Question

Which of the following buffer solutions is used for precipitation of cations of the III A group?

• A. \(\text{HF}+\text{NaF}\) in water.
• B. \(\text{HCOOH}+\text{HCOONa}\) in water
• C. \(\text{NH}_{4}\text{OH}+\text{NH}_{4}\text{Cl}\) in water.
• D. \(\text{CH}_{3}\text{COOH}+\text{CH}_{3}\text{COONa}\) in water.

Hint:

Group III A cations are precipitated as hydroxides, which needs a basic buffer with a limited hydroxide ion concentration.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
Group III A cations (\(\text{Fe}^{3+}\), \(\text{Al}^{3+}\), \(\text{Cr}^{3+}\)) are separated as hydroxides, which have very low solubility products.

Step 2: Which buffer is used
They are precipitated with \(\text{NH}_{4}\text{OH}\) in the presence of \(\text{NH}_{4}\text{Cl}\). The common ion \(\text{NH}_{4}^{+}\) suppresses ionisation of \(\text{NH}_{4}\text{OH}\), keeping \([\text{OH}^{-}]\) low.

Step 3: Result
This low \([\text{OH}^{-}]\) precipitates group III A hydroxides but not later-group hydroxides such as \(\text{Mg(OH)}_{2}\).

Final Answer:
The ammonium hydroxide and ammonium chloride buffer is used. \[ \boxed{\text{(C) }\text{NH}_{4}\text{OH}+\text{NH}_{4}\text{Cl}\text{ in water}} \]