Oxidation with permanganate in acid medium containing fluoride: determination of bisulphite

The factors affecting the success of both visual and potentiometric end-point detection in titration of bisulphite with permanganate in the presence of fluoride are examined. The optimum conditions are 0.02M H(2)SO(4) and 0.24-0.38M NaF. The oxidation product comprises dithionate and sulphate according to the overall reaction MnO(4)(-) + H(+) + 2HF(2)(-) + 3HSO(3)(-) right harpoon over left harpoon MnF(4)(-) + S(2)O(6)(2-) + SO(4)(2-) + 3H(2)O. The reverse titration is also satisfactory, but proceeds quantitatively according to MnO(4)(2-) + 2HF(2)(-) + 2HSO(3)(-) right harpoon over left harpoon MnF(4)(-) + 2SO(4)(2-) + 2H(2)O.