Instrumental research methods. General characteristics of instrumental methods of analysis Methods for determining concentrations in instrumental methods of analysis

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

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

Chemical methods are based on the use of chemical reactions (neutralization, 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 a reagent solution with a precisely known concentration spent on interaction with the substance being determined are used as an analytical signal.

Physical methods do not use chemical reactions, but measure any physical properties (optical, electrical, magnetic, thermal, etc.) of the 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 - a 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:

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

The most important among chemical methods of quantitative analysis are gravimetric and titrimetric methods, which are called classical methods of 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 methods of analysis are widely used at chemical industry enterprises to monitor the progress of the technological process, the quality of raw materials and finished products, and industrial waste. On the basis of these methods, pharmaceutical analysis is carried out - determining the quality of drugs and medicines that are produced by chemical and pharmaceutical enterprises.

Carrying out quantitative determinations by weight and volumetric (titrimetric) methods of chemical analysis is sometimes associated with great difficulties, the main of which are:

The need for preliminary separation of the part being determined from impurities;

Relatively low sensitivity, limiting the use of classical methods for analyzing small quantities of determined elements;

Large amounts of time (especially in the weight method) to conduct a complete analysis.

Physico-chemical methods They are distinguished by increased sensitivity and selectivity compared to classical methods, therefore, analysis by these methods, as a rule, requires a small amount of the analyte, and the content of a certain element in the sample can be extremely small.

Thus, the physicochemical methods of analysis differ expressiveness, selectivity, high sensitivity.

In terms of sensitivity, the first place is occupied by mass spectral and radioactivation methods of analysis. They are followed by the well-used spectral, spectrophotometric and polarographic methods.

For example, the sensitivity of determining some elements by different methods is as follows: Volumetric can be determined about 10-1 % ; weighing about 10 -2 % ; spectroscopic and photocolorimetric 10 -3 -10 -5 % ; fluorometric 10 -6 -10 -7%; kinetic 10 -6 -10 -8%; radio hee mic 10 -8 -10 -9%; method neutron activation analysis detect many impurities in quantities less than 10 -8 -10 -9 % .

In terms of accuracy, many physicochemical methods of analysis are inferior classic, and especially the weight method. Often, when the accuracy determined by hundredths and tenths of a percent is achieved by weight and volumetric methods, when performing analysis using physicochemical methods, the determination errors amount to 5-10 % , and sometimes much more.

Depending on the method of analysis, the accuracy of determinations is influenced by various factors.

For example, the accuracy of emission analysis is influenced by:

method of taking an average sample of the analyzed substance;

instability of the excitation source (electric arc, spark, burner flame);

the magnitude of the photometric measurement error;

inhomogeneity of the photographic emulsion (in the case of spectrography), etc.

In addition to relatively low accuracy, many physicochemical methods also have some other disadvantages. For example, emission spectroscopy is convenient only when carrying out mass analyses, since determining a particular element in a sample requires calibrating the instrument against a standard sample, which takes a lot of time. None of the physicochemical methods of analysis is universal.

It should be noted that, despite the progress of instrumental methods of analysis that make it possible to solve chemical analytical problems, classical methods of analysis have not lost their importance and are the basis of modern analytical chemistry.

All methods of quantitative analysis, physical and physical

Zicochemical methods of analysis are divided into the following groups: electrochemical; spectral (optical); chromatographic; radiometric; mass spectrometric.

Electrochemical methods of analysis. The group of electrochemical analysis methods includes the following types of analysis.

Electrical gravimetric analysis is based on the isolation from solutions of electrolytes of substances deposited on the electrodes when passing through solutions of direct electric current

ka. The metal or (oxide) released during electrolysis is weighed on an analytical balance and the content of the substance being determined in the solution is judged from the mass of the sediment.

Polarography is based on a change in current strength, varying depending on the voltage during the electrolysis process, under conditions where one of the electrodes (cathode) has a very small surface (polarizing electrode), and the other (anode) has a large surface (non-polarizing electrode). The polarizing cathode is mercury drops flowing from a thin hole in a capillary tube, as well as platinum (rotating), graphite, silver and other electrodes. The non-polarizing anode is “given” mercury or standard reference electrodes with a large surface area. The current strength at which a complete discharge of all analyte ions entering the near-electrode space as a result of diffusion is achieved is called the limiting diffusion current. The magnitude of this current is proportional to the initial concentration of the analyte (ions) in the solution.

Amperometric titration, which is a type of polarographic analysis, is based on a change in the process of titrating a solution of the substance being determined in the value of the maximum diffusion current passing through the solution at a constant voltage between the indicator polarizing electrode and the non-polarizing reference electrode.

Coulometry is based on a change in the amount of electricity spent on the electrolysis of a certain amount of a substance at a constant potential, which corresponds to the potential for the release of a given element. This method is based on Faraday's law.

A titration method in which the equivalence point corresponds to the moment when the electrolysis current reaches the “background” current is called coulometric titration. Typically the background current is equal to 0 , because the solution at this moment does not contain charged particles.

Conductometry is based on measuring the electrical conductivity of the analyzed solutions, which changes as a result of chemical reactions and depends on the nature of the electrolyte, its temperature and the concentration of the solution.

The titration method, in which the equivalence point is fixed by the intersection of two straight lines, reflecting the change in the equivalent electrical conductivity of the test solution as the titrant is added during the titration process, is called conductometric titration.

Spectral (optical) analysis methods. The group of spectral analysis methods includes the following methods.

Emission spectral analysis– a physical method based on the study of emission spectra of vapors of the analyzed substance (emission or study spectra) arising under the influence of strong sources of excitation (electric arc, high voltage spark); this method makes it possible to determine the elemental composition of a substance; those. judge what chemical elements are included in the composition of a given substance.

Flame photometry, which is a type of emission spectral analysis, is based on the study of the emission spectra of the elements of the analyzed substance,

arising under the influence of soft sources of excitation. In this method, the solution to be analyzed is sprayed into a flame. This method makes it possible to judge the content of mainly alkali and alkaline earth metals in the analyzed sample, as well as some other elements, for example, gallium, indium, thallium, lead, manganese, copper, phosphorus.

Absorption spectroscopy is based on the study of the absorption spectra of a substance, which is its individual characteristic. Distinguish spectrophotometric

method, based on determining the absorption spectrum or measuring light absorption (both in the ultraviolet and in the visible and infrared regions of the spectrum) at a strictly defined wavelength (monochromatic radiation), which corresponds to the maximum of the absorption curve of a given substance under study, as well as photocolorimetric method, based on determining the absorption spectrum or measuring light absorption in the visible part of the spectrum.

Turbodimetry is based on measuring the intensity of light absorbed by an uncolored suspension of a solid. In turbodimetry, the intensity of light absorbed by or transmitted through a solution is measured in the same way as in photocolometry of colored solutions.

Nephelometry is based on measuring the intensity of light reflected or scattered by a colored or uncolored suspension of a solid (sediment suspended in a given medium).

Luminescent, or fluorescent, the analysis method is based on measuring the intensity of visible light emitted by substances (fluorescence) when irradiated with ultraviolet rays.

Optical analysis methods also include refractometric method, based on refractive index measurement, and polarimetric, based on research

depending on the rotation of the plane of polarization.

Chromatographic methods of analysis. Based on the separation mechanism, there are several types of chromatographic analysis methods.

Adsorption liquid chromatography is based on the selective adsorption (absorption) of individual components of the analyzed mixture in a liquid medium. It is due to different adsorbability of dissolved components.

Adsorption gas chromatography is based on the use of differences in the adsorbability of gases and vapors. In

Depending on the main factor determining the separation, the following types of gas chromatography are distinguished: gas-liquid and gas-adsorption.

Partition chromatography is based on the use of differences in the distribution (sorbability) of individual components of the analyzed mixture between two immiscible liquid phases - mobile and stationary solvents.

Paper chromatography - a type of partition chromatography in which the carrier for a stationary solvent is strips or sheets of filter paper that do not contain mineral impurities.

Ion exchange chromatography is based on the use of ion exchange processes occurring between the moving fields of the adsorbent and the electrolyte fields contained in the analyzed solution.

Mass spectrometric methods of analysis. Mass spectrometric analysis methods are based on the determination of individual ionized atoms, molecules and radicals by separating sources of ions containing particles with different mass-to-charge ratios as a result of the combined action of electric and magnetic fields.

Physicochemical analysis according to N.S. Kurnakov. The method proposed by N.S. Kurzhakov, allows you to study physics

physical properties of systems depending on their chemical composition. For example, curves of the melting temperature versus composition of a lead-tin alloy can be used for analytical purposes.

This method is called physicochemical analysis. The concepts of “physico-chemical method of analysis” should not be confused.

for” with the concept of “physico-chemical analysis”.

If, during the process of heating or cooling the test substance, phase transformations associated with release or absorption are not observed in the analyzed object

heat, then the heating or cooling curves are characterized by a smooth progression. If phase transformations occur in the system, then on the temperature change curve, depending on the nature of these transformations, horizontal sections at a constant temperature or sharp bends in the curve are observed over a certain period of time. Such a cooling curve makes it possible to judge all phase transformations occurring in the sample under study during the cooling process.

Other methods of analysis.Electron paramagnetic resonance (EPR) method- is based on the use of the phenomenon of resonant absorption of electromagnetic waves by paramagnetic particles in a constant magnetic field and is successfully used to measure the concentration of paramagnetic substances, study redox reactions, study chemical kinetics and the mechanism of chemical reactions, etc.

Nuclear magnetic resonance (NMR) method is based on the use of resonant absorption of electromagnetic waves by the substance under study in a constant magnetic field, caused by nuclear magnetism. Method NMR used to study complex compounds, the state of ions in solution, to study chemical kinetics, etc.

Conclusion

Modern chemistry covers a large area of ​​human knowledge, since it is a science that studies substances and the laws of their transformation. Chemistry is in continuous development and deeply reveals the basic laws that make it possible to determine the behavior of electrons in atoms and molecules, to develop methods for calculating the structures of molecules and solids, the theory of chemical kinetics and chemical equilibrium. Guided by the basic laws of chemical thermodynamics, chemistry allows us to assess the direction of chemical processes and the depth of their occurrence. Important information is provided by studying the crystalline state of substances.

These questions will allow students to master areas of chemistry that were not studied in high school or were partially studied.

The knowledge acquired in this part of the chemistry course is necessary for studying special sections (properties of solutions, redox reactions, electrochemical processes, physical and chemical properties of substances)

The basic topics of the manual can be useful in the activities of specialists in any field of technology. Understanding the basic laws of chemistry and the ability to work with educational and specialized literature will allow specialists to find optimal solutions to the problems they face.

Sections of chemistry that are important in the practical activities of radio and electrical engineering specialists are also presented. Electrochemical processes (operation of galvanic cells, electrolysis) are considered, examples of chemical current sources and technical applications of electrolysis are given.

The reliability and durability of electronic products depends on the corrosion resistance of individual parts of devices, therefore, the manual examines the basic laws of corrosion processes, gives their classification, presents two mechanisms of their occurrence: chemical and electrochemical, and also provides methods and methods of protection against chemical and electrochemical corrosion.

Based on the information presented in this manual, some physical and chemical properties of metals and semiconductors (electrical conductivity, magnetic properties) are shown. The concept of chemical identification of substances based on qualitative and quantitative methods of analysis is given.

Knowledge is necessary when studying subsequent courses, such as materials science, strength of materials, theoretical foundations of various technological processes in electronics, electrical engineering, microelectronics, radio engineering, energy and other areas of specialist training.

Scientific and technological progress is not possible without the development of chemistry, which creates new substances with new properties that can be used in various industries.

When monitoring environmental pollution, analytical methods should allow the determination of both trace amounts of elements (at the level of n·10 -3 -n·10 -7%) and at high levels of pollution, preferably simultaneously, in a variety of objects with different physical properties and chemical composition.

When any method of analysis is compared with others, it is necessary to take into account a number of factors that collectively characterize the method. These include:

application area- objects of analysis and nomenclature of substances (inorganic and organic), the determination of which is possible using this method;

working range of determined concentrations– the range in which it is possible to determine the component without the use of additional dilution or concentration steps;

selectivity of determination– the ability to determine the substance of interest in the presence or influence of interfering components and factors, for example matrix effects;

metrological characteristics(determination sensitivity, detection limits, reproducibility and accuracy of the obtained measurement results, etc.);

the ability to recognize different physical and chemical forms of controlled substances in different matrices, for example, ions in different valence states;

equipment performance, suitability for performing mass measurements;

hardware- the complexity of the hardware and its cost, the possibility of application in production and field conditions;

requirements for training and qualifications of personnel(laboratory assistant, engineer, need for special training).

Methods that would equally satisfy all of the above requirements have not yet been developed, however, the basic conditions can be met when using modern physicochemical methods of analysis and their combinations.

1. Characteristics of the most common instrumental methods of analysis

Electroanalytical (electrochemical) methods. They are based on electrochemical processes in solutions. These methods have long been known and are often used in everyday monitoring of environmental objects; they have advantages in terms of low cost of equipment and the necessary costs for operating the devices. Advantages of electrochemical analysis methods:

High sensitivity and selectivity, rapid response to changes in the composition of the analyzed object;

Large range of determined chemical elements and substances;

Wide ranges of measured concentrations - from tens of % to n*10 -8%;

Accuracy and high reproducibility of results (relative standard deviation of analysis results in most EMAs is less than 0.3);

The ability to determine, along with the gross content, the physical and chemical forms of the elements being determined;

Simplicity of hardware design, availability of equipment and low cost of analysis;

Possibility of use in laboratory, production and field conditions, ease of automation and remote control.

They represent an area of ​​analytical chemistry that is very promising for improving hardware design and automation using microprocessors.

Table 1 Classification of instrumental methods of analysis

Method name and options	Defined Components	Detection limit, mg/l (mg/kg)	Linearity range
Electroanalytical methods
Voltammetry (polarography)	metal ions and their associated forms, gases		spec. but cf. feelings.
Potentiometry	inorganic ions
Ionometry with ion-selective electrodes	inorganic ions
Coulomb and conductometry	inorganic compounds, gases
Spectral analysis methods
Molecular spectrometry
Visible spectrophotometry	inorganic and organic compounds		simple and broad approx.
UV spectrophotometry	inorg. and organic ingredients
IR spectrometry Raman spectrometry	org identification substances		highly specialized
Atomic spectrometry
Atomic absorption spectrometry	chemical elements, mainly metals
Atomic emission spectrometry	more than 70 chemical elements
Atomic fluorescence spectrometry	organic substances and organometallic complexes
Radio spectroscopic methods
Electron paramagnetic resonance (EPR)	Macrocomponents, free radicals.		highly specific,
Nuclear magnetic resonance (NMR)	organic compounds containing nuclei H, C, F, P		insensitive.
Mass spectrometry
Mass spectrometry	Traces of the elements
Chromatographic methods
Gas chromatography	gases, volatile organic compounds	Depends on type	highly specific.
Gas-liquid chromatograph.	organic compounds	detector
High performance liquid chromatography	non-volatile organic compounds		apply.
Nuclear physics methods
Neutron activation analysis	chemical elements, with the exception of light ones		require special
-, - and - radiometry	radionuclides
-, - and - spectrometry

* - strongly depends on the element being defined; ** - depends on the detector used

Disadvantages - the effect of mutual influence of elements, the impossibility of multi-element determination, the influence of organic substances.

Spectral analysis methods based on usage interaction of atoms or molecules of analyte substances with electromagnetic radiation of a wide range of energies. In order of decreasing energy, these can be: gamma rays, x-rays, ultraviolet and visible, infrared, microwave and radio waves.

The interaction of molecules or atoms of a substance with various forms of energy is manifested in three closely related spectroscopic phenomena - emission, adsorption and fluorescence, which, one way or another, are used in analytical technology. An analytical signal can be the emission or absorption of radiation by a substance, therefore two types of spectral analysis are distinguished: absorption spectroscopy (uses absorption spectra) and emission spectroscopy (emission spectra).

Spectral analysis methods began to develop in the middle of the 19th century and have now become widespread in qualitative and quantitative analysis. The widespread use of spectral analysis methods is due to their versatility, selectivity, low detection limits, rapidity, and the ability to automate both individual stages and the entire analysis process as a whole. Modern spectral instruments have automated sample input systems, built-in microprocessors that control the analysis process, process experimental data and provide them in a form convenient for the consumer.

The group of spectral analysis methods includes:

molecular absorption spectral analysis in the visible, UV and IR regions;

method of analysis using Raman spectra;

luminescent or fluorescent assays;

atomic emission, atomic absorption and atomic fluorescence analyses;

radio spectroscopic methods of analysis (EPR spectroscopy, NMR spectroscopy).

Molecular spectrometry. Depending on the energy range used, optical analysis methods are divided into spectroscopy in the visible and ultraviolet regions of the spectrum (wavelength range from 200 to 700 nm, 1 nm = 10 -9 m) and infrared spectrometry (from wavelengths at which light becomes invisible to human eye ~ 780 nm to the area where the radiation already has the properties of high-frequency radio waves ~ 0.5 mm). Classical photometry and spectrophotometry are still widely used (microprocessor control allowing fully automated measurement processes). Infrared spectrometry is particularly useful for identifying and determining the structure of organic compounds. Raman spectrometry.

Atomic spectrometry. In the last 20-30 years, the role of atomic absorption and atomic emission spectrometry has grown. The methods require more complex and expensive equipment, but make it possible to perform mass analyzes and determine most chemical elements in matrices of a wide variety of compositions with extremely low detection limits (with an absolute content of ~ 10 -14 g). These instrumental methods of analysis are becoming common (routine) even in small environmental control laboratories, especially in the monitoring of air and natural water pollution, when simple preliminary sample preparation or concentration (extraction, evaporation of water samples or capture of atmospheric contaminants on a filter) help to increase the sensitivity of determinations.

Atomic fluorescent spectrometry also allows the determination of various elements, but based on the re-emission of light energy absorbed by free atoms.

EPR spectrometry. The EPR method is used to study molecules, atoms and radicals in gases, solutions and various types of matrices. EPR is one of the most sensitive methods for detecting and identifying free radicals, establishing their electronic configuration and geometry. The method is used to study complex compounds, in particular compounds of transition and rare earth metals.

Nuclear magnetic resonance spectroscopy- a method for measuring the relative energy and state of the nuclear spins of a molecule in a magnetic field. The method is suitable for studying atoms with nuclear spin and can be used for quantitative and qualitative analysis, especially when analyzing compounds with unknown structure. Most often used in relation to nuclei 1 H, 19 F and 31 P.

Mass spectrometry. This method analyzes a substance by converting it into ions and then separating them in an electric or magnetic field.

Molecular spectrometry methods (IR, UV, NMR, EPR and mass spectrometry) are more associated with establishing the structure and studying the mechanism of ongoing processes than with simple identification of the composition.

Chromatographic methods. Essentially, chromatography is a method for separating mixtures. After separating the mixture into components, they are identified and quantified. For this purpose, special devices are used, called a detector and based on different principles for measuring the amount or concentration of a substance - from the simplest thermoelements or photometers to high-resolution mass spectrometers in combination with a microprocessor. Instrumental chromatography is a hybrid method: a chromatographic column separates the sample components into separate zones, and a detector typically measures the concentration of the separated components in the carrier phase after they leave the column.

Chromatographic methods, especially gas-liquid and high-performance liquid chromatography, are often indispensable for the analysis of complex multicomponent mixtures, as well as for the identification and quantification of organic substances with similar structures. Methods that combine chromatographic separation of a mixture of analytes into components and their subsequent determination using mass or IR spectrometry (chromatography-mass spectrometry GLC-MS, gas-liquid chromatography - Fourier transform spectroscopy in the infrared region GLC-IR-FS) are developing especially rapidly.

Nuclear physics methods occupy a special position and are used more limitedly, since they require specially prepared laboratories, compliance with many radiation safety requirements and are suitable only for the determination of radioactive isotopes of chemical elements that have specific nuclear physical characteristics - the phenomenon of radioactive decay.

None of the listed methods of analysis is universal in terms of suitability for determining the content of all components of interest in any control objects.

When choosing a specific analysis method, the following questions should be considered first:

group characteristics and features of the physico-chemical properties of the pollutant subject to control;

Chemical composition and physical properties of controlled objects;

Possible range of changes in the concentrations of the analyte in the control objects;

Metrological characteristics of the method: sensitivity (detection limit), accuracy and correctness (selectivity, reproducibility of determination results, absence of interference with the determination from accompanying components, etc.);

Requirements for the method of preparing a sample of a substance before measurement;

Time spent on a single measurement;

The total duration of the analysis, taking into account sample preparation, measurement and issuance of results;

The ability to automate the process of sample preparation, measurement and delivery of analysis results.

The last four points are especially important when choosing a method suitable for performing mass analyses.

Instrumental methods of analysis are named due to the use of appropriate tools. According to the IUPAC (International Union of Pure and Applied Chemistry) definition, an instrument is a device that is used to observe a specific object, measure or report data on the state of a substance. The device replaces human actions, complements or increases his capabilities.

In instrumental methods of analysis, various types of instruments are used as instruments, designed to carry out basic analysis procedures, measure the physical and physicochemical properties of substances, and also to record measurement results. Thanks to modern computerized instruments, the sensitivity of the analysis can be significantly increased. Many physicochemical properties are specific.

All instrumental (physical and physicochemical) methods are based on the measurement of the corresponding physical quantities that characterize the substance being determined in the analyzed object.

For each instrumental method, the corresponding analytical signal is used. Table 1 shows examples of analytical signals and their corresponding methods, which belong to two important groups - electrochemical methods of analysis and optical methods of analysis. These groups also include some other methods not shown in the table. For example, optical methods include luminescence, atomic absorption and other spectroscopic methods, nephelometry, turbidimetry and polarimetry.

In addition to electrochemical and optical methods, other groups of methods are known. For example, methods that measure radioactivity are classified as nuclear physics methods. Mass spectrometric methods, thermal methods, etc. are also used. This classification is conditional and is not the only possible one.

Dependence of the analytical signal on the content of the analyte X called the calibration function. It is written as an equation of the form I=f(C). In this equation the symbol C indicate the content of the analyte X, which can be expressed in different units, for example, units of quantity of a substance (mol), units of mass (g, kg), units of molar concentration (mol/dm 3). These units are directly proportional to each other. The magnitude of the analytical signal is generally denoted by the symbol I, although some methods use specific designations (see Table 1). In each method, the calibration functions are of the same type, but the exact type of calibration function for a particular method depends on the nature of the substance being determined X and signal measurement conditions. Thus, in all versions of refractometric analysis, the analytical signal is the refractive index of the light beam (n), which linearly depends on the substance content X in the test solution ( I= n = a + k C). This means that in the refractometric determination of any substance, the calibration graph is linear, but does not pass through the origin of coordinates (Fig. 1). The numerical values ​​of the constants a and k depend on which component is determined and under what conditions (solvent, temperature, wavelength) the refractive index is measured.

Table 1. Examples of instrumental analysis methods

Electrochemical methods	Analytical signal	Type of calibration function
	Primary, I	Secondary, I*
Conductometry	Electrical resistance, R	Electrical conductivity, L	L= a + k
Potentiometry	E.D.S. electrochemical closet, E	Electrode potential E	E= a + log b
Voltammetry	Current strength i	Limit diffuse current, i d	i d= k
Coulometry	The amount of electricity Q
Electrogravimetry	Mass of electrolysis product, m
Optical methods	Analytical signal	Type of calibration function
	Primary, I	Secondary, I*
Atomic emission spectral analysis	Photocurrent, i; relative blackening, S	i= a C b S= a + k lgC
Spectrophotometry	Optical density, D	D = l C
Refractometry	Refractive index, n	n = n - n o	n = n 0 + kC

In many methods, the dependence of the signal on concentration is described by nonlinear functions, for example, in luminescent analysis it is an exponential function ( I= kC n), in potentiometry - logarithmic function (E = E 0 + k logC) etc. Despite these differences, all calibration functions are similar in that as the value of C increases (the content of the analyte X) the signal value changes continuously, and each value of C corresponds to only one value I.

Calibration functions are established experimentally using standard reference samples (standards), which contain different precisely known amounts of the analyte. X. The data obtained as a result of measuring the signal for each standard allows the calibration function to be presented in the form of a table, graph or algebraic formula. If we now measure the analytical signal of the test sample with the same device under the same conditions as the standard signal, then from the magnitude of such a signal it will be possible to determine the content X in the test sample using the calibration function.

It is easy to calculate the result of the analysis if the signal I directly proportional to the content of the analyte X. If such a proportional dependence does not exist, then the directly measured (primary) analytical signal I converted into a secondary analytical signal I*. Choose a conversion method such that the secondary analytical signal I* was directly proportional to the amount of the substance being determined X. For example, the electrical resistance of a solution ( R) depends in a certain way on the concentration of dissolved electrolyte (C). The resistance of the solution being analyzed is easy to measure, but R as an analytical signal it is inconvenient because as C increases, the value R decreases, and nonlinearly. Therefore, in conductometric analysis, the secondary signal is the electrical conductivity of the solution L, which is related to resistance R with the following formula:

Electrical conductivity of the solution L increases proportionally as the concentration of dissolved strong electrolyte increases. Moreover, from all the values L, obtained for the same type of solutions with different concentrations X, you can subtract the same quantity L 0 - electrical conductivity of a solution that does not contain X.“Corrected” conductivity value L* = L - L 0 is not simply linearly dependent on concentration X, and is directly proportional to the concentration of the electrolyte in the solution, that is L* = k C.

Rice. 1.1. Typical calibration graphs for some instrumental methods: 1 - refractometry; 2 - luminescent analysis; 3 - potentiometry

This technique is called background subtraction. It is used very often in instrumental methods. Before starting a measurement, many instruments are adjusted so that they immediately show a corrected signal directly proportional to C. The scale of such an instrument can be calibrated directly in concentration units. Sometimes, to ensure the linearity of calibration graphs, they transform not the ordinate, but the abscissa. For example, in potentiometric analysis, it is not the contents that are plotted along the horizontal axis. X, and its logarithm. And in some variants of spectral analysis, a double transformation is carried out - they logarithmize both the signal and the concentration, and then build a linear graphical dependence lgI from lgС.

Electrochemical methods. The most applicable electrochemical methods of analysis include potentiometric, polarographic and conductometric.

P o t e n t i o m e t r i c METHOD is based on the measurement of electrode potentials, which depend on the activity of ions, and in dilute solutions on the concentration of ions. The potentials of metal electrodes are determined by the Nernst equation

Accordingly, the ion concentration can be judged from the potential value. The measuring cell consists of a measuring (indicating) electrode and a reference electrode, which is not sensitive to the substance being determined.

They are increasingly being used ion-selective electrodes at the interfaces of which ion exchange reactions occur. The potential of the ion-selective electrode depends on the activity, and in dilute solutions, on the ion concentration in accordance with the Nernst equation. The most widely known are ion-selective glass electrodes for measuring pH. An ion exchange reaction occurs on the surface of the glass electrode

Кt st + +Н р + Н st + +Кt р +

Kt st – glass cations (K +, Na +, Li +), index p means solution.

At the interface between glass and solution, a potential jump occurs, the magnitude of which depends on the activity of hydrogen ions

A measuring cell with glass and auxiliary electrodes is connected to a pH meter designed for measuring the pH of solutions.

The industry also produces ion-selective electrodes for determining the concentration of Na + , K + , NH 4 + , Cl - ions (determination limit 10 -1 - 10 -6 mol/l) and Ca 2+ , Mg 2+ , NO 3 - ions (limit determination 10 -1 – 10 -4 mol/l).

Conductometry. The electrical conductivity of dilute solutions is proportional to the concentration of electrolytes. Therefore, by determining the electrical conductivity and comparing the obtained value with the value on the calibration graph, you can find the concentration of the electrolyte in the solution. The conductometry method, for example, determines the total content of impurities in high-purity water.

Chromatographic analysis. The analysis is based on chromatography, which makes it possible to separate two- and multicomponent mixtures of gases, liquids and dissolved substances by sorption methods under dynamic conditions. The analysis is carried out using special instruments - chromatographs. Several analysis methods have been developed, which are classified according to the mechanism of the process and the nature of the particles (molecular, ion exchange, precipitation, partition chromatography) and by forms of application (column, capillary, thin-layer and paper). Molecular chromatography is based on the different adsorbability of molecules on adsorbents, ion exchange chromatography is based on the different ability to exchange ions of a solution. Precipitation chromatography uses the different solubility of precipitates formed by the components of the analyzed mixture when interacting with reagents applied to the carrier. Partition chromatography is based on the differential distribution of substances between two immiscible liquids. Molecular (liquid adsorption), ion exchange and precipitation chromatography are usually carried out in chromatography columns, respectively, with an adsorbent, ion exchange material or an inert reagent carrier. Partition chromatography is usually performed on paper or a thin layer of absorbent.

The advantages of the chromatographic method of analysis include speed and reliability, the ability to determine several components of a mixture or solution.

Optical methods of analysis. These methods are based on measuring the optical properties of substances and radiation, the interaction of electromagnetic radiation with atoms or molecules of the analyzed substance, causing radiation, absorption or reflection of rays. They include emission, luminescence and absorption spectral methods.

Methods based on the study of emission spectra are called EMISSIONAL SPECTRAL METHODS analysis. In the emission spectroscopy method, a sample of a substance is heated to very high temperatures (2000-15000 °C). When a substance evaporates, it dissociates into atoms or ions, which give off radiation. Passing through the spectrograph, the radiation is decomposed into components in the form of a spectrum of colored lines. Comparing this spectrum with reference data on the spectra of elements makes it possible to determine the type of element, and from the intensity of the spectral lines, the amount of the substance. The method makes it possible to determine micro- and ultra-micro-quantities of a substance, analyze several elements, and in a short time.

A type of emission analysis is e m i s s i o n a fime

photometry, in which the test solution is introduced into a colorless burner flame. The type of substance is judged by the change in the color of the flame, and the concentration of the substance is judged by the intensity of the flame coloring. The analysis is performed using a device - a flame photometer. The method is mainly used for the analysis of alkali, alkaline earth metals and magnesium.

Methods based on the glow of the analyzed substance under the influence of ultraviolet (photoluminescence), x-rays (x-ray luminescence) and radioactive (radioluminescence) rays are called LUMINOSCENT. Some substances have luminescent properties, while other substances can luminesce after treatment with special reagents. The luminescent method of analysis is characterized by very high sensitivity (up to 10 -10 - 10 -13 g of luminescent impurities).

Methods based on the study of absorption spectra of rays by analyzed substances are called a b so r b t i o n o – s p e c t r a l x. As light passes through a solution, the light or its components are absorbed or reflected. The nature and concentration of a substance is judged by the amount of absorption or reflection of rays.

In accordance with the Bouguer–Lambert–Beer law, the dependence of the change in the intensity of the light flux passing through the solution on the concentration of the colored substance in the solution With expressed by the equation

Lg(I 0 / I)=  lc

where I 0 and I are the intensity of the light flux incident on the solution and passed through the solution,  is the light absorption coefficient, depending on the nature of the solute (molar absorption coefficient); l– thickness of the light-absorbing solution layer.

By measuring the change in light intensity, the concentration of the analyte can be determined. Determination is carried out using spectrophotometers and photocolorimeters.

IN with spectrophotometers use monochromatic radiation, and in PHOTO COLOR AND METTERS- visible light. The data obtained during the measurement are compared with graduated graphs built on standard solutions.

If the absorption of rays is measured by atoms of the analyte component, which are obtained by spraying a solution of the analyte into a burner flame, then the method is called a t o m o - a b so r b t i o n(atomic absorption spectroscopy). The method allows you to analyze substances in very small quantities.

An optical method based on the reflection of light by solid particles suspended in solution is called nephelometric. The analysis is carried out using nephelometer devices.

Thus, the use of the laws of electrochemistry, sorption, emission, absorption or reflection of radiation and the interaction of particles with magnetic fields has made it possible to create a large number of instrumental methods of analysis, characterized by high sensitivity, speed and reliability of determination, and the ability to analyze multicomponent systems.