In salt analysis cations of group I and II precipitated in acidic medium and therefore the solution remains acidic till group II. But from III group onwards cations precipitates in basic medium and hence filtrate of group II is to be boiled with con `HNO_(3)` before using it for III group.
Conc. `HNO_(3)` is added for proceeding to test III group radicals, because

Options

A. to form nitrates which gives granular precipitates.
B. oxidize any remaining `H_(2)S`.
C. increase ionization of `NH_(4)OH`
D. convert `Fe^(2+)` ion to `Fe^(3+)` ions.

Answer

D

Explanation

In presence of `H_(2)` gas `Fe^(3+)` reduced to `Fe^(2+)` , and therefore to convert it again to `Fe^(3+),HNO_(3)` is added.