The use of vanadium-sulphosalicylic acid complex as a redox indicator

In a previous communication it was reported that vanadium^V gives an intense turquoise-blue colour with sulphosalicylic acid in concentrated sulphuric acid and that the colour reaction is specific and very sensitive for the detection of vanadium^V. During our studies on the nature of the complex, it was found that vanadium^IV gives a negative colour test and that the blue colour developed by the reagent with vanadium^V is discharged on the addition of a reducing agent like iron^II, uranium^IV or hydroquinone. It was also observed that the blue colour is restored by the addition cerium^IV solution. This reversible property of the complex has been further investigated to see if the complex can be used as a redox indicator.