The use of brucine as a redox indicator in dichrometry

Abstract

1. The mechanism of the action of brucine as a redox indicator in the titration of iron(II) with potassium dichromate has been investigated. The speed of oxidation of brucine by potassium dichromate is slow in 1.0 N sulphuric acid but is strongly induced by the reaction between iron(II) and dichromate so that the pink color due to the intermediate oxidation product of brucine appears immediately on adding a trace of iron(II) to a mixture of brucine and potassium dichromate in 1.0 N sulphuric acid at the concentrations encountered in volumetric analysis. Chromium(III) salts strongly retard the oxidation of brucine while iron(III) salts accelerate the reaction. In the presence of a mixture of chromium(III), iron(III) and trace of iron(II) the pink color of the intermediate oxidation product of brucine appears quickly in 3 to 4 N sulphuric acid. For the successful titration of tron(II) with potassium dichromate using brucine as indicator, the overall concentration of sulphuric acid in the mixture must be about 3 to 4 N. At lower acidities, the indicator action is sluggish. 2. The use of brucine has been extended to the dichrometric titration of uranium(IV) and hydroquinone. For these titrations brucine has a distinct advantage over diphenylbenzidine. Uranium(IV) can be directly titrated with potassium dichromate in the cold in 4 N sulphuric acid medium, using 1 ml of 0.1 per cent brucine as indicator in a total Volume of 50–100 ml. Hydroquinone can be titrated with dichromate in the cold at an overall acid concentration of 5 N sulphuric acid and an overall dilution of 150–200 ml so that the color of quinone does not interefere with the detection of the pink color at the equivalence point.