Physico-chemical studies as a direction of analytical chemistry are widely used 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 composition of 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;
- interpretative;
- methods of qualitative and quantitative analysis.
Empirical research methods reflect the studied object from the side of 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 - comparative, longitudinal, integrated. The first involves a comparison of the state of the object obtained at different times and under different conditions. Longitudinal - observation of the object of study for a long period of time. Integrated is a combination of longitudinal and comparative methods.
Interpretation methods are genetic and structural. The genetic option involves the study of the development of an object from the moment it occurs. The structural method studies and describes the structure of the object.
The methods of qualitative and quantitative analysis involved analytical chemistry. Chemical studies are aimed at determining the composition of the object of study.
Quantitative Analysis Methods
Using 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 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. A complete analysis is distinguished by finding the quantitative composition of the substances contained in the sample. A partial version determines the content of only the components of interest in this study of chemicals.
Three groups of methods are distinguished, depending on the method of analysis: chemical, physical, and physicochemical. All of them are based on a change 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 necessarily present in the pharmaceutical, petrochemical, construction industries and many others.
Three types of chemical research can be distinguished. Gravimetry, or weight analysis, is based on a change in the quantitative characteristics of the analyte in the sample. This option is simple and gives accurate results, but is laborious. With this type of chemical research method, the desired 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 occurs by measuring the volume of the reagent that reacts with the test substance. Its concentration is known in advance. Reagent volume is measured when the equivalence point is reached. In gas analysis, the volume of released or absorbed gas is determined.
In addition, the study of chemical models is often used. That is, an analogue of the object under study is created that is more convenient to study.
Physical research
In contrast to chemical studies based on 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 angle change in this case 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.
For fluorimetry, lasers and mercury lamps are used, which create monochromatic radiation. Some substances are capable of fluorescence (absorb and give off absorbed radiation). Based on the fluorescence intensity, a conclusion is made on the quantitative determination of a substance.
Physico-chemical studies
Physico-chemical research methods record 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 studied object on its chemical composition. These methods require the use of some measuring instruments. As a rule, monitoring is carried out for thermal conductivity, electrical conductivity, light absorption, boiling point and melting point.
Physico-chemical studies of the substance are widespread due to the high accuracy and speed of obtaining results. In the modern world, in connection with the development of IT-technology, chemical methods have become difficult to apply. Physico-chemical methods are used in the food industry, agriculture, and forensics.
One of the main differences between physical and chemical methods from chemical ones is that the end of the reaction (equivalence point) is found using measuring instruments, and not visually.
The main methods of physicochemical research are considered spectral, electrochemical, thermal and chromatographic methods.
Spectral methods of analysis of substances
The spectral methods of analysis are based on the interaction of an object with electromagnetic radiation. The absorption, reflection, dispersion of the latter is investigated. Another name for the method is optical. It is a combination of qualitative and quantitative research. Spectral analysis allows you to evaluate the chemical composition, structure of the components, magnetic field and other characteristics of the 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. Using a spectroscope, you 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 allows one to study the emission spectra and provides information on the composition of the substance. To obtain data, it is subjected to an electric arc discharge. A variation of this method is flame photometry. Absorption spectra are studied by the absorption method. The above options relate to the qualitative analysis of the substance.
Quantitative spectral analysis compares the intensity of the spectral line of the studied object and the substance of known concentration. Such methods include atomic absorption, atomic fluorescence and luminescent analyzes, turbidimetry, nephelometry.
Basics of electrochemical analysis of substances
Electrochemical analysis uses electrolysis to investigate a substance. 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 conductivity) are placed, electrodes (substances with electronic conductivity). Electrodes and electrolytes interact with each other. In this case, the current is supplied from the outside.
Classification of electrochemical methods
Classify electrochemical methods based on the phenomena on which physicochemical studies are based. These are methods with and without imposing extraneous potential.
Conductometry is an analytical method and measures the electrical conductivity of G. In conductometric analysis, as a rule, an alternating current is used. Conductometric titration is a more common research method. The production of portable conductometers used for chemical research of water is based on this method.
When conducting potentiometry, the EMF of a reversible galvanic cell is measured. The coulometry method determines the amount of electricity consumed during electrolysis. Voltammetry investigates the dependence of the current value on the laid potential.
Thermal analysis methods
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 change 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 specific heat of a substance, and enthalpimetry, based on a study of specific heat. Also among them should be attributed dilatometry, which records the change in sample volume under the influence of temperature.
Chromatographic methods of analysis of substances
The chromatography method is a method of separation of substances. There are many types of chromatography, the main of them: gas, distribution, redox, sedimentary, ion-exchange.
The components in the test sample are divided 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 substance. The components of the sample move in the mobile phase along the stationary one. By 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 physico-chemical methods is sanitary-chemical and forensic chemical research. They have some differences. In the first case, accepted hygienic standards are used to evaluate the analysis. They are established by ministries. A sanitary-chemical study is carried out in the manner prescribed by the epidemiological service. The process uses media models that mimic the properties of food products. They also reproduce the operating conditions of the sample.
A forensic chemical study is aimed at the quantitative identification of narcotic, potent substances and poisons in the human body, food products, and medications. The examination is carried out by court order.