Physical and chemical research of substances. Application of physicochemical research methods

In the general case, the analysis of the composition of a substance is understood to mean the determination of their elementary, functional or molecular composition; in some cases it is necessary to determine the phase composition of the medium.

When controlling chemical technological processes, it is most often necessary to determine the molecular composition. The tasks of the analysis of substances are associated with the determination of the content of both one component of the analyzed mixture, and two or more of its components. Devices for determining the composition are called analyzers. Analyzers designed to determine the content of only one component in a mixture are sometimes also called concentrators.

Strictly speaking, the composition of substances is characterized by the number of particles of the individual components of the sample and can also be expressed by the number of moles, the mass of the components in grams, or other units of mass. However, for practical purposes, the composition is expressed in terms of concentrations WITH components: concentration is understood as the ratio of the amount T of the determined component in the sample to the total amount of the sample M:. The quantities T and M can be related in a certain way to the particle numbers of the components. The following concentration units are most common: for liquids - mg / cm 3; g / cm 3; % by weight or volume; for gases - mg / m 3; g / m 3; % by volume.

The properties of substances are characterized by numerical values ​​of physical or physicochemical quantities (for example, density, viscosity, electrical conductivity, etc.) that can be measured.

The practical implementation of analytical measurements is based on the use of the relationship between the composition of the analyte (concentrations of its components) and the quantities characterizing its physical and physicochemical parameters:

where - the measured parameter of the analyte; , , ..., -concentration of components; NS- the total number of components.

By the type of the measured parameter, analytical methods (instruments) can be based on the determination of optical, electrical, magnetic, thermal, kinetic, and mechanical properties of the medium. The measured parameters are, for example, spectral coefficients of radiation, absorption, scattering and reflection of radiation, refractive index, dielectric constant and magnetic susceptibility, density, viscosity and thermal conductivity, pressure and velocity of propagation of acoustic vibrations, etc. high accuracy in determining the values ​​of these parameters. For example, electrical conductivity, density, refractive index can be measured with an accuracy reaching 10 -4 -10 -5 of their values.

The analysis of the composition is based on the assumption that for each analyzed medium, it is possible to establish the minimum number of independent parameters characterizing it, allowing the concentration to be determined. However, for real media, finding a complete system of independent parameters is a very difficult task; therefore, in practice, an incomplete system of measured parameters is used and, therefore, concentrations are calculated with some error.

Let, for example, you need to determine the concentration th component ... Since, when monitoring and regulating technological processes, changes in the concentrations of components are usually small, the function in equation (1) can be considered additive in the first approximation. Then

(2)

where
at
;
at
; - concentration of the determined component; - the average content of components in the analyzed environment;
- deviation of the content of the corresponding components from the average value;
- a change in the measured parameter caused by a change
concentration of components.

From equation (2), you can determine the desired value

Hence it follows that the readings of the analyzer determining the concentration one component, depend to a certain extent on changes in content other components of the environment. The weaker this dependence, i.e., the smaller the relative values ​​of the term
, the higher the selectivity of determining the concentration , and the accuracy of the analysis.

The selectivity of analysis is one of the most important characteristics of an automatic analyzer.

In practice, the choice of analytical methods providing selective determination of a component by direct measurement of physical or physicochemical parameters of a sample is very limited. The selectivity of most of the analytical methods used is determined by the fact that the analyzed sample is subjected to a preliminary active influence, during which it changes qualitatively. The result of the impact on the sample can be, for example, a change in its aggregate or phase state, ionization, spatial or space-time separation of the sample, enrichment, change in its composition. After the transformation of the sample, its physical or physicochemical parameters are measured. In this case, the measurement of various parameters of the sample can be combined with the same types of its preliminary transformation. For example, in the chromatographic method of analysis, the analyzed mixture is separated into components in a chromatographic column, and then the concentrations of the components in the carrier gas are determined by measuring either density, or thermal conductivity, or ionization efficiency, etc.

To establish the relationship between analytical methods (analyzers) and determine their place in analytical instrumentation, various options for their classifications are used. Depending on the purposes of the classification, analytical instruments can be classified, for example, according to the following criteria: principle of operation (method of analysis); the properties of the analyzed environment; by the number of determined components; execution; the way to unify the output signal; the method of outputting measurement results.

Other signs of classification are also possible. Taking into account the preliminary transformation of the sample, it seems advisable to classify the analyzers according to the principle of operation within the framework of a two-dimensional set. With this approach, analytical methods and instruments can be characterized by the method of transformation of the sample and the measured physical parameter, i.e., the classification table should have, as it were, two coordinate axes: on one there are methods for converting the analyzed sample, and on the other - the types of the measured physical parameter of the converted sample.

In the simplest case, the analysis can be performed without transforming the sample, when the composition of the analyzed mixture can be judged directly by the measured parameter.

The measured parameters of the sample can be subdivided into mechanical (speed and absorption of sound, density), thermal and kinetic (specific heat, thermal conductivity, viscosity), electrical and magnetic (conductivity, potential, dielectric constant, magnetic susceptibility), optical (coefficients of absorption, reflection, refraction and scattering, radiation intensity, magneto-optical rotatability).

Measurement of mechanical parameters (speed and sound absorption) forms the basis of acoustic analysis methods. The methods of calorimetry, thermoconductometry and viscometry, respectively, are based on the measurement of thermal and kinetic parameters - specific heat, thermal conductivity and viscosity. A significant group of methods of analysis is based on the measurement of electrical and magnetic parameters: on the measurement of conductivity - conductometry, potential - potentiometry (pH-ve-try), polarography, dielectric constant - dielectomy, magnetic susceptibility - magnetomechanical methods of analysis.

Analysis methods based on direct measurement of the optical parameters of the analyzed sample are widely used in analytical practice: on the measurement of the absorption coefficient - absorption-optical, refractive index - refractometry, optical activity coefficient - polarimetry, scattering coefficient - nephelometry, turbidimetry.

Additional targeted transformation of the sample during analysis allows for increased selectivity of analytical measurement. Both physical and chemical methods can be used to convert the sample. If the impact on the sample leads to a significant change in its physical properties while the composition of the sample remains unchanged, then such a transformation will be called physical. If the impact on the sample leads to a significant change in its composition, then such a transformation will be called chemical.

The physical methods of transformation used in analytical instrumentation include: ionization (excitation), change in the state of aggregation, spatial and (or) time separation, enrichment (sorption, extraction). The chemical transformation of the sample is carried out on the basis of chemical reactions. For example, by preliminary ionization of the sample, the composition can be linked to the processes taking place in the ionized gas. The combination of ionization with the subsequent measurement of the conductivity of the ionized gas forms the basis of ionization methods of analysis, and the combination of ionization with the measurement of optical parameters is the basis of atomic absorption spectrophotometry. Chromatography and mass spectrometry are based on preliminary spatial and temporal separation of the sample into components followed by measurement of thermal conductivity, electrical conductivity, or optical parameters.

A chemical reaction followed by a measurement of the color effect (optical parameters) forms the basis of photocolorimetric methods, a preliminary chemical reaction followed by a measurement of the thermal effect (specific heat) is the basis of thermochemistry, and, for example, a preliminary chemical reaction in combination with measurement of the electrical parameters of the converted sample is the basis of electrochemical analysis methods.

With automatic control of the concentration (composition) and properties of liquids in the chemical industry, the following methods of analysis are most widely used (classification according to GOST 16851-71): without preliminary conversion of the sample - conductometric, potentiometric, polarographic, dielectric, optical (refractometric, absorption, luminescent, polarization , turbidimetric, nephelometric), by the magnitude of the temperature depression, by the pressure of saturated vapor, radioisotope, mechanical (density), kinetic (viscosity); with preliminary transformation of the sample - titrimetric.

For automatic analysis of gases: without preliminary conversion of the sample (classification according to GOST 13320-81) - absorption-optical (infrared and ultraviolet absorption), thermal conductometric, thermomagnetic, pneumatic; with preliminary transformation of the sample - electrochemical (conductometric, coulometric, polarographic, potentiometric), thermochemical, photocolorimetric, flame ionization, aerosol ionization, chromatographic, mass spectrometric. In the further presentation, the above classification is adopted. From the above classification, moisture meters are selected, united in a special raadel for their intended purpose.

Spectral analysis is one of the main methods for analyzing the chemical composition of a substance. The analysis of its composition is made based on the study of its spectrum. Spectral analysis - used in various studies. With its help, a complex of chemical elements was discovered: He, Ga, Cs. in the atmosphere of the sun. And also Rb, In and XI, the composition of the Sun and most other celestial bodies is determined.

Industries of application

Spectral examination, distributed in:

1. Metallurgy;
2. Geology;
3. Chemistry;
4. Mineralogy;
5. Astrophysics;
6. Biology;
7. medicine, etc.

Allows you to find in the studied objects the smallest amounts of the installed substance (up to 10 - MS) Spectral analysis is divided into qualitative and quantitative.

Methods

The spectrum-based method for determining the chemical composition of a substance is the basis of spectral analysis. Linear spectra have a unique personality, just like human fingerprints or snowflake patterns. The uniqueness of the patterns on the skin of the finger is a great advantage for tracing a criminal. Therefore, due to the peculiarities of each spectrum, it is possible to establish the chemical content of the body by analyzing the chemical composition of the substance. Even if its element mass does not exceed 10 - 10 g, using spectral analysis it can be detected in the composition of a complex substance. This is a rather sensitive method.

Emission Spectral Analysis

Emission spectral analysis is a series of methods for determining the chemical composition of a substance from its emission spectrum. The basis of the method for establishing the chemical composition of a substance - spectral examination, is based on patterns in the emission spectra and absorption spectra. This method allows you to identify millionths of a milligram of a substance.

There are methods of qualitative and quantitative examination, in accordance with the establishment of analytical chemistry as a subject, the purpose of which is the formation of methods for establishing the chemical composition of a substance. Methods for identifying a substance become extremely important within qualitative organic analysis.

By the line spectrum of vapors of any of the substances, it is possible to establish which chemical elements are contained in its composition, since any chemical element has a personal specific spectrum of radiation. This method of establishing the chemical composition of a substance is called qualitative spectral analysis.

X-ray spectrum analysis

There is another method for determining a chemical called X-ray spectral analysis. X-ray spectral analysis is based on the activation of atoms of a substance when it is irradiated with X-rays, the process is called secondary or fluorescent. And also activation is possible when irradiated with high-energy electrons, in this case the process is called direct excitation. As a result of the movement of electrons in the deeper inner electron layers, X-ray lines appear.

The Wolfe-Bragg formula allows you to set the wavelengths in the composition of X-rays when using a crystal of a popular structure with a known distance d. This is the basis of the method of determination. The substance under study is bombarded with fast electrons. It is placed, for example, on the anode of a collapsible X-ray tube, after which it emits characteristic X-rays that fall on a crystal of a known structure. The angles are measured and the corresponding wavelengths are calculated using the formula, after photographing the resulting diffraction pattern.

Receptions

Currently, all methods of chemical analysis are based on two techniques. Either on: physical reception, or on chemical reception comparison of the established concentration with its unit of measurement:

Physical

The physical technique is based on the method of correlating the unit of quantity of the component with the standard by measuring its physical property, which depends on its content in the sample of the substance. The functional dependence "Saturation of the property - the content of the component in the sample" is determined by the method of calibration of the measuring instrument of the given physical property by the component to be installed. From the calibration graph, quantitative relations are obtained, built in the coordinates: "the saturation of the physical property - the concentration of the component to be installed".

Chemical

The chemical technique is used in the method of correlating the unit of the quantity of the component with the standard. Here, the laws of conservation of the amount or mass of a component are used in chemical interactions. Chemical interactions are based on the chemical properties of chemical compounds. In a sample of a substance, a chemical reaction is carried out that meets the specified requirements to determine the desired component, and the volume or mass of the components participating in a specific chemical reaction is measured. Quantitative ratios are obtained, then the number of equivalents of the component for a given chemical reaction or the law of conservation of mass is written.

Devices

Devices for the analysis of the physical and chemical composition of a substance are:

1. Gas analyzers;
2. Signaling devices of maximum permissible and up to explosive concentrations of vapors and gases;
3. Concentration meters for liquid solutions;
4. Density meters;
5. Saline meters;
6. Moisture meters and other devices similar in purpose and completeness.

Over time, the range of analyzed objects is increasing and the speed and accuracy of the analysis increases. Spectral analysis is becoming one of the most important instrumental methods for establishing the atomic chemical composition of a substance.

Every year more and more complexes of devices appear for quantitative spectral analysis. They also produce the most advanced types of equipment and spectrum registration methods. Spectral laboratories are organized, initially in machine-building, metallurgical, and then in other areas of industry. The speed and accuracy of the analysis grows over time. In addition, the scope of the analyzed objects is expanding. Spectral analysis is becoming one of the main instrumental methods for establishing the atomic chemical composition of a substance.

Active Edition from 09.03.2004

"INSTRUMENTS, AUTOMATION MEANS AND COMPUTER EQUIPMENT. STATE ELEMENTAL ESTIMATE RATES FOR EQUIPMENT INSTALLATION. COLLECTION N 11. GESNM-2001-11) (approved by the Decree of the State Construction Committee of the Russian Federation of 05.28.2001 N 53.2004) (ed. Of 09.03.2001) (ed. From 09.03.2001).

Section 2. Devices for the analysis of the physical and chemical composition of a substance and special devices

Introductory instructions

1. In this section, instruments for analyzing the physical and chemical composition of a substance include gas analyzers, alarms of maximum allowable concentrations of vapors and gases, concentrators of liquid solutions, densitometers, salt meters, moisture meters and devices similar in purpose and completeness.

2. For submersible sensors and transducers of PH-meters, ordered and supplied separately from each other, the norms of Section 2 of this Book should be applied.

3. The rates take into account the costs of installing a complete set of instruments (sensors, measuring units, secondary instruments, display units, auxiliary devices).

4. When applying the standards, it is necessary to be guided by the following characteristics of the complexity category of the sets:

Category I - a set consisting of one transducer (receiver, measuring unit) and an indication unit (secondary device, signaling device). The set can include one or two simplest auxiliary devices (power or flow stabilizer, filter, etc.);

Category II - a set consisting of two transducer blocks (receiver and control unit, primary and normalizing transducers, etc.), or from one transducer and a set of auxiliary devices (for example, a set of sample preparation devices as part of a refrigerator, a flow stimulator, a filter etc.), as well as an indication unit;

5. The rates do not include installation costs:

communication lines and connection of wiring, which are determined according to the Collection of GESNm N 8 "Electrical installations" and the norms of sections 4 and 8 of this Collection;

flow sensors determined according to table 11-02-012.

Meter: set.

Physicochemical research as a direction of analytical chemistry has found wide application in every area of ​​human life. They allow you to study the properties of the substance of interest, determining the quantitative component of the components in the sample.

Substance research

Scientific research is the knowledge of an object or phenomenon in order to obtain a system of concepts and knowledge. According to the principle of action, the methods used are classified into:

• empirical;
• organizational;
• interpretive;
• methods of qualitative and quantitative analysis.

Empirical research methods reflect the object under study from the side of external manifestations and include observation, measurement, experiment, comparison. Empirical research is based on reliable facts and does not involve the creation of artificial situations for analysis.

Organizational methods - comparative, longitudinal, complex. The first implies a comparison of the states of an object obtained at different times and under different conditions. Longitudinal - observation of the object of study over a long period of time. Complex is a combination of longitudinal and comparative methods.

Interpretive methods - genetic and structural. The genetic variant involves the study of the development of an object from the moment of its inception. The structural method studies and describes the structure of an object.

Analytical chemistry deals with the methods of qualitative and quantitative analysis. Chemical research is aimed at determining the composition of the research object.

Quantitative analysis methods

With the help of quantitative analysis in analytical chemistry, the composition of chemical compounds is determined. Almost all the methods used are based on the study of the dependence of the chemical and physical properties of a substance on its composition.

Quantitative analysis can be general, complete and partial. The total determines the amount of all known substances in the object under study, regardless of whether they are present in the composition or not. A complete analysis is distinguished by finding the quantitative composition of the substances contained in the sample. The partial option determines the content of only the components of interest in a given chemical study.

Depending on the method of analysis, three groups of methods are distinguished: chemical, physical and physicochemical. All of them are based on changes in the physical or chemical properties of a substance.

Chemical research

This method is aimed at determining substances in various quantitatively occurring chemical reactions. The latter have external manifestations (color change, gas, heat, sediment). This method is widely used in many sectors of the life of modern society. A chemical research laboratory is a must-have in the pharmaceutical, petrochemical, construction industries and many others.

There are three types of chemical research. Gravimetry, or weight analysis, is based on changes in the quantitative characteristics of a test substance in a sample. This option is simple and accurate, but time consuming. With this type of chemical research methods, the required substance is released from the total composition in the form of a precipitate or gas. Then it is brought into a solid insoluble phase, filtered, washed, dried. After carrying out these procedures, the component is weighed.

Titrimetry is a volumetric analysis. The study of chemicals is carried out by measuring the volume of a reagent that reacts with the test substance. Its concentration is known in advance. Reagent volume is measured when the equivalence point is reached. Gas analysis determines the volume of emitted or absorbed gas.

In addition, chemical model research is often used. That is, an analogue of the object under study is created, which is more convenient to study.

Physical research

Unlike chemical research, based on carrying out appropriate reactions, physical methods of analysis are based on the properties of substances of the same name. To carry them out, special devices are required. The essence of the method is to measure changes in the characteristics of a substance caused by the action of radiation. The main methods of conducting physical research are refractometry, polarimetry, fluorimetry.

Refractometry is performed using a refractometer. The essence of the method is to study the refraction of light passing from one medium to another. The change in the angle in this case depends on the properties of the components of the environment. Therefore, it becomes possible to identify the composition of the medium and its structure.

Polarimetry is which uses the ability of certain substances to rotate the plane of oscillation of linearly polarized light.

For fluorimetry, lasers and mercury lamps are used, which produce monochromatic radiation. Some substances are capable of fluorescent (absorbing and giving off absorbed radiation). Based on the fluorescence intensity, a conclusion is made about the quantitative determination of the substance.

Physical and chemical research

Physicochemical research methods register changes in the physical properties of a substance under the influence of various chemical reactions. They are based on the direct dependence of the physical characteristics of the investigated object on its chemical composition. These methods require the use of some measuring instruments. As a rule, observation is carried out for thermal conductivity, electrical conductivity, light absorption, boiling and melting points.

Physicochemical studies of a substance are widespread due to the high accuracy and speed of obtaining results. In the modern world, due to development, methods have become difficult to apply. Physicochemical methods are used in the food industry, agriculture, and forensic science.

One of the main differences between physical and chemical methods from chemical ones is that the end of the reaction (the point of equivalence) is found using measuring instruments, and not visually.

The main methods of physical and chemical research are considered to be spectral, electrochemical, thermal and chromatographic methods.

Spectral methods of analysis of substances

Spectral analysis methods are based on the interaction of an object with electromagnetic radiation. Absorption, reflection, scattering of the latter is investigated. Another name for the method is optical. It is a collection of qualitative and quantitative research. Spectral analysis allows you to evaluate the chemical composition, structure of components, magnetic field and other characteristics of a substance.

The essence of the method is to determine the resonant frequencies at which a substance reacts to light. They are strictly individual for each component. With the help of a spectroscope, you can see the lines in the spectrum and determine the constituents of the substance. The intensity of the spectral lines gives an idea of ​​the quantitative characteristic. The classification of spectral methods is based on the type of spectrum and the objectives of the study.

The emission method allows one to study the emission spectra and provides information on the composition of a substance. To obtain data, it is subjected to an electric arc discharge. A variation of this method is flame photometry. The absorption spectra are investigated by the absorption method. The above options refer to the qualitative analysis of the substance.

Quantitative spectral analysis compares the intensity of the spectral line of an object under study and a substance of known concentration. These methods include atomic absorption, atomic fluorescence and luminescence analyzes, turbidimetry, nephelometry.

Fundamentals of Electrochemical Analysis of Substances

Electrochemical analysis uses electrolysis to investigate a substance. The reactions are carried out in an aqueous solution on electrodes. One of the available characteristics is subject to measurement. The study is carried out in an electrochemical cell. This is a vessel in which electrolytes (substances with ionic conduction), electrodes (substances with electronic conduction) are placed. Electrodes and electrolytes interact with each other. In this case, the current is supplied from the outside.

Classification of electrochemical methods

Electrochemical methods are classified based on the phenomena on which physicochemical studies are based. These are methods with and without the imposition of extraneous potential.

Conductometry is an analytical method and measures electrical conductivity G. Conductometric analysis typically uses alternating current. Conductometric titration is a more common research method. This method is the basis for the manufacture of portable conductometers used for chemical studies of water.

When carrying out potentiometry, the EMF of a reversible galvanic cell is measured. Coulometry measures the amount of electricity consumed during electrolysis. Voltammetry examines the dependence of the current on the laid potential.

Thermal methods for the analysis of substances

Thermal analysis is aimed at determining the change in the physical properties of a substance under the influence of temperature. These research methods are performed for a short period of time and with a small amount of the studied sample.

Thermogravimetry is one of the methods of thermal analysis, which accounts for the registration of changes in the mass of an object under the influence of temperature. This method is considered one of the most accurate.

In addition, thermal research methods include calorimetry, which determines the heat capacity of a substance, and enthalpimetry, based on the study of heat capacity. They also include dilatometry, which records the change in sample volume under the influence of temperature.

Chromatographic methods for the analysis of substances

Chromatography is a method for separating substances. There are many main ones: gas, distribution, redox, sedimentary, ion-exchange.

The components in the test sample are separated between the mobile and stationary phases. In the first case, we are talking about liquids or gases. The stationary phase is a sorbent - a solid. The components of the sample move in the mobile phase along the stationary one. According to the speed and time of passage of the components through the last phase, their physical properties are judged.

Application of physicochemical research methods

The most important area of ​​physical and chemical methods is sanitary-chemical and forensic chemical research. They have some differences. In the first case, the accepted hygienic standards are used to assess the analysis performed. They are established by ministries. Sanitary-chemical research is carried out in accordance with the procedure established by the epidemiological service. The process uses media models that simulate the properties of food. They also reproduce the operating conditions of the sample.

Forensic chemical research is aimed at the quantitative identification of narcotic, potent substances and poisons in the human body, food products, medications. The examination is carried out by a court order.