What is done in iron group (Group III) precipitation?

In qualitative analysis, cations of the iron group such as $\mathrm{Fe^{3+}},\ \mathrm{Al^{3+}},\ \mathrm{Cr^{3+}}$ are precipitated as hydroxides by adding $\mathrm{NH_4OH}$.

$\mathrm{Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3 \downarrow}$

Why is $\mathrm{NH_4Cl}$ added first?

$\mathrm{NH_4Cl}$ is a strong electrolyte and dissociates as:

$\mathrm{NH_4Cl \rightarrow NH_4^+ + Cl^-}$

$\mathrm{NH_4OH}$ is a weak base:

$\mathrm{NH_4OH \rightleftharpoons NH_4^+ + OH^-}$

When $\mathrm{NH_4Cl}$ is added, the concentration of $\mathrm{NH_4^+}$ increases. By the common ion effect, the ionization of $\mathrm{NH_4OH}$ is suppressed, so the concentration of $\mathrm{OH^-}$ decreases.

Why is lower $\mathrm{OH^-}$ concentration important?

A controlled, low concentration of $\mathrm{OH^-}$ ensures that only Group III hydroxides precipitate, while other cations that would precipitate at higher $\mathrm{OH^-}$ remain in solution.

Checking the options

• A: decrease concentration of $\mathrm{OH^-}$ ions — Correct, this is the direct purpose via common ion effect.
• B: increase concentration of $\mathrm{Cl^-}$ ions — Not the purpose of adding $\mathrm{NH_4Cl}$ here.
• C: prevent interference by phosphate ions — This is handled by other reagents/procedures, not the primary reason here.
• D: increase concentration of $\mathrm{NH_4^+}$ ions — This does happen, but it is the means, not the final purpose asked in the question. The intended answer is decrease in $\mathrm{OH^-}$ concentration.

Final answer

$\boxed{\text{A}}$