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Question
You are provided with PbCO3, ZnCO3 and CaCO3. How will you identify these cations?
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Solution
By use of Ammonium hydroxide we can identify the ions PbCO3, ZnCO3 and CaCO3 as :
(i) \[\ce{PbCO3 + 2NH4OH -> Pb(OH)2 v + 2NH4NO3}\]
Pb(OH)2 forms white precipitate which are insoluble in excess ammonium hydroxide.
(ii) \[\ce{ZnCO3 + NH4OH -> Zn(OH)2 v + 2Na2CO3}\]
Zn(OH)2 forms white gelatinous precipitate which are soluble in excess ammonium hydroxide.
(iii) \[\ce{CaCO3 + NH4OH -> Ca(OH)2 + 2Na2CO3}\]
No precipitation of Ca(OH)2 occurs even with addition of excess of NH4OH. Because the concentration of hydroxide ion from ammonium hydroxide is so low that it cannot precipitate the hydroxide of calcium.
Concept Insight : Some precipitated metallic hydroxides by ammonium hydroxide become soluble hydroxides when treated with excess of ammonium hydroxide due to the formation of a soluble complex salt in the presence of excess of ammonium hydroxide.
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