Inorganic Qualitative Analysis

Qualitative analysis of inorganic salts means the identification of cations and anions present in the salt or a mixture of salts. Inorganic salts may be obtained by complete or partial neutralisation of acid with base or vice-versa. Inorganic qualitative wet chemical analysis includes the analysis of cations and anions found in the sample under examination. The classical procedure for a full systematic analysis of an inorganic sample consists of several parts. First, a preliminary dry test can be performed, which involves heating the sample to detect the presence of components such as carbon or water, or introducing the sample into a flame and noting the resulting color. Certain elements can be identified by their characteristic flame color. After the preliminary tests have been performed, the sample is usually dissolved in water for subsequent determination of the anionic constituents and the cationic constituents. The procedure followed is based on the principle of treating the solution with successive reagents so that each reagent separates a group of components. When a constituent has been separated, it is further examined to confirm its presence and determine its quantity (quantitative analysis). Parts of the material are dissolved separately and different procedures are used for each material to detect cationic and anionic constituent.

Figure 1. Steps in the qualitative analysis of an unknown mixture of cations in an aqueous solution.

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In order to perform a complete analysis, BOC Sciences already has a good knowledge of all the reactions in which an anion or a cation may be involved (theory). In addition, our chemists have acquired good manual skills (practice) to avoid making mistakes. We have designed different methods depending on the different chemical/physical properties of the analytes to be identified. Qualitative analysis is carried out through the reactions which are easily perceptible to our senses such as sight and smell. Such reactions involve:

• Formation of a precipitate
• Change in colour
• Evolution of gas

At BOC Sciences, a systematic analysis of an inorganic salt involves the following steps:

• Preliminary examination of solid salt and its solution

Preliminary examination of a salt often furnishes important information, which simplifies further course of analysis. They give quite important clues for the presence of certain anions or cations. These tests involve noting the general appearance and physical properties, such as colour, smell, solubility etc. of the salt. Solubility of a salt in water and the pH of aqueous solutions give important information about the nature of ions present in the salt. If a solution of the salt is acidic or basic in nature, this means that it is being hydrolysed in water.

• Determination of anions by reactions carried out in solution (wet tests)
• Determination of cations by reactions carried out in solution (wet tests)

Analysis of cations

The common cations to be analyzed are: Ag⁺ , Pb²⁺ , Hg²⁺ , Al³⁺ , Fe³⁺ , Cr³⁺ , Mn²⁺. These ions are not separated individually, but are grouped into groups, called groups or selective reactants, according to their behavior towards specific reactants, and they cause the same reaction in all elements belonging to the group. A systematic study of cations using wet analysis requires that the substance be first dissolved in a suitable solvent. Prior to this, we need to check for the presence of certain substances, such as complex cyanides, silicates and silica, which interfere with the analysis and can be removed if necessary. If the initial sample is a solid, it is also necessary to convert it into a solution, so it needs to be done gradually: cold water, hot water, acidified water, aqua regia and sulfur nitrate.

Figure 2. Steps in a Typical Qualitative Analysis Scheme for a Solution That Contains Several Metal Ions. (ELDREDGE, Patricia A. 2011)

• Solubility test of dissolved samples
• Addition of group reactivity: hydrochloric acid (group VI)
• Once the sample is turned into a solution, the systematic analysis can begin
• Digestion and filtration of precipitates

Analysis of anions

In general, the analysis of anions begins with an alkaline Na₂CO₃ solution, which is used to eliminate any color interference from the presence of iron, manganese or nickel. This solution is then analyzed for the presence of reducing and oxidizing anions.

• Search for reducing ions
• Search for oxidizing anions