While much may be said in favor of the systematic analysis for acid ions, based on the preparation of a solution containing only the alkali salts of the anions, and while one should be familiar with the plan and be able to have recourse to it at will, yet the drawbacks mentioned suggest another basis for the analysis.
Systematic Analysis for Acid Ions in Acid Solution.
Desirability of Experience with Both Methods.
The Groups of Acid Ions.
Since the grouping for both methods of analysis may be made the same, the following grouping of acid ions has been adopted. Only the group characteristics are given; the members of the groups are described in detail in the Laboratory Manual (Part III).
I. Ions of Amphoteric Acids and of Related Acids. This group includes those acids whose amphoteric character, or whose ready reduction by hydrogen or ammonium sulphide, leads to their being found, or indicated, in the systematic analysis for metal ions.
II. The Carbonate Group. This group includes those acids whose physical properties (insolubility), or the physical properties of decomposition products of which (carbon dioxide is a decomposition product of carbonic acid), usually lead to their discovery in the course of the preparation of solutions or of the analysis for cations.
III. The Sulphate Group. The barium salts of this group of acid ions are insoluble in acid solution. Barium nitrate, added to a solution acidified with nitric acid, is the group reagent.
IV. The Chloride Group. The anions of this group form silver salts, which are insoluble in nitric acid. Silver nitrate, added to a solution acidified with nitric acid, is the group reagent.
V. The Phosphate Group. A test for this group, as a whole, can be made only if cations other than the alkali metal ions are absent: the barium salts of the acid ions of the group are insoluble in neutral, but soluble in strongly acid, solutions. Barium nitrate, used with a neutral solution, is the group reagent in the absence of metal ions other than the alkali metal ions. In the presence of other cations, the three members of the group, which are not found in some other group,[595] namely: phosphate, borate and fluoride ions, are tested for specifically, and the group test omitted. Phosphate-ion is tested for in nitric acid solution, in the same solution as is used for the tests for groups III and IV. [p303]