Kinetic Photometric Determination of Ascorbic Acid by Its Reductive Decoloration of Toluidine Blue

In an acidic medium of 0.9 mol·L-1 HCl,toluidine blue (TB) (λmax=585 nm) was reduced by ascorbic acid leading to decoloration of the thiazine dye. Speed of the decoloration reaction was directly proportional to the concentration of ascorbic acid. Linear relationship beteen the magnitude of decrease in absorbance (ΔA) due to redective decoloration and concentration of ascorbic acid in its reductive species was obtained under fixed time intervals as shown by the linear regression equation ΔA=0.010 C-0.009 (r=0.999) where C is the concentration of ascorbic acid in μg (10 mL)-1,with molar absorptivity of (ε585) 1.8×104L·mol-1·cm-1,and detection limit of 2.0 μg/10 mL. In the application of the proposed method to the determination of ascorbic acid in 6 different samples,the results obtained were in consistency with those obtained by the GB standard method.