Paper: CSIR-NET June 2019 · Subject: Inorganic Chemistry · Chapter: Analytical Chemistry · Topic: Analytical Chemistry – General · Marks: 2 · Difficulty: Medium
Under review — part of this question (a structure or an option) is not clear in the source scan. 4AB is checking it against the original book; the answer shown may change.
The role of $\mathrm{H_3PO_4}$ in the estimation of Fe (II) with $\mathrm{K_2Cr_2O_7}$ using diphenylamine sulphonate as indicator is to
(a)avoid aerial oxidation of Fe (II)
(b)reduce the electrode potential of $\mathrm{Fe}^{3+}-\mathrm{Fe}^{3+}$
(c)stabilize the indicator
(d)stabilize $\mathrm{K_2Cr_2O_7}$
Answer
Under review — not yet confirmed.
Answer: B
Explanation
H₃PO₄ complexes Fe³⁺ as colourless phosphate complexes, lowering the Fe³⁺/Fe²⁺ potential so the indicator changes colour sharply at the true end point (b).