16.15: Qualitative Analysis
For solutions containing mixtures of different cations, the identity of each cation can be determined by qualitative analysis. This technique involves a series of selective precipitations with different chemical reagents, each reaction producing a characteristic precipitate for a specific group of cations. Metal ions within a group are further separated by varying the pH, heating the mixture to redissolve a precipitate, or adding other reagents to form complex ions.
For instance, group IV cations, which consist of insoluble carbonates and phosphatases such as Ba2+, Ca2+, and Mg2+, all form white precipitates in the presence of diammonium hydrogen phosphate ((NH4)2HPO4) in a basic solution. The precipitates are dissolved in dilute acetic acid. To identify each cation, a confirmatory test is performed.
All three cations form bright yellow chromate salts upon the addition of potassium chromate (K2CrO4); however, only barium chromate (BaCrO4) is insoluble in acetic acid. The solution can be filtered, and the filtrate contains Ca2+ and Mg2+.
The filtrate can now be divided into two parts to test for the remaining cations. If the solution forms a white precipitate in the presence of ammonium oxalate ((NH4)2C2O4) solution, Ca2+ ions can be confirmed. The white precipitate is that of calcium oxalate, which is insoluble in both water and acetic acid.
Mg2+ is identified by a charcoal cavity test. In this test, metallic carbonates are decomposed into the corresponding metal oxide in a charcoal cavity. The color of the residue indicates the possible cation. Magnesium oxide (MgO) leaves a white residue in the charcoal cavity. This residue is treated with a few drops of Cobalt nitrate (Co(NO3)2) solution. With heat, cobalt nitrate decomposes into cobalt (II) oxide, which forms a pink amalgam (CoO-MgO), confirming the presence of Mg2+.