Victoria Blue BX as Internal Indicator in Ceriometry

Abstract

Several internal oxidation-reduction indicators have been proposed for titrations with standard solutions of potassium bichromate and ceric sulfate (1, 2). Some of those which are applicable with potassium bichromate cannot be used with ceric sulfate solutions because of the high oxidation-reduction potential of the latter. Hence it was thought desirable to continue the search for other indicators of high oxidation-reduction potential. Victoria blue BX,* a diphenylnaphthylmethane derivative manufactured by the Du Pont Company, has a more pronounced color change than any indicator we have studied, going from sky blue to a rather bright pink, which fades at once to a light pink. Although it is not a reversible indicator, the color change at the end point is exceptionally rapid and sharp. Overstepped end points can be redetermined by adding standard ferrous solution and a few more drops of indicator solution. Like ortho-phenanthroline, Victoria blue BX does not give a satisfactory color change when the stannous chloride method of reduction has been used. It serves admirably, however, when the iron has been reduced with cadmium, aluminum or zinc. An end point correction of 0.02 ml. should be made for accurate analysis, although for an accuracy of 0.20 per cent this correction is unnecessary when the usual volume of standard solution is used. The correction does not vary with volume within large limits. The end point may be approached at any reasonable rate. Nearness to the end point is indicated first by a slight fading of the blue solution to a clear sky-blue, and a few tenths of a milliliter before the end point is reached, a red spot can be seen for an instant as each drop of standard solution is added. At the end point as little as 0.02 ml. of 0.1 molar cerie sulfate solution will easily produce the entire color change. As a standard oxidizing solution, ceric sulfate has several advantages over potassium permanganate and potassium bichromate, but some disadvantages also. The first cost is considerably greater. Our standard solutions of ceric salts have not become constant in less than five or six weeks. During this period a precipitate slowly settles out. A solution made up with conductivity water in place of ordinary distilled water showed the same tendency. Cooling does not seem to hasten the separation.