Question:
Which from the following buffers is used for precipitation of cations of IIIA group in qualitative analysis?
Which from the following buffers is used for precipitation of cations of IIIA group in qualitative analysis?
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Group IIIA rule: Hydroxide precipitation → use ammonium buffer.
Updated On: May 4, 2026
- \(\text{NH}_4\text{OH}\) and \(\text{NH}_4\text{Cl}\)
- \(\text{CH}_3\text{COOH}\) and \(\text{CH}_3\text{COONa}\)
- \(\text{C}_6\text{H}_5\text{COOH}\) and \(\text{C}_6\text{H}_5\text{COONa}\)
- \(\text{HCOOH}\) and \(\text{HCOONa}\)
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The Correct Option is A
Solution and Explanation
Concept:
In qualitative inorganic analysis, specific buffer solutions are used to control pH for selective precipitation of cations.
Step 1: Understand IIIA group. Group IIIA cations (Fe$^{3+}$, Al$^{3+}$, Cr$^{3+}$):
• Precipitate as hydroxides
• Require controlled basic medium
Step 2: Role of buffer. \[ \text{NH}_4\text{OH} + \text{NH}_4\text{Cl} \]
• Maintains pH around 8–9
• Prevents precipitation of other groups
Step 3: Analyze options.
• (A) NH$_4$OH + NH$_4$Cl → Correct buffer \checkmark
• (B), (C), (D) → Acidic buffers, not suitable $\times$
Step 4: Conclusion. \[ \text{Correct buffer = NH}_4\text{OH + NH}_4\text{Cl} \]
In qualitative inorganic analysis, specific buffer solutions are used to control pH for selective precipitation of cations.
Step 1: Understand IIIA group. Group IIIA cations (Fe$^{3+}$, Al$^{3+}$, Cr$^{3+}$):
• Precipitate as hydroxides
• Require controlled basic medium
Step 2: Role of buffer. \[ \text{NH}_4\text{OH} + \text{NH}_4\text{Cl} \]
• Maintains pH around 8–9
• Prevents precipitation of other groups
Step 3: Analyze options.
• (A) NH$_4$OH + NH$_4$Cl → Correct buffer \checkmark
• (B), (C), (D) → Acidic buffers, not suitable $\times$
Step 4: Conclusion. \[ \text{Correct buffer = NH}_4\text{OH + NH}_4\text{Cl} \]
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