Question

To avoid precipitation of hydroxides of Ni²⁺, Co²⁺ and Mn²⁺ along with those of the third group cations, the solutions should be:

A. heated with few drops of conc. HNO₃
B. heated with excess of ammonium chloride
C. concentrated to small volume
D. none of these

Answer 1

To avoid unwanted precipitation of hydroxides for Ni²⁺, Co²⁺, and Mn²⁺, maintain a slightly acidic to neutral pH before adding sulfide ions to precipitate desired cations, and control the hydroxide ion concentration to avoid complex ion formation.

Step-by-step explanation:

To avoid precipitation of hydroxides of Ni²⁺, Co²⁺, and Mn²⁺ along with those of the third group cations, the solution should be maintained in such a way to prevent the formation of insoluble substances. Typically, this is done by controlling the pH and avoiding excess hydroxide ions, as too high a concentration can lead to complex ion formation, which may hinder the separation process. Also, it is crucial to avoid conditions that would precipitate these cations as hydroxides or as base-insoluble sulfides when adding sulfide ions to precipitate group 3 cations like Fe³⁺.

Therefore, to avoid the unwanted precipitation, the solution should be maintained at a slightly acidic to neutral pH before the addition of sulfide ions to ensure that only the desired cations precipitate. Furthermore, the subsequent steps often involve controlling the concentration of precipitating agents and selectively redissolving some cations by altering the solution's pH or through complexation, which can provide an effective separation.