Question

You are given a mixture of ions which may contain any four of the following ions :Fe3+, Cu2+, Zn2+, Mg2+, Sr2+, and Ni2+. Based on your knowledge of qualitative analysis of ions, solubility and complex formation , devise a method to separate and identify the components of the unknown mixture.

Answer 1

In order to identify and separate the cations we perform a qualitative test as follows,

Since the above radicals have different solubility products and get precipitated by different reagents in their solutions.

On reactions with ammonium hydroxide(NH₄OH) and NH₄Cl

Fe3+ ions precipitate first as Fe(OH)₃ .This is because NH4Cl decrease ionisation degree of NH4OH and furnish low [OH^-] and as group (iii) cations Fe³⁺ precipitates.

On reaction with H2S in ammoniacal solution Zn2+ and Ni2+ precipitates as ZnS and NiS. Reaction is feasible as basic medium increases ionisation of H2S increasing [S^2-]. which accounts for high value of Ksp.

In case of Cu²⁺ we use H2S gaseous form in prescence of dilute HCl.

Thus CuS is precipated as Ksp is high and HCl deceases ionisation of

H2S which furnishes low [S2-] due to common ion effect of H+.

Similarly, for Mg2+ and Sr2+ which belong to group (vi) and (v)

reaction with NH4OH + Na2HPO4 (in case of Mg2+) and (NH4)2CO3+NH4Cl yield MgHPO₄ and SrCO3 respectively.

Thus , we may conclude performing the above step we can identify the desired four ions from the mixture whichever they are.