Analytical ChemistryHard

Question

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

Options

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

Solution

Function of strong electrolyte NH4Cl is to suppress the ionisation of NH4OH so that the concentration of OH- ions in the solution is decreased but it is sufficient to precipitate the third group basic radicals because the solubility product of group III hydroxides is lower than IV, V and VI group hydroxides. The Cr(OH)3↓ is slightly soluble in excess of precipitant, upon boiling the solution, Cr(OH)3 is precipitated.

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