Q1(i). Role of H₃PO₄ in the dichromate titration of Fe²⁺

Answer in brief

H₃PO₄ complexes Fe³⁺ and lowers the Fe³⁺/Fe²⁺ potential, so diphenylamine (0.76 V) changes colour at 99.7 % titration, inside the steep jump; it also removes the yellow colour of Fe³⁺.

Key points

• Diphenylamine is a redox indicator: colourless → violet at EIn ≈ 0.76 V.
• Without H₃PO₄: E°′ ≈ 0.68 V (1 M H₂SO₄); E = 0.76 V is reached at 95.7 % titration: a premature end point.
• H₃PO₄ forms stable, colourless phosphato complexes of Fe³⁺; the uncomplexed [Fe³⁺] falls, so E°′ falls to about 0.6 V or lower.
• With 0.61 V, E = 0.76 V needs 99.7 % titration: the colour change falls inside the potential jump.
• Second role: the yellow Fe³⁺ (chloro complexes) is decolourised, so the violet end point is seen clearly.