REDUCTION OF DICHROMATE BY HYDROBROMIC AND NITROUS ACIDS IN PRESENCE OF MANGANOUS SULPHATE

Hydrohromic and nitrous acids can be titrated pateutiometrically with potassium dichromate in presence of strong sulphuric acid (8-12N) and excess nianganous sulphate at room temperature (20-22°) Hydrochloric acid is not suitable. Steady potentials are easily attained. Under the ex­perimental conditions the manganoue ion is assumed to be first oxidised to manganic ion in spite of the standard oxidation potential of the corresponding electrode being less than that of the Cr2+—Cr2O72- electrod, The manganic ion can then oxidise bromide or nitrite ions rapidly. When the sulphuric acid concentration is lower or higher, the figures obtained are less than the correct ones, namely three moles of nitrous acid and six of bromide ions per mole of potassium dichromate