The Solubility Product Principle Can Be Applied to Qualitative Analysis

The increase in solubility of Al(OH)3 in a basic medium is the result of the formation of the complex ion Al(OH)4 in which Al(OH)3 acts as the Lewis acid and OH acts as the Lewis base. Other amphoteric hydroxides behave in a similar manner. 

The Solubility Product Principle Can Be Applied to Qualitative Analysis 

Here we will briefly discuss qualitative analysis, the determination of the types of ions present in a solution. We will focus on the cations. There are some 20 common cations that can be analyzed readily in aqueous solution. These cations can be divided into five groups according to the solubility products of their insoluble salts (Table 12.6). Because an unknown solution may contain from 1 to all 20 ions, any analysis must be carried out systematically from group 1 gqgedure 

Adding acid to the solution shifts this equilibrium to the left so that only the least soluble metal sulfides, that is, those with the smallest Ksp values, will precipitate out of solution. These are 31283, CdS, CuS, HgS, and SnS (see Table 12.6). 

Group 3 Cations. At this stage, sodium hydroxide is added to the solution to make it basic. In a basic solution, the equilibrium between and MS shifts to the right. Therefore, the more soluble sulfides (CoS, FeS, MnS, NiS, and ZnS) now precipitate out of solution. Note that the Al and Cr ions actually precipitate as the hydroxides Al(OH)3 and Cr(OH)3, rather than as the sulfides, because the hydroxides are less soluble. The solution is then filtered to remove the insoluble sulfides and hydroxides. 

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