Reversible indicators for use in potassium bromate titrations

A review has been given of reversible indicators for use in bromate titrations. An examination has been made of four reversible indicators, α-naphthoflavone, p-ethoxy-chrysoidine. fuchsin and apomorphine.' All four indicators are satisfactory, but α-naphtho-flavone gives the sharpest end-points. Fuchsin and p-cthoxychrysoidine are gradually destroyed when the end-point is passed and re-passed several times. Apomorphine is only suitable for the titration of trivalent antimony and the end-point is the least sharp of all four indicators.Only α-naphthoflavone is suitable for the titration of 8-hydroxyquinoline solutions. Although it is possible to titrate directly and even back-titrate with standard 8-hydroxyquinoline, the method is not suitable because of the slow reaction between bromine and 8-hydroxyquinoline. The titration can be carried out satisfactorily, however, by adding excess bromate, then excess arsenite and re-titrating with bromate. The other two indicators are not suitable for this purpose; in the direct titration they tend to be destroyed by prolonged contact wth the excess bromine which is always present and, in any case, their colours are obscured by that of the 8-hydroxyquinoline so that the back-titration method is unsatisfactory.Aluminium can be determined satisfactorily after precipitations as the 8-hydroxyquinolate by titration with bromate and arsenite using α-naphthoflavone as indicator.