Titrimetric determination of indigo sulphonate with potassium iodate

Abstract

Conditions have been established for the direct titration of indigo sulphonate with potassium iodate at room temperature. The titration can be carried out in a medium which is 6–8 N with respect to hydrochloric acid. The blue colour of the indigo disappears sharply at the end-point, giving a clear yellow solution. It has been observed that 1 g mole of indigo consumes very nearly 1 g mole of potassium iodate. The reaction of indigo sulphonate with potassium iodate is very slow, but is markedly accelerated by chloride ion. Direct titration of indigo sulphonate has also been found possible in mixed acid media containing sulphuric and hydrochloric acids at suitable concentrations. The stoichiometry observed in these titrations is in striking contrast with the reported deviations in the titrations of indigo sulphonate with potassium permanganate and cerium IV sulphate. Evidence has also been obtained for the formation of a new type of iodo derivative of isatin sulphonate in the reaction between indigo sulphonate and potassium iodate. The new iodo-isatin sulphonate is capable of liberating iodine from potassium iodide quantitatively.