Question
What is the function of NH4Cl in group II analysis?
Solution
The cations of group III (Al3+, Fe3+, Cr3+, Cr3+ are precipitated as their hydroxides by adding NH4OH to their salt solutions containing NH4Cl. This is explained on the basis of common ion effect.
Due to the common ion effect of
ions, the degree of dissociation of NH4OH is suppressed, so the concentration of hydroxide ions decreases and thereby the ionic product of the hydroxides of group III cations decreases. Even with their suppressed concentration of OH– ions, their ionic products exceed the solubility product and hence radicals of group III get precipitated. The solubility products of hydroxides of higher group cations are relatively high and thus these cations are not precipitated in group III.
Due to the common ion effect of
ions, the degree of dissociation of NH4OH is suppressed, so the concentration of hydroxide ions decreases and thereby the ionic product of the hydroxides of group III cations decreases. Even with their suppressed concentration of OH– ions, their ionic products exceed the solubility product and hence radicals of group III get precipitated. The solubility products of hydroxides of higher group cations are relatively high and thus these cations are not precipitated in group III.
