Qualitative Analysis of Cobalt (II) Ion

To the original solution of the salt and add NH₄Cl and NH₄OH. Now add a pinch of KNO₂ acidify with dil CH₃COOH. Yellow precipitate indicates the presence of CO²⁺ ion.

CH₃COOH + KNO₂ → CH₃COOH + HNO₂

Co(NO₂)₂ + 2HNO₂ → Co(NO₂) + H₂O + NO

Co(NO₂)₃ + 3KNO₂ → K₃[Co(NO₂)₆]

On passing H₂S gas, a solution with black precipitate is formed. Dissolve this precipitate in aqua regia (HCl : HNO₃) 3 : 1. Neutralize by adding aqueous NH₃. Now add aqueous NH₄SCN. Appearance of blue colour indicates the presence of Co²⁺ ion.