Question

In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to : 

(1) prevent interference by phosphate ions 

(2) decrease concentration of \(^-OH\) ions 

(3) increase concentration of \(Cl^-\) ions 

(4) increase concentration of \({NH _4} ^+\) ions

Answer 1

Correct option is (2) decrease concentration of \(^-OH\) ions 

\(NH_4 OH \rightleftharpoons NH^+ _4 + OH ^-\)

\(NH_4 Cl \rightarrow NH ^+ _4 + Cl\)

Due to common ion effect of \(NH ^+ _4 ,\)

\([OH ^-]\) decreases in such extent that only group-III cation can be precipitated, due to their very low \(K_{sp}\) in the range of \(10 ^{-38}\).