Qualitative analysis is a method used in analytical chemistry to determine the ions inside a substance. Let us analyze its features, as well as options for the practical use of elements in analytical chemistry.
Classification
Qualitative analysis is a method that involves dividing into several groups:
- chemical, which are based on any external effect;
- physical, allowing to determine the composition using thermal, magnetic, electrical properties;
- physico-chemical, based on the analysis of physical processes that occur as a result of chemical interactions.
During the experiments, the variety that is most suitable in a particular case is selected.
Destination
Qualitative analysis is the discovery of ions, chemical elements, molecules, groups in an analyzed sample of a substance. Its purpose is to detect certain ions or elements that are in the composition of the compound.
The definition of qualitative analysis explains the application in its framework of the physical and chemical properties of a substance.
Selectivity
Of the many chemical reactions used to detect ions or elements, only those processes that are characterized by an external result. Qualitative analysis methods are effective in the formation of sediment, gas evolution, discoloration, energy release. All processes underlying the method are called analytical reactions.
Qualitative analysis is a method that is based on selective (specific) processes manifested in a particular ion (group of elements).
Reaction Requirements
There are certain requirements that apply to interactions in a qualitative analysis:
- speed and irreversibility of the course;
- external signs (sediment, gas, color);
- high sensitivity
Specific is called such a reaction that makes it possible to detect the necessary element even in the case of its minimum concentrations, and in the presence of other elements in the mixture.
Sensitivity is determined by the minimum amount of detectable element at which it is detected without additional enrichment of the solution. This is an important characteristic of a qualitative analysis that allows us to talk about the effectiveness of the conducted (planned) experiment.
Methods
The following methods of qualitative and quantitative analysis are distinguished:
- by the number of detected particles: elementary, functional, phase, isotopic, molecular;
- by the amount of the compound taken for analysis: macro- (more than 100 mg, 5 ml), micro- (not more than 0.1 ml, 1 - mg), half-micro - (medium range), ultramicromethods (substances less than 0.1 mg, 0.05 ml);
- on the determined object: organic and inorganic.
A little help
A brief description of the qualitative and quantitative analysis allows us to understand their main differences. In a qualitative analysis, the sample may be in the form of a solution or a dry material in which several compounds are present at once. The analysis of the sample is carried out by a fractional technique, revealing ions using certain qualitative reactions.
First, the presence of an ammonium cation is detected, since it is most easily removed using reagents from the mixture. Next, anions are identified, and the composition of the existing sample is summarized.
How to determine ammonium cation
In order to discover these ions, two methods are used in qualitative analysis. The first option is based on the addition of an alkali solution (active metal hydroxide). A few drops of the solution in question or the ammonium salt are treated with a solution of sodium hydroxide (potassium). As a result of a qualitative reaction, evolution of gaseous ammonia is observed. To detect it, indicator paper is used (phenolphthalein becomes raspberry).
The second method for determining ammonium cations in a sample involves the use of Nessler's reagent. Excess alkali is treated with a few drops to precipitate colored bases, which interfere with the observation of the precipitate resulting from the interaction with the Nessler reagent of the ammonium cation.
This reagent is a complex salt of potassium and mercury, which gives a red-brown precipitate with an ammonium cation. It is used to determine calcium cations in a mixture.
Determination of calcium cations
For a qualitative analysis, the use of a microscopic reaction in the form of gypsum is permissible. One drop of concentrated sulfuric acid is added to a few drops of the analyzed solution. After a couple of minutes, transfer it to a glass slide, evaporate until a rim is formed. The result of the experiment is examined under a microscope.
The second variant of the detection of calcium cations in the analyzed sample is based on staining the colorless flame of the spirit lamp with a saturated brick-red color with the volatile salts of calcium.
Determination of iron cations
Bivalent and trivalent iron is detected from the initial solution, since under the influence of certain chemicals (hydrogen peroxide, alkali, hydrogen sulfide), the oxidation state changes from +2 to +3. To detect an iron cation with an oxidation state of +2, it is necessary to add a mixture of potassium hexacyanoferrate (3) and hydrochloric acid to the test solution. When a saturated blue precipitate of Prussian blue appears, we can talk about the content of Fe 2+ in the solution.
In order to determine ferric iron, it is necessary to add K 4 [Fe (CN) 6 ] salt solution and hydrochloric acid solution to the solution. The appearance of saturated blue color confirms the presence of Fe 3+ in the solution.
Methods for detecting Co2 +, Ni2 +, Cr3 +
These cations have specific reactions, so they can be detected in portions of the initial solution. To detect the cobalt cation, sodium acetate is added dropwise to the existing solution until an acidic medium is obtained. Then, sodium fluoride (or ammonium) and NH4NCS are added to the solution so that the iron cation binds to the complex, then it is extracted with isoamyl alcohol.
During the formation of the complex compound, the organic layer of the solvent is colored blue.
To open the nickel cation in the analyzed solution, a solution of sodium fluoride is used (the cations of ferrous iron and copper are removed), then ammonia and a few drops of dimethylglyoxime are added. In the presence of a nickel cation, a saturated red precipitate appears.
To identify the trivalent chromium cation, a few drops of a solution of acetic acid and sodium acetate, as well as an excess of complexon 3 (EDTA) are added to the solution. Then the whole mixture is heated in a water bath. The appearance of a violet color indicates the presence of a trivalent chromium cation in the analyzed solution.
Determination of anions
To identify sulfate using interaction with barium chloride. To a few drops of a solution in which there is a given anion, add a few drops of barium chloride. Barium sulfate is treated with a solution of nitric or hydrochloric acid, the resulting precipitate does not dissolve.
Carbonate can be detected in solution using a specific reaction with hydrochloric acid. In the presence of carbonate in the solution, carbon dioxide evolution is observed.
Chloride can be found in the analyzed solution using silver nitrate. The appearance of a white curdled sediment is a confirmation of the presence of chloride.