Spectrophotometric Determination of Ascorbic Acid in Pharmaceutical Preparations with Tungstomolybdophosphoric Acid

A simple and sensitive spectrophotometric method is established for the determination of ascorbic acid based on the reaction of ascorbic acid with tungstomolybdophosphorus heterpoly compound.The maximum absorption is at 730nm. Beer's law is followed in the range of 0.08-10 mg L^-1 ascorbic acid. The apparent molar absorptivity is 1.05×10⁴ L·mol^-1·cm^-1,and the linear regression equation is A=1.05×10⁴C+0.017 with a linear correlation coefficient of 0.9995. The detdrion limit of 0.04mg·L^-1,the recovery of 94%-102% and the relative standard deviation of less than 1.5% (n=8) were estimated, respectively. Instead of heating,the color reaction can be completed by standing at room temperature for 30 minutes. The absorption of excess tungstomolybdophosphorus heteropoly yellow can be removed by the addihon of a certain amount of sodium hydroxide solution, thus the sensitivity and selectivity of the method are improved.The proposed method has been applied to the determination of ascorbic acid content in vitamin C tablets.