neutralize harmful components. It is necessary to know the speed and completeness of reactions, their dependence on external conditions, be able to calculate the concentration of reagents that provide the required degree of purification. Physicochemical purification methods are also widely used, including rectification, extraction, sorption, ion exchange, and chromatography.
Studying the course of physical and colloidal chemistry by students of environmental specialties (№№) includes mastering a theoretical (lecture) course, seminars on analytical chemistry, including physical and chemical methods of analysis, methods of separation and purification, chromatography and sections of colloidal chemistry, laboratory work and practical exercises , as well as independent work, including completing three homework assignments. During laboratory and practical work, students acquire skills in conducting physical and chemical experiments, plotting graphs, mathematical processing of measurement results and error analysis. When completing laboratory, practical and homework assignments, students acquire skills in working with reference literature.
Seminars on analytical and colloidal chemistry
Seminar 1. Subject of analytical chemistry. Classification of analysis methods. Metrology. Classic methods of quantitative analysis.
Specialists working in the field of environmental engineering need fairly complete information about the chemical composition of raw materials, production products, production waste and the environment - air, water and soil; special attention must be paid to identifying harmful substances and determining their quantities. This problem is solved analytical chemistry - the science of determining the chemical composition of substances. Chemical analysis is the main thing and necessary remedy pollution control.
A super-brief study of this section of chemistry cannot qualify an analytical chemist; its goal is to become familiar with a minimum amount of knowledge sufficient to set specific tasks for chemists, focusing on the capabilities of certain analytical methods, and understand the meaning of the obtained analytical results.
A distinction is made between qualitative and quantitative analysis. The first determines the presence of certain components, the second - their quantitative content. When studying the composition of a substance, qualitative analysis always precedes quantitative analysis, since the choice of a quantitative analysis method depends on the qualitative composition of the object being studied. Analysis methods are divided into chemical and physicochemical. Chemical methods of analysis are based on the transformation of the analyte into new compounds that have certain properties. The composition of a substance is determined by the formation of characteristic compounds of elements.
Qualitative analysis inorganic compounds based on ionic reactions and allows the detection of elements in the form of cations and anions. For example, Cu 2+ ions can be identified by the formation of a bright complex ion 2+ of blue color. When analyzing organic compounds, C, H, N, S, P, Cl and other elements are usually determined. Carbon and hydrogen are determined after burning the sample, recording the released carbon dioxide and water. There are a number of techniques for detecting other elements.
Qualitative analysis is divided into fractional and systematic.
Fractional analysis is based on the use of specific and selective reactions, with the help of which the desired ions can be detected in any sequence in individual portions of the test solution. Fractional analysis makes it possible to quickly determine a limited number of ions (one to five) contained in a mixture whose composition is approximately known.
Systematic analysis is a specific sequence of detecting individual ions after all other interfering ions have been found and removed from solution.
Individual groups of ions are isolated using similarities and differences in the properties of ions using so-called group reagents - substances that react equally with a whole group of ions. Groups of ions are divided into subgroups, and those, in turn, into individual ions, which are detected using the so-called. analytical reactions characteristic of these ions. Such reactions are necessarily accompanied by an analytical sign, that is, an external effect - the formation of a precipitate, the release of gas, a change in the color of the solution.
The analytical reaction has the properties of specificity, selectivity and sensitivity.
Specificity allows you to detect a given ion under certain conditions in the presence of other ions by one or another characteristic feature (color, smell, etc.). There are relatively few such reactions (for example, the reaction of detecting the NH 4 + ion by the action of an alkali on a substance when heated). Quantitatively, the specificity of the reaction is assessed by the value of the limiting ratio, equal to the ratio of the concentrations of the determined ion and interfering ions. For example, a droplet reaction to the Ni 2+ ion by the action of dimethylglyoxime in the presence of Co 2+ ions is possible with a limiting ratio of Ni 2+ to Co 2+ equal to 1: 5000.
The selectivity (or selectivity) of a reaction is determined by the fact that a similar external effect is possible only with a limited number of ions with which the reaction gives a positive effect. The degree of selectivity (selectivity) is greater, the smaller the number of ions with which the reaction produces a positive effect.
The sensitivity of a reaction is characterized by a number of mutually related quantities: the detection limit and the dilution limit. For example, the detection limit in the microcrystalloscopic reaction to the Ca 2+ ion under the action of sulfuric acid is 0.04 μg Ca 2+ in a drop of solution. The maximum dilution (V pre, ml) is calculated by the formula: V pre = V · 10 2 / C min, where V is the volume of the solution (ml). The limiting dilution shows the volume of solution (in ml) that contains 1 g of the ion being determined. For example, in the reaction of the K + ion with sodium hexanitrosocobaltate - Na 3, a yellow crystalline precipitate of K 2 Na is formed. The sensitivity of this reaction is characterized by a dilution limit of 1:50000. This means that using this reaction it is possible to open a potassium ion in a solution containing at least 1 g of potassium in 50,000 ml of water.
Chemical methods of qualitative analysis are of practical importance only for a small number of elements. For multielement, molecular, as well as functional (determining the nature of functional groups) analysis, physicochemical methods are used.
Components are divided into main (1 - 100% by weight), minor (0.01 - 1% by weight) and impurity or trace (less than 0.01% by weight).
Depending on the mass and volume of the analyzed sample, macroanalysis is distinguished (0.5 - 1 g or 20 - 50 ml),
semi-microanalysis (0.1 - 0.01 g or 1.0 - 0.1 ml),
microanalysis (10 -3 - 10 -6 g or 10 -1 - 10 -4 ml),
ultramicroanalysis (10 -6 - 10 -9 g, or 10 -4 - 10 -6 ml),
submicroanalysis (10 -9 - 10 -12 g or 10 -7 - 10 -10 ml).
The analyzed components can be atoms and ions, isotopes of elements, molecules, functional groups and radicals, phases.
Classification according to the nature of the particles being determined:
1.isotopic (physical)
2. elemental or atomic
3. molecular
4. structural-group (intermediate between atomic and molecular) - determination of individual functional groups in molecules of organic compounds.
5. phase - analysis of inclusions in heterogeneous objects, for example minerals.
Other types of classification analysis:
Gross and local.
Destructive and non-destructive.
Contact and remote.
Discrete and continuous.
Important characteristics of the analytical procedure are the rapidity of the method (speed of analysis), the cost of analysis, and the possibility of its automation.
MOSCOW AUTOMOBILE AND ROAD ROAD INSTITUTE (STATE TECHNICAL UNIVERSITY)
Department of Chemistry
I approve the Head. professor at the department
I.M. Papisov "___" ____________ 2007
A.A. LITMANOVICH, O.E. LITMANOVICH
ANALYTICAL CHEMISTRY Part 1. Qualitative chemical analysis
Toolkit
for second year students of the specialty “Engineering Environmental Protection”
MOSCOW 2007
Litmanovich A.A., Litmanovich O.E. Analytical chemistry: Part 1: Qualitative chemical analysis: Methodological manual / MADI
(GTU) - M., 2007. 32 p.
The main chemical laws qualitative analysis of inorganic compounds and their applicability for determining the composition of environmental objects. The manual is intended for students of the specialty “Engineering Environmental Protection”.
© Moscow Automobile and Highway Institute (state Technical University), 2008
CHAPTER 1. SUBJECT AND TASKS 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. Using these methods, it is established which chemical elements, in what form and in what quantities are contained in the object being studied. In analytical chemistry there are two large sections - qualitative and quantitative analysis. Analytical chemistry solves the assigned problems using chemical and instrumental methods (physical, physicochemical).
IN chemical methods analysis the element being determined is converted into a compound that has properties that can be used to establish the presence of this element or measure its quantity. One of the main ways to measure the amount of a compound formed is to determine the mass of the substance by weighing on an analytical balance - the 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.
A current direction in the development of modern analytical chemistry is the development of methods for analyzing environmental objects, waste and waste waters, gas emissions from industrial enterprises and road transport. Analytical control makes it possible to detect excess content of particularly 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 equilibrium in water
solutions; redox equilibria and properties of substances; patterns of complex formation reactions; conditions for the formation and dissolution of the solid phase (precipitates).
1.2. Analytical reactions. Conditions and methods of their implementation
Qualitative chemical analysis is carried out using analytical reactions, accompanied by noticeable external changes: for example, the release of gas, a change in color, the 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 a component in a solution sample at which this analysis technique allows one to reliably 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 a 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 a given ion in the presence of other ions. As examples of specific reactions(of which there are few) the following can be cited.
a) Interaction of ammonium salts with excess alkali when heated:
NH4 Cl + NaOH → NH3 + NaCl + H2 O. (1)
The ammonia released is easily recognized by its characteristic odor (“ ammonia") or by a change in the color of 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) The interaction of ferrous iron salts with potassium hexacyanoferrate (III) K3 with the formation of a blue precipitate (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 because the presence of unknown ions can be determined by a 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 that thermodynamic equilibrium is achieved in the system within a short time(almost at the rate of mixing of components during reactions in solution).
When performing analytical reactions, it is necessary to remember what determines the shift in the equilibrium of the reaction in the desired direction and its occurrence to a large depth of transformation. For reactions occurring in aqueous solutions of electrolytes, the shift in thermodynamic equilibrium is influenced by the concentration of ions of the same name, pH of the medium, and temperature. In particular, it depends on temperature the value of the equilibrium constants – constants
dissociation for weak electrolytes and solubility product (SP) for poorly soluble salts and bases
These factors determine the depth of the reaction, the yield of the product and the accuracy of determining the analyte (or the very possibility of detecting a specific 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 significantly lower than in an aqueous one (the PR value is smaller), 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 analyzing the solution.
In qualitative chemical analysis, a rational sequence in the separation and detection of ions is developed - a systematic flow (scheme) of analysis. In this case, ions are separated from the mixture in groups based on their equal treatment 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 precipitates and solutions, in which individual ions are then detected . The use of group reagents makes it possible to decompose difficult task qualitative analysis into a number of simpler ones. The ratio of ions to the action of certain
group reagents are the basis analytical classification ions.
1.3. Preliminary analysis of an aqueous solution containing a mixture of salts by color, smell, pH value
The presence of color in a transparent 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 H2 O molecules as ligands, for which the color of the solution is indicated in Table. 1 .
Table 1 |
||
Solution color | Possible cations | Possible |
Turquoise | Cu2+ | |
Cr3+ | ||
Ni2+ | MnO4 2- |
|
Fe3+ (due to hydrolysis) | CrO4 2- , Cr2 O7 2- |
|
Co2+ | MnO4 - |
Measuring the pH 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 |
|||||
water pH | |||||||
nogo sol- | |||||||
Hydrolysis | Na+ , K+ , Ba2+ , | SO3 2- , S2- , CO3 2- , |
|||||
educated | Ca2+ | CH3 COO- | |||||
metals s- | (corresponding |
||||||
basis | electronic | acids – 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 or
strong acid (HCl, H2 SO4), you can gently sniff the solution (Table 3).
Table 3
pH of the sample solution | Corresponding ion |
||
after adding | |||
in solution |
|||
Ammonia | NH4+ |
||
(smell of ammonia) | |||
unpleasant | SO3 2- |
||
smell (SO2) | |||
"Vinegar" | (acetic | CH3 COO- |
|
acid CH3 COOH) | |||
(hydrogen sulfide H2 S) | |||
The reason for the appearance of the odor (see Table 3) is the well-known property of reactions in electrolyte solutions - the displacement of weak acids or bases (often aqueous solutions gaseous substances) from their salts with strong acids and bases, respectively.
CHAPTER 2. QUALITATIVE CHEMICAL ANALYSIS OF CATIONS
2.1. Acid-base method for classifying cations according to 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. Cations are classified according to the following criteria:
a) solubility of chlorides, sulfates and hydroxides; b) basic or amphoteric nature of hydroxides;
c) the ability to form stable complex compounds with ammonia (NH3) - ammonia (i.e. ammine 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 actions |
|||
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– |
||
NH4OH (conc.) | Fe2+, Fe3+, Mg2+, | Precipitate: Fe(OH)2, |
|
Mn2+ | Fe(OH)3, Mg(OH)2, |
||
Mn(OH)2 |
|||
NH4OH (conc.) | Cu2+, Ni2+, Co2+ | Solution (colored): |
|
2+, blue |
|||
2+, blue |
|||
2+, yellow (on |
|||
the air turns blue due to |
|||
oxidation to Co3+) |
|||
Absent | NH4 + , Na+ , K+ |
Obviously, the given 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 analytical groups.
2.2. Intragroup analysis of cations and analytical reactions for their detection
2.2.1. First group (Ag+, Pb2+)
Test solution containing cations Ag+, Pb2+
↓ + 2M solution of HCl + C 2 H5 OH (to reduce the solubility of PbCl2)
If PC > PR, 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)
It is obvious that at low concentrations of precipitated cations, the concentration of Cl– anions should be relatively high
↓ To part of the sediment + H2 O (distilled) + boiling
Partially goes into solution | The sediment contains 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 the sediment: | complexation: | ||
1. With reagent KI (after | AgCl↓+ 2NH4 OH(g) → | ||
cooling): | →+ +Cl– +2H2 O |
||
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 a mixture of chlorides + 30%
Its subject as a science is the improvement of existing and development of new methods of analysis, their practical application, and the study of the theoretical foundations of analytical methods.
Depending on the task, analytical chemistry is subdivided into qualitative analysis, aimed at determining whether What or which substance, in what form it is in the sample, and quantitative analysis aimed at determining How many of a given substance (elements, ions, molecular forms, etc.) is in the sample.
Determining the elemental composition of material objects is called elemental analysis. Establishing the structure of chemical compounds and their mixtures at the molecular level is called molecular analysis. One of the types of molecular analysis of chemical compounds is structural analysis, aimed at studying the spatial atomic structure of substances, establishing empirical formulas, molecular masses, etc. The tasks of analytical chemistry include determining the characteristics of organic, inorganic and biochemical objects. Analysis of organic compounds by functional groups is called functional analysis.
Analytical chemistry has existed as long as chemistry has existed in its modern sense, and many of the techniques used in it date back to an even earlier era, the era of alchemy, one of the main tasks of which was precisely determining the composition of various natural substances and studying the processes of their mutual transformations. But, with the development of chemistry as a whole, the methods of work used in it were significantly improved, and, along with its purely auxiliary significance as one of the auxiliary departments of chemistry, analytical chemistry now has the significance of a completely independent department of chemical knowledge with very serious and important theoretical tasks. Modern physical chemistry had a very important influence on the development of analytical chemistry, which enriched it with a number of completely new methods of work and theoretical foundations, which include the doctrine of solutions (see), the theory of electrolytic dissociation, the law of mass action (see Chemical equilibrium) and the whole doctrine of chemical affinity.
Totality traditional methods Determining the composition of a substance by its sequential chemical decomposition is called “wet chemistry” (“wet analysis”). These methods have relatively low accuracy, require relatively low qualifications of analysts and are now almost completely replaced by modern ones. instrumental methods(optical, mass spectrometric, electrochemical, chromatographic and other physicochemical methods) determining the composition of a substance. However, wet chemistry has its advantage over spectrometric methods - it allows, through standardized procedures (systematic analysis), to directly determine the composition and different oxidative states of elements such as iron (Fe +2, Fe +3), titanium, etc.
Analytical methods can be divided into gross and local. Bulk methods of analysis usually require a separated, subdivided substance (a representative sample). Local Methods determine the composition of a substance in a small volume in the sample itself, which makes it possible to compile “maps” of the distribution of the chemical properties of the sample over its surface and/or depth. Methods should also be highlighted direct analysis, that is, not related to the preliminary preparation of the sample. Sample preparation is often necessary (eg crushing, pre-concentration or separation). Statistical methods are used when preparing samples, interpreting results, and estimating the number of analyzes.
To determine the qualitative composition of a substance, it is necessary to study its properties, which, from the point of view of analytical chemistry, can be of two types: the properties of the substance as such, and its properties in chemical transformations.
The first include: physical state (solid, liquid, gas), its structure in the solid state (amorphous or crystalline substance), color, smell, taste, etc. In this case, it is often already based on external properties alone, determined with the help of organs human feelings, it seems possible to establish the nature of a given substance. In most cases, it is necessary to transform a given substance into some new one with clearly defined characteristic properties, using for this purpose some specially selected compounds called reagents.
The reactions used in analytical chemistry are extremely diverse and depend on the physical properties and degree of complexity of the composition of the substance being studied. In the case where a obviously pure, homogeneous chemical compound is subject to chemical analysis, the work is done relatively easily and quickly; when you have to deal with a mixture of several chemical compounds, the question of its analysis becomes more complicated, and when doing work you need to adhere to some specific system in order not to overlook a single element included in the substance. There are two types of reactions in analytical chemistry: wet reactions(in solutions) and dry reactions.
In qualitative chemical analysis, only reactions in solutions are used that are easily perceived by human senses, and the moment of occurrence of the reaction is recognized by one of the following phenomena:
Formation of sediment in reactions of chemical analysis depends on the formation of some water-insoluble substance; if, for example, sulfuric acid or a water-soluble salt is added to a solution of any barium salt, a white powdery precipitate of barium sulfate is formed:
BaCl 2 + H 2 SO 4 = 2HCl + BaSO 4 ↓
Keeping in mind that some other metals can give a similar reaction to the formation of a white precipitate under the influence of sulfuric acid, for example, lead, which can form the insoluble sulfate salt PbSO 4, to be completely sure that this is exactly one or another metal, it is necessary to produce more calibration reactions, subjecting the precipitate formed in the reaction to appropriate research.
To successfully carry out the reaction of precipitation formation, in addition to selecting the appropriate reagent, it is also necessary to observe a number of very important conditions regarding the strength of solutions of the salt and reagent being studied, the proportion of both, temperature, duration of interaction, etc. When considering precipitation formed in chemical reactions analysis, it is necessary to pay attention to their appearance, that is, color, structure (amorphous and crystalline precipitates), etc., as well as their properties in relation to the influence of heat, acids or alkalis, etc. When interacting weak solutions Sometimes it is necessary to wait for the formation of sediment for up to 24-48 hours, provided that they are kept at a certain temperature.
The reaction of precipitate formation, regardless of its qualitative significance in chemical analysis, is often used to separate certain elements from each other. For this purpose, a solution containing compounds of two or more elements is treated with an appropriate reagent capable of converting some of them into insoluble compounds, and then the resulting precipitate is separated from the solution (filtrate) by filtration, further studying them separately. If we take, for example, the salts of potassium chloride and barium chloride and add sulfuric acid to them, an insoluble precipitate of barium sulfate BaSO 4 and water-soluble potassium sulfate K 2 SO 4 are formed, which can be separated by filtration. When separating a precipitate of a water-insoluble substance from a solution, care must first be taken to ensure that it receives an appropriate structure that allows the filtering work to be carried out without difficulty, and then, having collected it on the filter, it is necessary to thoroughly wash it from foreign impurities. According to the research of V. Ostwald, it must be borne in mind that when using a certain amount of water for washing, it is more advisable to rinse the sediment many times in small portions of water than, on the contrary, several times in large portions. As for the success of the separation reaction of any element in the form of an insoluble precipitate, then, based on the theory of solutions, W. Ostwald established that for a sufficiently complete separation of any element in the form of an insoluble precipitate, it is always necessary to take an excess of the reagent used for precipitation .
Change in solution color is one of the very important signs in the reactions of chemical analysis and is very important, especially in connection with the processes of oxidation and reduction, as well as in work with chemical indicators (see below - alkalimetry and acidimetry).
Examples color reactions in qualitative chemical analysis the following can be used: potassium thiocyanate KCNS gives a characteristic blood-red color with iron oxide salts; with ferrous oxide salts the same reagent does not produce anything. If you add any oxidizing agent, for example, chlorine water, to a solution of slightly green ferric chloride FeCl 2, the solution turns yellow due to the formation of ferric chloride, which is the highest oxidation state of this metal. If you take potassium dichromate K 2 Cr 2 O 7 orange in color and add to it in solution a little sulfuric acid and some reducing agent, for example, wine alcohol, the orange color changes to dark green, corresponding to the formation of a lower oxidation state of chromium in the form of a salt chromium sulfate Cr 3 (SO 4) 3.
Depending on the progress of the chemical analysis, it is often necessary to carry out these processes of oxidation and reduction. The most important oxidizing agents are: halogens, nitric acid, hydrogen peroxide, potassium permanganate, potassium dihydroxide; the most important reducing agents are: hydrogen at the time of release, hydrogen sulfide, sulfurous acid, tin chloride, hydrogen iodide.
Gas evolution reactions in solutions during the production of qualitative chemical analysis most often have no independent significance and are auxiliary reactions; most often we encounter the release of carbon dioxide CO 2 - during the action of acids on carbon dioxide salts, hydrogen sulfide - during the decomposition of sulfur metals with acids, etc.
These reactions are used in chemical analysis, mainly in the so-called. “preliminary testing”, when testing sediments for purity, for verification reactions and when studying minerals. The most important reactions of this kind consist of testing a substance in relation to:
To carry out these tests, in most cases, a non-luminous flame of a gas burner is used. The main components of illuminating gas (hydrogen, carbon monoxide, swamp gas and other hydrocarbons) are reducing agents, but when it burns in air (see Combustion), a flame is formed, in various parts of which the conditions necessary for reduction or oxidation can be found, and equals for heating to a more or less high temperature.
Fusibility test It is carried out mainly when studying minerals, for which a very small fragment of them, fixed in a thin platinum wire, is introduced into the part of the flame that has the highest temperature, and then, using a magnifying glass, they observe how the edges of the sample are rounded.
Flame color test is made by introducing a small sepia sample of a small sample of the substance on a platinum wire, first into the base of the flame, and then into the part of it with the highest temperature.
Volatility test is produced by heating a sample of a substance in an assay cylinder or in a glass tube sealed at one end, and volatile substances turn into vapors, which then condense in the colder part.
Oxidation and reduction in dry form can be produced in balls of fused borax ( 2 4 7 + 10 2 ) The substance tested is introduced in small quantities into balls obtained by melting these salts on a platinum wire, and they are then heated in the oxidizing or reducing part of the flame. Restoration can be done in a number of other ways, namely: heating on a stick charred with soda, heating in a glass tube with metals - sodium, potassium or magnesium, heating in charcoal using a blowpipe, or simple heating.
The classification of elements adopted in analytical chemistry is based on the same division that is accepted in general chemistry - into metals and non-metals (metalloids), the latter being most often considered in the form of the corresponding acids. To carry out a systematic qualitative analysis, each of these classes of elements is divided in turn into groups with some common group characteristics.
Metals in analytical chemistry are divided into two departments, which in turn are divided into five groups:
Aluminum and chromium do not form sulfur compounds by water; other metals form sulfur compounds, which, like their oxides, are soluble in weak acids. Hydrogen sulfide does not precipitate them from an acidic solution; ammonium sulfide precipitates oxides or sulfur compounds. Ammonium sulphide is a common reagent for this group, and an excess of its sulfur compounds does not dissolve. 4th group: silver, lead, bismuth, copper, palladium, rhodium, ruthenium, osmium. Sulfur compounds are insoluble in weak acids and are precipitated by hydrogen sulfide in an acidic solution; they are also insoluble in ammonium sulphide. Hydrogen sulfide is a common reactant for this group. 5th group: tin, arsenic, antimony, gold, platinum. Sulfur compounds are also insoluble in weak acids and are precipitated by hydrogen sulfide from an acidic solution. But they are soluble in ammonium sulphide and form water-soluble sulfasalts with it.
Nonmetals (metalloids) always have to be discovered in chemical analysis in the form of the acids they form or their corresponding salts. The basis for dividing acids into groups is the properties of their barium and silver salts in relation to their solubility in water and partly in acids. Barium chloride is a general reagent for group 1, silver nitrate in nitrate solution is for group 2, barium and silver salts of group 3 acids are soluble in water. 1st group: in a neutral solution, barium chloride precipitates insoluble salts; Silver salts are insoluble in water, but soluble in nitric acid. These include acids: chromic, serous, sulfurous, aqueous, carbonic, silicon, sulfuric, hydrofluorosilicic (barium salts, insoluble in acids), arsenic and arsenic. 2nd group: in a solution acidified with nitric acid, silver nitrate gives a precipitate. These include acids: hydrochloric, hydrobromic and hydroiodic, hydrocyanic, hydrogen sulfide, ferric and ferric hydrocyanide and iodine. 3rd group: nitric acid and perchloric acid, which are not precipitated by either silver nitrate or barium chloride.
However, it must be borne in mind that the reagents indicated for acids are not general reagents that could be used to separate acids into groups. These reagents can only give an indication of the presence of an acidic or other group, and to discover each individual acid one must use the private reactions belonging to them. The above classification of metals and nonmetals (metalloids) for the purposes of analytical chemistry was adopted in Russian schools and laboratories (according to N.A. Menshutkin); in Western European laboratories another classification was adopted, based, however, essentially on the same principles.
The theoretical foundations for reactions of qualitative chemical analysis in solutions must be sought, as already indicated above, in the departments of general and physical chemistry about solutions and chemical affinity. One of the first, most important issues is the state of all minerals in aqueous solutions, in which, according to the theory of electrolytic dissociation, all substances belonging to the classes of salts, acids and alkalis dissociate into ions. Therefore, all reactions of chemical analysis occur not between whole molecules of compounds, but between their ions. For example, the reaction of sodium chloride NaCl and silver nitrate AgNO 3 occurs according to the equation:
Na + + Cl - + Ag + + (NO 3) - = AgCl↓ + Na + + (NO 3) - sodium ion + chlorine ion + silver ion + nitric acid anion = insoluble salt + nitric acid anion
Consequently, silver nitrate is not a reagent for sodium chloride or hydrochloric acid, but only for chlorine ion. Thus, for each salt in solution, from the point of view of analytical chemistry, its cation (metal ion) and anion (acid residue) must be considered separately. For a free acid, hydrogen ions and an anion must be considered; finally, for each alkali - a metal cation and a hydroxyl anion. And essentially the most important task of qualitative chemical analysis is to study the reactions of various ions and how to discover them and separate them from each other.
To achieve the latter goal, by the action of appropriate reagents, ions are converted into insoluble compounds that precipitate from solution in the form of precipitation, or are isolated from solutions in the form of gases. In the same theory of electrolytic dissociation, one must look for an explanation for the action of chemical indicators, which often find application in chemical analysis. According to the theory of W. Ostwald, all chemical indicators are relatively weak acids, partially dissociated in aqueous solutions. Moreover, some of them have colorless whole molecules and colored anions, others, on the contrary, have colored molecules and a colorless anion or an anion of a different color; When exposed to the influence of free hydrogen ions of acids or hydroxyl ions of alkali, chemical indicators can change the degree of their dissociation, and at the same time their color. The most important indicators are:
Chemical indicators have very important applications in volumetric chemical analysis (see below). In reactions of qualitative chemical analysis, one often encounters the phenomenon of hydrolysis, that is, the decomposition of salts under the influence of water, and the aqueous solution acquires a more or less strong alkaline or acidic reaction.
In a qualitative chemical analysis, it is important to determine not only what elements or compounds are included in the composition of a given substance, but also in what, approximately, relative quantities these components are found. For this purpose, it is always necessary to proceed from certain quantities of the analyzed substance (usually it is enough to take 0.5-1 grams) and, when performing the analysis, compare the amount of individual precipitation with each other. It is also necessary to use solutions of reagents of a certain strength, namely: normal, half-normal, one tenth of normal.
Every qualitative chemical analysis is divided into three parts:
Regarding the nature of the analyte, four cases may occur:
When analyzing solid non-metallic substance First of all, an external examination and microscopic examination are carried out, as well as a preliminary test using the above methods of analysis in dry form. Initially, a sample of a substance is dissolved, depending on its nature, in one of the following solvents: water, hydrochloric acid, nitric acid and aqua regia (a mixture of hydrochloric and nitric acids). Substances that are unable to dissolve in any of the above solvents are transferred into solution using some special techniques, such as: fusion with soda or potash, boiling with soda solution, heating with certain acids, etc. The resulting solution is subjected to systematic analysis with preliminary isolation of metals and acids into groups and their further separation into individual elements, using their characteristic private reactions.
When analyzing metal alloy a certain sample of it is dissolved in nitric acid (in rare cases in aqua regia), and the resulting solution is evaporated to dryness, after which the solid residue is dissolved in water and subjected to systematic analysis.
If the substance is liquid, first of all, attention is paid to its color, smell and reaction to litmus (acidic, alkaline, neutral). To verify the presence of any solids in the solution, a small portion of the liquid is evaporated on a platinum plate or watch glass. After these preliminary tests, the liquid is apalized using conventional methods.
Analysis gases produced by some special methods indicated in the quantitative analysis.
Quantitative chemical analysis aims to determine the relative amounts of the individual constituents of any chemical compound or mixture. The methods used in it depend on the qualities and composition of the substance, and therefore quantitative chemical analysis must always be preceded by qualitative chemical analysis
To perform quantitative analysis, two different methods can be used: gravimetric and volumetric. With the weight method, the bodies being determined are isolated in the form of, if possible, insoluble or poorly soluble compounds of known chemical composition, and their weight is determined, on the basis of which the amount of the desired element can be found by calculation. In volumetric analysis, the volumes of titrated (containing a certain amount of reagent) solutions used for analysis are measured. In addition, a number of special methods of quantitative chemical analysis differ, namely:
Represents a very special group medical chemical analysis, covering a number of different methods for studying blood, urine and other waste products of the human body.
Methods of gravimetric quantitative chemical analysis are of two types: direct analysis method And method of indirect (indirect) analysis. In the first case, the component to be determined is isolated in the form of some insoluble compound, and the weight of the latter is determined. Indirect analysis is based on the fact that two or more substances subjected to the same chemical treatment undergo unequal changes in their weight. Having, for example, a mixture of potassium chloride and sodium nitrate, you can determine the first of them by direct analysis, precipitating the chlorine in the form of silver chloride and weighing it. If there is a mixture of potassium and sodium chloride salts, you can determine their ratio indirectly by precipitating all the chlorine in the form of silver chloride and determining its weight, followed by calculation.
Analysis method name the principles underlying the analysis of matter, that is, the type and nature of the energy that causes disturbance of the chemical particles of the substance.
The analysis is based on the relationship between the detected analytical signal and the presence or concentration of the analyte.
Analytical signal is a fixed and measurable property of an object.
In analytical chemistry, analytical methods are classified according to the nature of the property being determined and the method of recording the analytical signal:
1.chemical
2.physical
3.physical and chemical
Physicochemical methods are called instrumental or measuring methods, since they require the use of instruments and measuring instruments.
Let's consider the complete classification of chemical methods of analysis.
Chemical methods of analysis- are based on measuring the energy of a chemical reaction.
During the reaction, 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 by quantities such as reagent consumption, mass of product formed, reaction time, etc.
By goals chemical analysis methods are divided into two groups:
I.Qualitative analysis– consists of detecting the individual elements (or ions) that make up the analyte.
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 components of a complex substance.
Quantitative chemical methods classify:
1. Gravimetric(weight) method of analysis is based on isolating the analyte in its pure form and weighing it.
Gravimetric methods are divided according to the method of obtaining the reaction product:
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 analysis methods are based on measuring the volume of the reagent spent on interaction with the substance.
Volumetric methods, depending on the state of aggregation of the reagent, are divided into:
a) gas-volumetric methods, which are based on selective absorption of the determined component of the gas mixture and 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 liquid reagent consumed for interaction with the substance being determined.
Depending on the type of chemical reaction, volumetric analysis methods are distinguished:
· protolitometry – a method based on the occurrence of a neutralization reaction;
· redoxometry – a method based on the occurrence of redox reactions;
· complexometry – a method based on the occurrence of a complexation reaction;
· precipitation methods – methods based on the occurrence of precipitation formation reactions.
3. Kinetic analytical methods 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 to the analytical problem is carried out by performing an 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
Chemical analysis begins with the selection and preparation of a sample for analysis. It should be noted that all stages of analysis are interconnected. Thus, a carefully measured analytical signal does not provide correct information about the content of the component being determined if the sample is selected or prepared for analysis incorrectly. Sampling error often determines the overall accuracy of component determination and makes the use of highly accurate methods pointless. In turn, sample selection and 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 when conducting chemical analysis that they are usually prescribed by the State Standard (GOST).
Let's consider the basic rules for sampling:
· The result can only be correct if the sample is sufficiently representative, that is, it accurately reflects the composition of the material from which it was selected. The more material selected for the sample, the more representative it is. However, very large samples are difficult to handle and increase analysis time and costs. Thus, the sample must be taken so that it is representative and not very large.
· The optimal sample mass is determined by 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.
· To ensure the representativeness of the sample, batch homogeneity must be ensured. If it is not possible to form a homogeneous batch, then the batch should be separated into homogeneous parts.
· When taking samples, the aggregate state of the object is taken into account.
· The condition for the uniformity of sampling methods must be met: random sampling, periodic, chess, multi-stage sampling, “blind” sampling, systematic sampling.
· One of the factors that must be taken into account when choosing a sampling method is the possibility of changes in the composition of the object and the content of the component being determined over time. For example, the variable composition of water in the river, changes in the concentration of components in food products, etc.
