Determination of L-Ascorbic Acid in Fruits, Vegetables and Beverages by Phosphorus Bismuth Molybdenum Blue Spectrophotometry

In sulphuric acid medium, Bi³⁺, Mo(Ⅵ) and PO₄^3- can react to produce yellow phosphorus bismuth molybdenum heteropoly acid, which is then reduced to phosphorus bismuth molybdenum blue by L-ascorbic acid. Based on this reaction, a spectrophotometric method for determination of L-ascorbic acid in fruits, vegetables and beverages was established. The optimized conditions found were as follows:① wavelength of determination was 710 nm; ② amount of 1.95×10^-2 mol·L^-1 KH₂PO₄ solution was 3.0 mL; ③ amount of 6.51×10^-2 mol·L^-1 ammonium molybdate solution was 4.0 mL; ④ amount of 2.06×10^-4 mol·L^-1 bismuth nitrate solution was 1 mL; ⑤ concentration of H₂SO₄ was 0.16 mol·L^-1; ⑥ temperature of reaction was room temperature. Linear relationship between values of absorbance and mass concentration of L-ascorbic acid was obtained in the range of 4-120 mg·L^-1, with apparent molar absorptivity of 3.59×10³L·mol^-1·cm^-1 and detection limit (3s/k) of 0.2 mg·L^-1. The proposed method was applied to the analysis of samples of fruits, vegetables and beverages, giving values of recovery and RSD's (n=11) in the ranges of 98.0%-102% and 1.1%-2.3% respectively.