Chemical analysis in Ukraine

There are many types of chemical analysis. They can be classified according to different criteria:

• by the nature of the information received. A distinction is made between qualitative analysis (in this case, they find out what a given substance consists of, which components are included in its composition) and quantitative analysis (determine the content of certain components, for example, in% by weight, or the ratio of different components). The line between qualitative and quantitative analysis is rather arbitrary, especially in the study of trace impurities. So, if in the course of a qualitative analysis a certain component was not detected, then it is necessary to indicate the minimum amount of this component that could be detected using this method. Possibly, the negative result of the qualitative analysis is connected not with the absence of the component, but with the insufficient sensitivity of the method used! On the other hand, quantitative analysis is always performed taking into account the previously found qualitative composition of the material under study;
• classification by the objects of analysis: technical, clinical, forensic, etc.
• classification according to the objects of definition.

Classification of types of chemical analysis

Classification "by object of determination" is very important, as it helps to choose the appropriate method of analysis (analytical method). Thus, for elemental analysis, spectral methods are often used, based on the registration of the radiation of atoms at different wavelengths. Most spectral methods involve complete destruction (atomization) of the analyte. If it is necessary to establish the nature and quantitative content of different molecules that make up the organic matter under study (molecular analysis), then one of the most suitable methods will be chromatographic, which does not imply the destruction of molecules.

In the course of elemental analysis, elements are identified or quantified regardless of their oxidation state or whether they are included in the composition of certain molecules. The complete elemental composition of the test material is determined in rare cases. Usually, it is sufficient to identify some elements that significantly affect the properties of the object under study.

The material analysis began to be distinguished as an independent form relatively recently, earlier it was considered as part of the elemental analysis. The purpose of material analysis is to separately determine the content of different forms of the same element. For example, chromium (III) and chromium (VI) in waste water. In petroleum products, "sulfate sulfur", "free sulfur" and "sulfide sulfur" are determined separately. Investigating the composition of natural waters, they find out what part of mercury exists in the form of strong (non-dissociating) complex and organoelement compounds, and what part - in the form of free ions. These tasks are more difficult than the tasks of elemental analysis.

Molecular analysis is especially important in the study of organic substances and materials of biogenic origin. An example would be the determination of benzene in gasoline or acetone in exhaled air. In such cases, it is necessary to take into account not only the composition, but also the structure of the molecules. Indeed, the test material may contain isomers and homologues of the determined component. Thus, it is often necessary to determine the content of glucose in the presence of many of its isomers and other related compounds, such as sucrose.

When it comes to determining the total content of all molecules that have some common structural features, the same functional groups, and therefore similar chemical properties, use the term structural-group (or functional) analysis. For example, the sum of alcohols (organic compounds having an OH group) is determined by conducting a reaction common to all alcohols with metallic sodium, and then measuring the volume of hydrogen evolved. The amount of unsaturated hydrocarbons (with double or triple bonds) is determined by oxidizing them with iodine. The total content of the same type of components is sometimes established in inorganic analysis - for example, the total content of rare earth elements.

Phase analysis is a specific type of analysis. So, carbon in cast irons and steels can dissolve in iron, can form chemical compounds with iron (carbides), and can form a separate phase (graphite). The physical properties of the product (strength, hardness, etc.) depend not only on the total carbon content, but also on the distribution of carbon between these forms. Therefore, metallurgists are interested not only in the total carbon content in cast iron or steel, but also in the presence of a separate graphite phase (free carbon) in these materials, as well as in the quantitative content of this phase.

The accuracy, sensitivity, and other characteristics of individual methods belonging to the same analytical method differ, but not as much as the characteristics of different methods. Any analytical problem can always be solved by several different methods (for example, chromium in alloy steel can be determined by the spectral method, and titrimetric, and potentiometric). The analyst chooses a method based on the known capabilities of each and the specific requirements for the analysis. It is impossible to choose the “best” and “worst” methods once and for all, everything depends on the problem being solved, on the requirements for the analysis results. Thus, gravimetric analysis, as a rule, gives more accurate results than spectral, but requires a lot of labor and time. Therefore, gravimetric analysis is good for arbitrage analysis, but not suitable for rapid analysis.

The determination methods are divided into three groups: chemical, physical and physicochemical. Often, physical and physicochemical methods are united under the general name “instrumental methods”, since in both cases the instruments are used, and the same ones. In general, the boundaries between groups of methods are rather arbitrary.

Chemical methods are based on carrying out a chemical reaction between the analyte and a specially added reagent. The group of chemical methods includes classical (long-known and well-studied) methods of determination, primarily gravimetry and titrimetry. The number of chemical methods is relatively small. As an analytical signal in chemical methods, the mass or volume of a substance is usually measured. Complex physical instruments, with the exception of analytical balances, and special standards of chemical composition are not used in chemical methods.

Physical methods are not associated with chemical reactions and the use of reagents. Their main principle is the comparison of the same type of analytical signals of a component in the test material and in a certain standard (sample with a precisely known concentration). Having built a calibration graph in advance (the dependence of the signal on the concentration or mass) and measuring the signal value for the sample of the test material, the concentration in this material is calculated. Physical methods are usually more sensitive than chemical ones, therefore, the determination of trace impurities is carried out mainly by physical methods. These methods are easy to automate and require less time for analysis. However, physical methods require special standards and require rather complicated, expensive, and highly specialized equipment.

Physicochemical methods of analysis occupy an intermediate place between chemical and physical methods in terms of their principles and capabilities. In this case, the analyst conducts a chemical reaction, but monitors its progress or its result not visually, but with the use of physical devices. For example, it gradually adds to the test solution another one with a known concentration of the dissolved reagent, and at the same time controls the potential of the electrode dipped into the titrated solution (potentiometric titration). By the jump in potential, the analyst judges the end of the reaction, measures the volume of titrant spent on it, and calculates the result of the analysis. Such methods are usually as accurate as chemical methods and almost as sensitive as physical methods.

Instrumental methods are often divided according to another, more clearly expressed feature - the nature of the measured signal. In this case, subgroups of optical, electrochemical, resonance, activation and other methods are distinguished. There are also few biological and biochemical methods that are still underdeveloped.