Such a branch of science, as analytical chemistry, studies the composition and structure of chemicals. It is used to study an unknown compound or mixture. Qualitative analysis allows you to determine the presence or absence of elements or radicals in a chemical compound, and quantitative analysis includes determining the chemical content in the analyzed sample, that is, the number of components of the mixture. For a qualitative analysis, chemical reactions characteristic of certain elements are used that are easy to carry out. Qualitative reactions allow one to observe the expected effects or their absence.
Analytical chemistry is engaged in the development of new and improvement of existing methods of analysis, their practical application, as well as the study of the theoretical foundations of analytical processes. Specialists in this field are familiar with chemical, physical and physico-chemical methods of analysis. Many of them are based on qualitative reactions that are used to detect radicals and elements, as well as compounds that make up the studied samples. To study the qualitative and quantitative composition are used:
- elementary analysis (determined by elemental composition);
- molecular analysis (the structure of chemical compounds at the molecular level is established);
- structural analysis (one of the types of molecular analysis that studies the spatial structure of atoms and molecules, their molecular masses and empirical formulas);
- functional analysis (organic compounds are studied by functional groups).
Thus, both inorganic and organic compounds can be recognized. In the presence of specific elements, color may appear or disappear, precipitate may precipitate or dissolve, gas bubbles may be released, etc. If the qualitative reactions are chosen correctly, that is, they are selective (selective) with respect to a particular cation or anion, and are also highly sensitive (that is, the detection threshold allows you to set small amounts), then the result will be a reliable result - a conclusion about the presence or absence of an element or substance in the sample . Such an analysis of aqueous solutions is based on known ionic qualitative reactions.
For inorganic compounds, they often occur in aqueous solutions, but in the case of alkali metal cations, detection is carried out by adding dry salts to the middle (hottest) part of the flame of the spirit lamp. Lithium cations (Li +) will color the flame dark pink. Potassium cations (K +) - in purple, sodium (Na +) - in yellow, rubidium (Rb +) - in red, cesium (Cs +) - in blue. Qualitative reactions to cations can be carried out for barium salts: the presence of barium cations (Ba2 +) is established by adding a reagent with sulfate ions (SO42-), since the resulting barium sulfate will precipitate in white, which does not dissolve in acids: Ba2 + + SO42- → BaSO4 ↓. The presence of lead cations (Pb2 +) is detected when salt is exposed to an aqueous solution of sulfide (S2-), resulting in the formation of lead sulfide, which precipitates as a black precipitate: Pb2 + + S2- → PbS ↓. There are a lot of known qualitative reactions, both to cations and to anions, and they are described in analytical chemistry.
When choosing high-quality reactions for testing a sample, it is useful to know the general rules for the solubility of chemical compounds:
- All nitrates are soluble.
- Almost all salts of potassium, sodium and ammonium are soluble.
- All chlorides, bromides and iodides are soluble, with the exception of silver, mercury (I) and lead (II) halides.
- All sulfates are soluble, with the exception of barium, strontium and lead (II) sulfates , which are insoluble, and calcium and silver sulfates , which are moderately soluble.
- All carbonates, sulfites and phosphates are not soluble, with the exception of carbonates, sulfites and phosphates of potassium, sodium and ammonium.
- All sulfides are insoluble, with the exception of sulfides of alkali metals, alkaline earth metals and ammonium.
- All hydroxides are insoluble, with the exception of alkali metal hydroxides. Strontium, calcium and barium hydroxides are sparingly soluble.
Organic substances such as alkanes (saturated hydrocarbons) or alkenes (unsaturated hydrocarbons) can be detected using a solution of potassium permanganates, which in the first case will not change color, since paraffin hydrocarbons do not react with permanganate in the cold. In the second case, the solution becomes colorless due to the Wagner reaction (for example, with ethylene): 2KMnO4 + 3C2H4 + 4H2O - → 2KOH + 3CH2OH-CH2OH + 2MnO2 ↓. As a result, a precipitate of manganese dioxide having a brown color. Proteins are complex organic compounds that provide the vital activity of any living organism. There are a lot of them, their definition is of great practical importance. For these purposes, qualitative reactions to proteins are used, they are divided into colored and registered. With their help, it is not the proteins themselves that are determined, but the amino acids that make up their composition.