Q1(h). Manganese in pyrolusite by redox back-titration

Answer in brief

The ore (impure MnO₂) is reduced with a known excess of standard Fe(II) in dilute H₂SO₄; unused Fe²⁺ is back-titrated with standard KMnO₄, and the Fe²⁺ consumed gives MnO₂ and hence Mn.

Key points

• Weigh about 0.2 g of powdered ore; add a measured excess of standard ferrous ammonium sulfate and dilute H₂SO₄; warm until no black particles remain.
• MnO₂ + 2 Fe²⁺ + 4 H⁺ → Mn²⁺ + 2 Fe³⁺ + 2 H₂O.
• Titrate the unused Fe²⁺ with standard KMnO₄ to the first permanent pale pink (self-indicator); run a blank.
• mmol Fe²⁺ used = (MV)Fe − 5(MV)KMnO₄; mmol MnO₂ = ½ of that; % Mn = mmol MnO₂ × 54.94 × 100 / mg of sample.
• Alternative reductant: standard sodium oxalate at 60–70 °C, excess titrated hot with KMnO₄.