A decantate contains the cations Zn2+, Mg2+ and Ni2+. The solution is treated wi
ID: 815192 • Letter: A
Question
A decantate contains the cations Zn2+, Mg2+ and Ni2+. The solution is treated with sodium hydroxide, which causes the precipitation of magnesium hydroxide and nicket hydroxide. After filtration, the solid is then added to aqueous ammonia. Nickel forms a soluble complex in aqueous ammonia and this results in the separation of magnesium hydroxide. Magnesium hydroxide is dissolved in the presence of hydrochloric acid and ad attempt is made to re-precipitate the solid using sodium hydroxide in the presence of magnesium reagent but there is no formation of a blue lake. Explain.
Explanation / Answer
Magnesium Hydroxide when precipitated in presence of magnesium reagent [4-(p-nitrophenylazo)resorcinol] the characteristic "Blue Lake" is formed.
Bu as ammonia was used in the previous step for solubilizing Ni2+ ion the solution contains ammonia. While acidifying the solution ammonium chloride is formed. The presence of ammonium chloride in the solution interferes with the blue lake formation.