Q1(g). Volumetric manganese in steel

Answer in brief

Oxidise dissolved manganese quantitatively to permanganate and titrate it with a standard reducing reagent such as arsenite.

Key points

• Dissolve a weighed sample under suitable oxidising acidic conditions and remove or control interfering species.
• Use ammonium persulfate with an appropriate silver-ion catalyst to oxidise Mn(II) to Mn(VII). Destroy excess oxidant by the prescribed treatment before titration.
• Titrate the cooled permanganate with standard arsenite to discharge the permanganate colour.
• 2MnO₄⁻ + 5H₃AsO₃ + 6H⁺ → 2Mn²⁺ + 5H₃AsO₄ + 3H₂O. Thus n_(Mn)=(2/5)n_(As(III)).
• Convert manganese moles to mass and divide by steel sample mass.