Physicochemical studies of substances

Physico-chemical research as a direction of analytical chemistry has found wide application in every sphere of human activity. They allow us to study the properties of the substance of interest, determining the quantitative component of the components in the composition of the sample.

Investigation of substances

Scientific research is the cognition 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;
• Interpretative;
• Methods of qualitative and quantitative analysis.

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

Organizational methods are comparative, longitudinal, complex. The first implies a comparison of the states of the object, obtained at different times and conditions that differ from each other. Longitude - observation of the object of research over a long period of time. Complex is a combination of longitudinal and comparative methods.

Interpretation methods are genetic and structural. The genetic variant assumes the study of the development of the object from the moment of its appearance. The structural method studies and describes the structure of the object.

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

Methods of quantitative analysis

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

Quantitative analysis is general, complete and partial. The total determines the amount of all known substances in the studied object, regardless of whether they are present in the composition or not. The complete analysis differs in finding the quantitative composition of the substances contained in the sample. A partial version defines the content of only components of interest in this study of chemicals.

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

Chemical research

This method is aimed at the determination of substances in various quantitatively occurring chemical reactions. The latter have external manifestations (color change, release of gas, heat, sediment). This method is widely used in many branches of life of modern society. The laboratory of chemical research is necessarily present in the pharmaceutical, petrochemical, construction industry and many others.

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

Titrimetry is a volumetric analysis. Investigation of chemical substances occurs by measuring the volume of a reagent that reacts with the substance under study. Its concentration is known in advance. The volume of the reagent is measured when the equivalence point is reached. In gas analysis, the volume of the released or absorbed gas is determined.

In addition, the study of chemical models is often used. That is, an analog of the studied object is created, more convenient in the study.

Physical research

Unlike chemical research, based on the conduct of appropriate reactions, physical methods of analysis are based on the same properties of substances. For their conduct, special devices are required. The essence of the method consists in measuring the changes in the characteristics of a substance caused by the action of radiation. The main methods of carrying out physical research are refractometry, polarimetry, fluorimetry.

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

Polarimetry is an optical research method that uses the ability of certain substances to rotate the plane of oscillation of linearly polarized light.

Fluorimetry uses lasers and mercury lamps, which produce monochromatic radiation. Some substances are able to fluoresce (absorb and give up absorbed radiation). Based on the fluorescence intensity, a conclusion is made about the quantitative determination of the substance.

Physico-chemical research

Physico-chemical research methods record the change in physical properties of a substance under the action 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 made for thermal conductivity, electrical conductivity, light absorption, boiling point and melting point.

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

One of the main differences between physical and chemical methods from chemical is that the end of the reaction (the point of equivalence) is found with the help of 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 substance analysis

The spectral methods of analysis are based on the interaction of the object with electromagnetic radiation. The absorption, reflection and dispersion of the latter are investigated. Another method name is optical. It is a combination of qualitative and quantitative research. Spectral analysis makes it possible to evaluate the chemical composition, the structure of the components, the magnetic field, and other characteristics of the substance.

The essence of the method is to determine the resonant frequencies at which the substance reacts to light. They are strictly individual for each component. With the help of the spectroscope one can see the lines on the spectrum and determine the constituent substances. 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 purpose of the study.

The emission method makes it possible to study emission spectra and gives information on the composition of matter. To obtain data, it is subjected to an electric arc discharge. A variation of this method is flame photometry. Absorption spectra are studied by absorption method. The above options refer to a qualitative analysis of the substance.

The quantitative spectral analysis compares the intensity of the spectral line of the object under study and a substance of known concentration. Such methods include atomic absorption, atomic fluorescent and luminescent analyzes, turbidimetry, nephelometry.

Fundamentals of electrochemical analysis of substances

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

Classification of electrochemical methods

Classify electrochemical methods based on the phenomena on which physico-chemical studies are based. These are methods with the imposition of extraneous potential and without it.

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

During the potentiometry, the EMF of the reversible cell is measured. The coulometry method determines the amount of electricity consumed during electrolysis. Voltamperometry examines the dependence of the magnitude of the current on the paved potential.

Thermal methods for the analysis of substances

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

Thermogravimetry is one of the methods of thermal analysis, which involves recording the change in the mass of an object under the influence of temperature. This method is considered one of the most accurate.

In addition, the thermal methods of investigation include calorimetry, which determines the specific heat of the substance, enthalpy- metry, based on the study of the heat capacity. Also to their number should be attributed dilatometriyu, which fixes the change in the volume of the sample under the influence of temperature.

Chromatographic methods of substance analysis

The chromatography method is a method of separation of substances. There are many types of chromatography, the main ones are: 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 sample components move in the mobile phase along the stationary phase. The speed and time of the passage of components through the last phase are judged on their physical properties.

Application of physical and chemical methods of research

The most important area of physico-chemical methods is sanitary-chemical and forensic chemical research. They have some differences. In the first case, the adopted hygienic standards are used to evaluate the analysis. They are set by the ministries. Sanitary and chemical research is conducted in the order established by the epidemiological service. In the process, medium models are used that mimic the properties of food products. They also reproduce the conditions of operation of the sample.

Forensic-chemical research is aimed at quantitative detection of narcotic, potent substances and poisons in the human body, food products, medicinal preparations. Examination is carried out by judicial decision.