Analytical chemistry- this is a section that allows you to control the production and quality of products in various sectors of the economy. The exploration of natural resources is based on the results of these studies. Analytical chemistry methods are used to control the degree of contamination environment.
Analysis is the main option for determining the chemical composition of feed, fertilizers, soils, and agricultural products, which is important for the normal functioning of the agro-industrial industry.
Qualitative and quantitative chemistry are indispensable in biotechnology and medical diagnostics. The efficiency and effectiveness of many scientific fields depends on the degree of equipment of research laboratories.
Analytical chemistry is a science that allows one to determine the composition and chemical structure of a substance. Her methods help answer questions related not only to the constituent parts of a substance, but also to their quantitative relationship. With their help, you can understand in what form a specific component is found in the substance under study. In some cases, they can be used to determine the spatial arrangement of constituent components.
When thinking through methods, information is often borrowed from related fields of science and adapted to a specific area of research. What questions does analytical chemistry solve? Analysis methods make it possible to develop theoretical foundations, set the boundaries of their use, evaluate metrological and other characteristics, and create methods for analyzing various objects. They are constantly updated, modernized, becoming more versatile and efficient.
When talking about a method of analysis, a principle is assumed that is implied in the expression of the quantitative relationship between the property being determined and the composition. Selected techniques, including identifying and eliminating interference, devices for practical activities and options for processing measurements taken.
There are three main areas of knowledge:
Modern analytical chemistry is a combination of qualitative and quantitative analysis. The first section addresses the issue of the components included in the analyzed object. The second provides information about the quantitative content of one or several parts of the substance.
They are divided into the following groups: sampling, sample decomposition, component separation, identification and determination. There are also hybrid methods that combine separation and definition.
Determination methods are of maximum importance. They are divided according to the nature of the analyzed property and the option of recording a certain signal. Analytical chemistry problems often involve the calculation of certain components based on chemical reactions. To carry out such calculations, a solid mathematical basis is required.
Among the main requirements for analytical chemistry methods, we highlight:
When choosing an analysis method, it is important to clearly know the purpose and objectives of the study and evaluate the main advantages and disadvantages of the available methods.
The chemical method of analytical chemistry is based on qualitative reactions characteristic of certain compounds.
After sample collection and preparation are completed, the chemical analysis stage is carried out. It is associated with the detection of components in a mixture and determination of its quantitative content.
Analytical chemistry is a science in which there are many methods, one of them is signal. An analytical signal is considered to be the average of several measurements of a physical quantity at last stage analysis, which is functionally related to the content of the required component. If it is necessary to detect a certain element, an analytical signal is used: sediment, color, line in the spectrum. Determination of the amount of a component is related to the mass of the deposit, the intensity of the spectral lines, and the magnitude of the current.
Masking is the inhibition or complete suppression of a chemical reaction in the presence of substances that can change its speed or direction. There are two masking options: equilibrium (thermodynamic) and nonequilibrium (kinetic). For the first case, conditions are created under which the reaction constant decreases so much that the process proceeds insignificantly. The concentration of the masked component will be insufficient to reliably detect the analytical signal. Kinetic masking is based on the increase in the difference between the speeds of the detected and masked substance with a constant reagent.
Concentration and separation are carried out due to certain factors:
Separation is the process by which the components present in the original mixture can be separated from each other.
Concentration is an operation due to which the ratio of the number of small elements to the number of macrocomponents increases.
Sedimentation is suitable for the separation of several It is used in combination with determination methods designed to obtain an analytical signal from solid samples. The division is based on the different solubility of substances used in aqueous solutions.
The Department of Analytical Chemistry involves conducting laboratory research related to extraction. It refers to the physical and chemical process of distributing a substance between immiscible liquids. Extraction is also the name given to the process of mass transfer during chemical reactions. Such research methods are suitable for the extraction and concentration of macro- and microcomponents, as well as for group and individual isolation in the analysis of various natural and industrial facilities. Such methods are simple and fast to perform, guarantee excellent concentration and separation efficiency, and are fully compatible with a variety of determination methods. Thanks to extraction, it is possible to consider the state of the component in solution at different conditions, as well as to identify its physical and chemical characteristics.
It is used for concentrating and separating substances. Sorption technologies provide good selectivity for mixture separation. This is the process of absorption of vapors, liquids, gases by sorbents (solid-based absorbers).
What else does analytical chemistry do? The textbook contains information about the electroremoval technique, in which a concentrated or separated substance is deposited on solid electrodes in the form simple substance or as part of a compound.
Electrolysis is based on the precipitation of a specific substance using electric current. The most common option is cathodic deposition of low-active metals. The material for the electrode can be platinum, carbon, copper, silver, tungsten.
It is based on differences in the speeds of movement of particles of different charges in electric field when changing tension, particle size. Currently, in analytical chemistry there are two forms of electrophoresis: simple (frontal) and on a carrier (zone). The first option is suitable for a small volume of solution containing the components to be separated. It is placed in a tube containing solutions. Analytical chemistry explains all the processes occurring at the cathode and anode. In zone electrophoresis, the movement of particles occurs in a stabilizing medium that holds them in place after the current is turned off.
The cementation method consists of restoring components on metals having a significant negative potential. In such a case, two processes occur at once: cathodic (with the release of a component) and anodic (the cementing metal dissolves).
Distillation is based on different volatilities chemical substances. A transition occurs from a liquid form to a gaseous state, then condenses, again passing into the liquid phase.
With simple distillation, a single-step process of separation and then concentration of the substance occurs. In the case of evaporation, those substances that are present in a volatile form are removed. For example, they may include macro- and microcomponents. Sublimation (sublimation) involves transferring a substance from a solid phase to a gas, bypassing the liquid form. A similar technique is used in cases where the substances being separated are poorly soluble in water or do not melt well.
In analytical chemistry, there are many ways to isolate one substance from a mixture and detect its presence in the sample under study. Among the most used analytical methods is chromatography. It allows you to detect liquid, gaseous, and solid substances with a molecular weight from 1 to 106 a. e.m. Thanks to chromatography, it is possible to obtain comprehensive information about the properties and structure of organic substances various classes. The method is based on the distribution of components between the mobile and stationary phases. Stationary is a solid substance (sorbent) or a film of liquid that is deposited on a solid substance.
The mobile phase is a gas or liquid that flows through the stationary part. Thanks to this technology, it is possible to identify individual components, quantify the composition of the mixture, and separate it into components.
In addition to chromatography, gravimetric, titrimetric, and kinetic methods are used in qualitative and quantitative analysis. All of them are based on physical and chemical properties substances, allow the researcher to detect certain compounds in a sample and calculate their quantitative content. Analytical chemistry can rightfully be considered one of the most important branches of science.
ANALYTICAL CHEMISTRY, the science of determining the chemical composition of substances and materials and, to some extent, chemical structure connections. Analytical chemistry develops the general theoretical foundations of chemical analysis, develops methods for determining the components of the sample under study, and solves problems of analyzing specific objects. The main goal of analytical chemistry is the creation of methods and means that, depending on the task at hand, provide accuracy, high sensitivity, rapidity and selectivity of analysis. Methods are also being developed that make it possible to analyze micro-objects, carry out local analysis (at a point, on a surface, and so on), analysis without destroying the sample, at a distance from it (remote analysis), continuous analysis (for example, in a flow), and also install, in what chemical compound and in what physical form does the component being determined exist in the sample (material chemical analysis) and what phase does it belong to (phase analysis). Important trends in the development of analytical chemistry are automation of analyses, especially in the control of technological processes, and mathematization, in particular the widespread use of computers.
Structure of science. Three major areas of analytical chemistry can be distinguished: general theoretical foundations; development of analysis methods; analytical chemistry of individual objects. Depending on the purpose of the analysis, a distinction is made between qualitative chemical analysis and quantitative chemical analysis. The task of the first is to detect and identify the components of the analyzed sample, the task of the second is to determine their concentrations or masses. Depending on which components need to be detected or determined, there are isotopic analysis, elemental analysis, structural group (including functional) analysis, molecular analysis, material analysis, and phase analysis. According to the nature of the analyzed object, the analysis of inorganic and organic substances, as well as biological objects, is distinguished.
IN theoretical foundations In analytical chemistry, a significant place is occupied by the so-called chemometrics, including the metrology of chemical analysis. The theory of analytical chemistry also includes teachings about the selection and preparation of analytical samples, about drawing up an analysis scheme and choosing methods, about the principles and ways of automating analysis, the use of computers, as well as the principles rational use chemical analysis results. The peculiarity of analytical chemistry is the study of individual, rather than general, specific properties and characteristics of objects, which ensures the selectivity of many analytical methods. Thanks to close connections with the achievements of physics, mathematics, biology and various areas technology (this especially concerns methods of analysis), analytical chemistry is turning into a discipline at the intersection of sciences. Other names for this discipline are often used - analytics, analytical science, etc.
In analytical chemistry, there are methods of separation, determination (detection) and hybrid methods of analysis, usually combining methods of the first two groups. Determination methods are conveniently divided into chemical methods of analysis (gravimetric analysis, titrimetric analysis, electrochemical methods of analysis, kinetic methods of analysis), physical methods of analysis (spectroscopic, nuclear physical, etc.), biochemical methods of analysis and biological method analysis. Chemical methods are based on chemical reactions (interaction of matter with matter), physical methods are based on physical phenomena (interaction of matter with radiation, energy flows), biological methods use the response of organisms or their fragments to changes in the environment.
Almost all determination methods are based on the dependence of any measurable properties of substances on their composition. Therefore, an important direction in analytical chemistry is the search and study of such dependencies in order to use them to solve analytical problems. In this case, it is almost always necessary to find an equation for the relationship between property and composition, develop methods for recording the property (analytical signal), eliminate interference from other components, and eliminate the interfering influence of various factors (for example, temperature fluctuations). The magnitude of the analytical signal is converted into units characterizing the amount or concentration of components. The measured properties can be, for example, mass, volume, light absorption, current strength.
Much attention is paid to the theory of analysis methods. Theory chemical methods is based on the concepts of several basic types of chemical reactions widely used in analysis (acid-base, redox, complexation), and several important processes(precipitation, dissolution, extraction). Attention to these issues is due to the history of the development of analytical chemistry and the practical significance of the corresponding methods. Since, however, the share of chemical methods is decreasing, and the share of physical, biochemical and biological is growing, improving the theory of methods is of great importance last groups and integration theoretical aspects individual methods in the general theory of analytical chemistry.
History of development. Testing of materials was carried out in ancient times; for example, ores were examined to determine their suitability for smelting, and various products were examined to determine their gold and silver content. Alchemists of the 14th-16th centuries carried out a huge amount of experimental work to study the properties of substances, laying the foundation for chemical methods of analysis. In the 16th-17th centuries (the period of iatrochemistry), new chemical methods detection of substances based on reactions in solution (for example, the discovery of silver ions by the formation of a precipitate with chloride ions). R. Boyle, who introduced the concept of “chemical analysis,” is considered the founder of scientific analytical chemistry.
Until the mid-19th century, analytical chemistry was the main branch of chemistry. During this period many were discovered chemical elements, the constituent parts of some natural substances were identified, the laws of constancy of composition and multiple ratios, and the law of conservation of mass were established. The Swedish chemist and mineralogist T. Bergman developed a systematic scheme qualitative analysis, actively used hydrogen sulfide as an analytical reagent, proposed methods of flame analysis to obtain pearls. In the 19th century, systematic qualitative analysis was improved by the German chemists G. Rose and K. Fresenius. The same century was marked by enormous strides in the development of quantitative analysis. A titrimetric method was created (French chemist F. Decroisille, J. Gay-Lussac), gravimetric analysis was significantly improved, and gas analysis methods were developed. The development of elemental analysis methods was of great importance organic compounds(Yu. Liebig). At the end of the 19th century, the theory of analytical chemistry was developed, which was based on the doctrine of chemical equilibrium in solutions with the participation of ions (mainly W. Ostwald). By this time, methods for analyzing ions in aqueous solutions occupied a predominant place in analytical chemistry.
In the 20th century, methods for microanalysis of organic compounds were developed (F. Pregl). The polarographic method was proposed (Ya. Heyrovsky, 1922). Many physical methods have appeared, for example mass spectrometry, X-ray, nuclear physics. Of great importance was the discovery of chromatography (M. S. Tsvet, 1903) and the creation different options this method, in particular partition chromatography (A. Martin and R. Singh, 1941).
In Russia and the USSR great importance for analytical chemistry I had I. A. Menshutkin’s textbook “Analytical Chemistry” (it went through 16 editions). M.A. Ilyinsky and L.A. Chugaev introduced organic analytical reagents into practice (late 19th - early 20th centuries), N.A. Tananaev developed the drop method of qualitative analysis (simultaneously with the Austrian chemist F. Feigl, 1920s). In 1938 N.A. Izmailov and M.S. Schreiber were the first to describe thin layer chromatography. Russian scientists made a great contribution to the study of complex formation and its analytical use (I.P. Alimarin, A.K. Babko), to the theory of action of organic analytical reagents, to the development of mass spectrometry, photometric methods, atomic absorption spectrometry (B.V. . Lvov), in analytical chemistry individual elements, especially rare and platinum, and a number of objects - substances of high purity, mineral raw materials, metals and alloys.
The requirements of practice have always stimulated the development of analytical chemistry. Thus, in the 1940-1970s, in connection with the need to analyze nuclear, semiconductor and other materials of high purity, such sensitive methods as radioactivation analysis, spark mass spectrometry, chemical spectral analysis, stripping voltammetry, providing the determination of up to 10 -7 -10 -8% of impurities in pure substances, i.e. 1 part of an impurity per 10-1000 billion parts of the main substance. For development ferrous metallurgy, especially in connection with the transition to high-speed converter steel production, rapid analysis has become crucial. The use of so-called quantometers - photoelectric devices for multi-element optical spectral or x-ray analysis - allows analysis to be carried out during melting.
The need to analyze complex mixtures of organic compounds has led to the intensive development of gas chromatography, which makes it possible to analyze complex mixtures containing several tens and even hundreds of substances. Analytical chemistry has significantly contributed to the mastery of the energy of the atomic nucleus, the study of space and the ocean, the development of electronics, and the progress of biological sciences.
Subject of study. An important role is played by the development of the theory of sampling of analyzed materials; Usually, sampling issues are resolved jointly with specialists in the substances being studied (for example, geologists, metallurgists). Analytical chemistry develops methods for sample decomposition - dissolution, fusion, sintering, etc., which should ensure complete “opening” of the sample and prevent loss of the determined components and contamination from the outside. The tasks of analytical chemistry include the development of techniques for such general analytical operations as volume measurement, filtration, and calcination. One of the tasks of analytical chemistry is to determine the directions for the development of analytical instrumentation, the creation of new circuits and designs of devices (which most often serves as the final stage in the development of an analysis method), as well as the synthesis of new analytical reagents.
For quantitative analysis, the metrological characteristics of methods and instruments are very important. In this regard, analytical chemistry studies the problems of calibration, production and use of reference samples (including standard samples) and other means of ensuring the accuracy of the analysis. Processing of analysis results, especially computer processing, occupies a significant place. To optimize analysis conditions, information theory, pattern recognition theory, and other branches of mathematics are used. Computers are used not only for processing results, but also for controlling instruments, taking into account interference, calibrating, and planning experiments; There are analytical problems that can only be solved with the help of computers, for example, the identification of molecules of organic compounds using expert systems.
Analytical chemistry determines general approaches to the choice of ways and methods of analysis. Methods for comparing methods are being developed, conditions for their interchangeability and combination, principles and ways of automating analysis are being determined. For the practical use of analysis, it is necessary to develop ideas about its result as an indicator of product quality, the doctrine of express control of technological processes, and the creation of cost-effective methods. Unification and standardization of methods are of great importance for analysts working in various sectors of the economy. A theory is being developed to optimize the amount of information needed to solve analytical problems.
Analysis methods. Depending on the mass or volume of the analyzed sample, separation and determination methods are sometimes divided into macro-, micro- and ultra-micro methods.
The separation of mixtures is usually resorted to in cases where direct determination or detection methods do not allow one to obtain correct result due to the interfering influence of other sample components. Particularly important is the so-called relative concentration, the separation of small quantities of analyte components from significantly larger quantities of the main components of the sample. The separation of mixtures can be based on differences in the thermodynamic, or equilibrium, characteristics of the components (ion exchange constants, stability constants of complexes) or kinetic parameters. The separation methods used are mainly chromatography, extraction, precipitation, distillation, as well as electrochemical methods such as electrodeposition. Determination methods are the main group of analytical chemistry methods. Quantitative analysis methods are based on the dependence of any measurable property, most often physical, on the composition of the sample. This dependence must be described in a certain and known way. Hybrid analytical methods that combine separation and determination are rapidly developing. For example, gas chromatography with various detectors is the most important method for analyzing complex mixtures of organic compounds. For the analysis of mixtures of poorly volatile and thermally unstable compounds, high-performance liquid chromatography is more convenient.
A variety of methods are needed for analysis, as each has its own advantages and limitations. Thus, extremely sensitive radioactivation and mass spectral methods require complex and expensive equipment. Simple, accessible and very sensitive kinetic methods do not always provide the required reproducibility of results. When evaluating and comparing methods, when choosing them for solving specific problems, many factors are taken into account: metrological parameters, scope of possible use, availability of equipment, analyst qualifications, traditions, etc. The most important among these factors are metrological parameters such as detection limit or concentration range (quantities) in which the method gives reliable results, and the accuracy of the method, i.e. the correctness and reproducibility of the results. In some cases, “multicomponent” methods are of great importance, allowing the determination of a large number of components at once, for example, atomic emission and X-ray spectral analysis, chromatography. The role of such methods is increasing. All other things being equal, methods of direct analysis are preferred, i.e., not associated with chemical sample preparation; however, such preparation is often necessary. For example, preliminary concentration of the component under study makes it possible to determine its lower concentrations, eliminating difficulties associated with the inhomogeneous distribution of the component in the sample and the lack of reference samples.
Local analysis methods occupy a special place. A significant role among them is played by X-ray microanalysis (electron probe), secondary ion mass spectrometry, Auger spectroscopy and other physical methods. They are of great importance, in particular when analyzing the surface layers of solid materials or inclusions in rocks.
A specific group consists of methods of elemental analysis of organic compounds. Organic matter is decomposed in one way or another, and its components in the form of simple inorganic compounds(CO 2, H 2 O, NH 3, etc.) are determined by conventional methods. The use of gas chromatography has made it possible to automate elemental analysis; For this purpose, C-, H-, N-, S-analyzers and other automatic devices are produced. The analysis of organic compounds by functional groups (functional analysis) is performed by various chemical, electrochemical, spectral (NMR or IR spectroscopy) or chromatographic methods.
In phase analysis, i.e., determining the chemical compounds that form separate phases, the latter are first isolated, for example, using a selective solvent, and then the resulting solutions are analyzed by conventional methods; Physical methods of phase analysis without preliminary phase separation are very promising.
Practical significance . Chemical analysis provides control of many technological processes and product quality in various industries, plays a huge role in the search and exploration of minerals, and in the mining industry. Using chemical analysis, the cleanliness of the environment (soil, water and air) is monitored. Achievements of analytical chemistry are used in various branches of science and technology: nuclear energy, electronics, oceanology, biology, medicine, forensics, archeology, space research. Great economic importance chemical analysis. Thus, accurate determination of alloying additives in metallurgy allows saving valuable metals. The transition to continuous automatic analysis in medical and agrochemical laboratories makes it possible to dramatically increase the speed of analyzes (blood, urine, soil extracts, etc.) and reduce the number of laboratory staff.
Lit.: Fundamentals of Analytical Chemistry: In 2 books / Edited by Yu. A. Zolotov. M., 2002; Analytical chemistry: In 2 vols. M., 2003-2004.
Analysis method name the principles underlying the analysis of matter, that is, the type and nature of the energy causing the disturbance chemical particles substances.
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 the isolation of the analyte in 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, carrying out each analysis consists of next 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 techniques and procedures for sample collection and preparation are so important in chemical analysis that they are usually prescribed 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.
· Optimal weight the sample 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 a river, changes in the concentration of components in food products etc.
Subject and tasks of analytical chemistry.
Analytical chemistry is the science of methods for qualitative and quantitative research into the composition of substances (or mixtures thereof). The task of analytical chemistry is to develop the theory of chemical and physicochemical methods of analysis and operations in scientific research.
Analytical chemistry consists of two main sections: qualitative analysis consists in “opening”, i.e. detection of individual elements (or ions) that make up the analyte. Quantitative Analysis consists in determining the quantitative content of individual components of a complex substance.
The practical importance of analytical chemistry is great. Using chemical methods. analysis discovered laws: constancy of composition, multiple ratios, determined atomic masses elements, chemical equivalents, formulas of many compounds have been established.
Analytical chemistry contributes to the development of natural sciences - geochemistry, geology, mineralogy, physics, biology, technological disciplines, medicine. Chemical analysis is the basis of modern chemical-technological control of all industries in which raw materials, products and production waste are analyzed. Based on the results of the analysis, the flow is judged technological process and about product quality. Chemical and physico-chemical methods analysis form the basis for establishing state standards for all manufactured products.
The role of analytical chemistry in organizing environmental monitoring is great. This is pollution monitoring surface waters, soils, HMs, pesticides, petroleum products, radionuclides. One of the tasks of monitoring is to create criteria that establish the limits of possible environmental damage. For example MPC - maximum permissible concentration- this is such a concentration, when exposed to the human body, periodically or throughout life, directly or indirectly through environmental systems, there are no diseases or changes in health status that are detectable modern methods immediately or later in life. For each chem. substances have their own MPC value.
Classification of methods of qualitative analysis.
When studying a new compound, they first determine what elements (or ions) it consists of, and then the quantitative ratios in which they are found. Therefore, qualitative analysis usually precedes quantitative analysis.
All analytical methods are based on obtaining and measuring analytical signal, those. any manifestation of the chemical or physical properties of a substance that can be used to determine quality composition the analyzed object or to quantify the components it contains. The analyzed object can be an individual connection in any aggregate state. mixture of compounds natural object(soil, ore, mineral, air, water), products industrial production and food. Before analysis, sampling, grinding, sifting, averaging, etc. are carried out. An object prepared for analysis is called sample or sample.
Depending on the task at hand, a method is chosen. Analytical methods of qualitative analysis are divided into: 1) “dry” analysis and 2) “wet” analysis.
Dry analysis carried out with solids. It is divided into pyrochemical and grinding methods.
Pyrochemical (Greek - fire) type of analysis is carried out by heating the test sample in the flame of a gas or alcohol burner, performed in two ways: obtaining colored “pearls” or coloring the burner flame.
1. “Pearls”(French - pearls) are formed when salts NaNH 4 PO 4 ∙ 4 H 2 O, Na 2 B 4 O 7 ∙ 10 H 2 O - borax) or metal oxides are dissolved in a melt. By observing the color of the resulting glass pearls, the presence of certain elements in the sample is established. So, for example, chromium compounds make pearl green, cobalt - blue, manganese - violet-amethyst, etc.
2. Flame coloring- volatile salts of many metals, when introduced into the non-luminous part of the flame, color it different colors, for example, sodium is intensely yellow, potassium is purple, barium is green, calcium is red, etc. These types of analysis are used in preliminary tests and as an “express” method.
Analysis by rubbing method. (1898 Flavitsky). The test sample is ground in a porcelain mortar with an equal amount of solid reagent. The color of the resulting compound is used to determine the presence of the ion being determined. The method is used in preliminary tests and “express” analysis in the field for the analysis of ores and minerals.
2.Wet analysis - This is the analysis of a sample dissolved in some solvent. The solvent most often used is water, acids or alkalis.
According to the method of conducting, methods of qualitative analysis are divided into fractional and systematic. Fractional Analysis Method- this is the determination of ions using specific reactions in any sequence. It is used in agrochemical, factory and food laboratories, when the composition of the test sample is known and it is only necessary to check the absence of impurities or during preliminary tests. Systematic analysis - This is an analysis in a strictly defined sequence, in which each ion is detected only after interfering ions have been detected and removed.
Depending on the amount of substance taken for analysis, as well as on the technique of performing operations, methods are divided into:
- macroanalysis - carried out in comparatively large quantities substances (1-10 g). The analysis is performed in aqueous solutions and in test tubes.
- microanalysis - examines very small quantities of a substance (0.05 - 0.5 g). It is performed either on a strip of paper, a watch glass with a drop of solution (droplet analysis) or on a glass slide in a drop of solution, crystals are obtained, according to the shape of which the substance is determined under a microscope (microcrystalscopic).
Basic concepts of analytical chemistry.
Analytical reactions - These are reactions accompanied by a clearly visible external effect:
1) precipitation or dissolution of sediment;
2) change in the color of the solution;
3) gas release.
In addition, two more requirements are imposed on analytical reactions: irreversibility and sufficient reaction rate.
Substances under the influence of which analytical reactions occur are called reagents or reagents. All chem. reagents are divided into groups:
1) by chemical composition(carbonates, hydroxides, sulfides, etc.)
2) according to the degree of purification of the main component.
Conditions for performing chem. analysis:
1. Reaction medium
2. Temperature
3. Concentration of the ion being determined.
Wednesday. Acidic, alkaline, neutral.
Temperature. Most chem. reactions are performed at room conditions“in the cold”, or sometimes you need to cool it under the tap. Many reactions occur when heated.
Concentration- this is the amount of a substance contained in a certain weight or volume of a solution. A reaction and reagent capable of causing a noticeable external effect characteristic of it even at a negligible concentration of the substance being determined are called sensitive.
The sensitivity of analytical reactions is characterized by:
1) extreme dilution;
2) maximum concentration;
3) a minimum volume of an extremely dilute solution;
4) detection limit (opening minimum);
5) sensitivity indicator.
Limit dilution Vlim – the maximum volume of solution in which one gram of a given substance can be detected (in more than 50 experiments out of 100 experiments) using a given analytical reaction. The dilution limit is expressed in ml/g.
For example, when copper ions react with ammonia in an aqueous solution
Cu 2+ + 4NH 3 = 2+ ¯bright blue complex
The limiting dilution of the copper ion is (Vlim = 2.5 10 5 mg/l), i.e. Copper ions can be opened by this reaction in a solution containing 1 g of copper in 250,000 ml of water. In a solution containing less than 1 g of copper (II) in 250,000 ml of water, these cations cannot be detected by the above reaction.
Limit concentration Сlim (Cmin) – the lowest concentration at which the analyte can be detected in solution by a given analytical reaction. Expressed in g/ml.
The maximum concentration and maximum dilution are related by the relation: Clim = 1 / V lim
For example, potassium ions in an aqueous solution are opened using sodium hexanitrocobaltate (III)
2K + + Na 3 [ Co(NO 2) 6 ] ® NaK 2 [ Co(NO 2) 6 ] ¯ + 2Na +
The limiting concentration of K + ions for this analytical reaction is C lim = 10 -5 g/ml, i.e. Potassium ion cannot be opened by this reaction if its content is less than 10 -5 g in 1 ml of the analyzed solution.
Minimum volume of extremely diluted solution Vmin– the smallest volume of the analyzed solution required to detect the discovered substance by a given analytical reaction. Expressed in ml.
Detection limit (opening minimum) m– the smallest mass of the analyte that can be unambiguously discovered by a given an. reaction in a minimal volume of extremely dilute solution. Expressed in µg (1 µg = 10 -6 g).
m = C lim V min × 10 6 = V min × 10 6 / V lim
Sensitivity index analytical reaction is determined
pС lim = - log C lim = - log(1/Vlim) = log V lim
An. the reaction is more sensitive, the smaller its opening minimum, the minimum volume of the extremely diluted solution, and the greater the maximum dilution.
The detection limit depends on:
1. Concentrations of the test solution and reagent.
2. Duration of the course of an. reactions.
3. Method of observing the external effect (visually or using a device)
4. Compliance with the conditions for fulfillment of an. Reactions (t, pH, amount of reagent, its purity)
5. Presence and removal of impurities, foreign ions
6. Individual characteristics analytical chemist (accuracy, visual acuity, ability to distinguish colors).
Types of analytical reactions (reagents):
Specific- reactions that allow the determination of a given ion or substance in the presence of any other ions or substances.
For example: NH4 + + OH - = NH 3 (odor) + H 2 O
Fe 3+ + CNS - = Fe(CNS) 3 ¯
blood red
Selective- reactions allow you to selectively open several ions at once with the same external effect. The fewer ions a given reagent opens, the higher its selectivity.
For example:
NH 4 + + Na 3 = NH 4 Na
K + + Na 3 = NaK 2
Group reactions (reagents) allow you to detect a whole group of ions or some compounds.
For example: group II cations - group reagent (NH4)2CO3
CaCI 2 + (NH 4) 2 CO 3 = CaCO 3 + 2 NH 4 CI
BaCI 2 + (NH 4) 2 CO 3 = BaCO 3 + 2 NH 4 CI
SrCI 2 + (NH 4) 2 CO 3 = SrCO 3 + 2 NH 4 CI
Depending on the task at hand, there are 3 groups of analytical chemistry methods:
Chemical methods are based on the use of chemical reactions (neutralization, oxidation-reduction, complexation and precipitation) into which the analyte enters. A qualitative analytical signal in this case is the visual external effect of the reaction - a change in the color of the solution, the formation or dissolution of a precipitate, the release of a gaseous product. In quantitative determinations, the volume of the released gaseous product, the mass of the formed precipitate, and the volume of the reagent solution with exact accuracy are used as an analytical signal. known concentration, spent on interaction with the analyte.
Physical methods do not use chemical reactions, but measure some physical properties(optical, electrical, magnetic, thermal, etc.) of the analyzed substance, which are a function of its composition.
Physicochemical methods use changes in the physical properties of the analyzed system as a result of chemical reactions. Physicochemical methods also include chromatographic methods of analysis, based on the processes of sorption-desorption of a substance on a solid or liquid sorbent under dynamic conditions, and electrochemical methods (potentiometry, voltammetry, conductometry).
Physical and physicochemical methods are often combined under the general name instrumental methods of analysis, since analytical instruments and devices that record physical properties or their changes are used to carry out the analysis. When conducting a quantitative analysis, the analytical signal is measured - physical quantity associated with the quantitative composition of the sample. If quantitative analysis is carried out using chemical methods, then the basis of the determination is always a chemical reaction.
There are 3 groups of quantitative analysis methods:
The most important among chemical methods of quantitative analysis are gravimetric and titrimetric methods, which are called classical methods analysis. These methods are standard for assessing the accuracy of a determination. Their main area of application is the precision determination of large and medium quantities of substances.
Classical analysis methods are widely used in enterprises chemical industry to control the progress of the technological process, the quality of raw materials and finished products, industrial waste. Based on these methods, pharmaceutical analysis is carried out - determining the quality of drugs and medicines, which are produced by chemical and pharmaceutical enterprises.
