Physico-chemical research as a branch of analytical chemistry is widely used in every sphere of human life. They allow you to study the properties of a substance of interest by 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 object under study 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, complex. The first implies a comparison of the states of an object obtained at different times and under different conditions. Longitudinal - observation of the objectresearch over a long period of time. Complex is a combination of longitudinal and comparative methods.
Interpretation methods - genetic and structural. The genetic variant involves the study of the development of an object from the moment of its occurrence. The structural method studies and describes the structure of an object.
Analytical chemistry deals with methods of qualitative and quantitative analysis. Chemical studies are aimed at determining the composition of the object of study.
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 is general, complete and partial. General 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 defines the content of only the components of interest in this chemical study.
Depending on the method of analysis, there are three groups of methods: chemical, physical and physico-chemical. All of them are based on a change in the physical or chemical properties of a substance.
Chemical Research
This method is aimed at the determination of substances in various quantitatively occurring chemicalreactions. The latter have external manifestations (discoloration, release of gas, heat, sediment). This method is widely used in many branches of life of modern society. The chemical research laboratory is a must-have in the pharmaceutical, petrochemical, construction and many other industries.
There are three types of chemical research. Gravimetry, or weight analysis, is based on the 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 isolated 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 these procedures, the component is weighed.
Titrimetry is a volumetric analysis. The study of chemicals occurs by measuring the volume of a reagent that reacts with the substance under study. Its concentration is known in advance. The reagent volume is measured when the equivalence point is reached. In gas analysis, the volume of released or absorbed gas is determined.
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 same properties of substances. For theircarrying out requires special devices. 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 physical examination 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. Changing the angle in this case depends on the properties of the medium components. 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 the absorbed radiation). Based on the intensity of fluorescence, a conclusion is made about the quantitative determination of the substance.
Physical and chemical research
Physico-chemical 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 object under study on its chemical composition. These methods require the use of some measuring instruments. As a rule, thermal conductivity, electrical conductivity, light absorption, boiling point and melting point are monitored.
Physico-chemical studies of matterare widely used due to their high accuracy and speed of obtaining results. In the modern world, due to the development of IT technologies, chemical methods have become difficult to apply. Physico-chemical methods are used in the food industry, agriculture, forensics.
One of the main differences between physical and chemical methods and chemical ones is that the end of the reaction (the equivalence point) 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 substance analysis
The basis of spectral methods of analysis is the interaction of an object with electromagnetic radiation. The absorption, reflection, and scattering of the latter are studied. 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 components, magnetic field and other characteristics of a 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 a spectroscope, you can see the lines on the spectrum and determine the constituents of a 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 purpose of the study.
Emission methodallows you to study the emission spectra and gives information about the composition of the substance. To obtain data, it is subjected to an electric arc discharge. A variation of this method is flame photometry. The absorption spectra are studied 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 the object under study and a substance of known concentration. These methods include atomic absorption, atomic fluorescence and luminescence analyses, 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 (substances with ionic conductivity), electrodes (substances with electronic conductivity) are placed. 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 physical and chemical studies are based. These are methods with and without extraneous potential.
Conductometry is an analytical method and measures electrical conductivity G. Conductometric analysis generally uses alternating current. Conductometric titration – morecommon 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. The coulometry method determines the amount of electricity consumed during electrolysis. Voltammetry investigates the dependence of the magnitude of the current on the applied potential.
Thermal methods of substance analysis
Thermal analysis is aimed at determining the change in the physical properties of a substance under the influence of temperature. These test methods are performed within 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, enthalpymetry, based on the study of heat capacity. They also include dilatometry, which captures the change in sample volume under the influence of temperature.
Chromatographic methods for the analysis of substances
The chromatography method is a way of separating substances. There are many types of chromatography, the main ones are: gas, distribution, redox, precipitation, ion exchange.
The components in the test sample are separated between moving and stationaryphases. 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. By the speed and time of passage of the components through the last phase, their physical properties are judged.
Application of physical and chemical 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, accepted hygienic standards are used to evaluate the analysis performed. They are set by the ministries. Sanitary-chemical research is carried out in accordance with the procedure established by the epidemiological service. The process uses environmental models that mimic the properties of food products. They also reproduce the operating conditions of the sample.
Forensic chemical research is aimed at the quantitative detection of narcotic, potent substances and poisons in the human body, food products, medications. The examination is carried out according to a court order.
