Why sometimes, a black coloured precipitate obtained in group IV even if nickel and cobalt ions are absent?

Explanation

### Step-by-Step Text Solution:

1. **Understanding the Context**: In qualitative inorganic analysis, different groups of cations are separated based on their solubility and precipitation reactions. Group IV typically includes cations like nickel (Ni²⁺) and cobalt (Co²⁺), which can form colored precipitates.

2. **Identifying the Black Precipitate**: The question asks why a black precipitate is sometimes observed in Group IV even in the absence of nickel and cobalt ions. The black precipitate in question is iron sulfide (FeS).

3. **Role of Iron**: Iron can exist in two oxidation states: Fe²⁺ (ferrous) and Fe³⁺ (ferric). In Group III analysis, iron is usually precipitated as ferric hydroxide (Fe(OH)₃) when ammonium hydroxide is added. However, if there is insufficient ammonium hydroxide, Fe³⁺ may not fully precipitate.

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