Determination of Ascorbic Acid in Vitamin C Tablets and Fruit Juice Drinks by Fluorescence Quenching Method

Rhodamine B (RB) solution when excited at (λ_ex) 365 nm shows fluorescence emission at its maximum at (λ_em) 580 nm. And the fluorescence will be quenched or weakened by addition of iodine (I₃^-) to the solution due to association of RB with I₃^-, but site of the fluorescence peak is not changed. However, if a definite amount of ascorbic acid (AA) is added before the quenching reaction, due to reduction of I₃^- by AA, the quenching effect will be ceased to a definite extent or completely eliminated as related to the amount of AA added, and re-appearance of the fluorescene emission will happen. Optimum conditions for the fluorescence quenching reaction were found as follows:in a total volume of 50 mL, an amount of AA was added to the solution before addition of 2.0 mL of 1×10^-3mol·L^-1 iodine solution, 5.00 mL of 1.0×10^-4mmol·L^-1 RB solution and 5.0 mL of pH 4.7 acetic acid-sodium acetate buffer solution. The solution was kept under normal temperature for 10 min. Linear relationship between values of fluorescence intensity and concentration of AA was obtained in the range within 40 μmol·L^-1. Detection limit (3s/k) found was 5.6×10^-9mol·L^-1. Based on these facts, it was proposed to use this indirect fluorescence quenching method to determine AA in Vitamin C tablets and injections. For the analysis, 5 tablets were taken, powdered and mixed thoroughly by grinding, and an aliquot of 25.0 mg was taken and dissolved in water, and its volume was made up to 250.0 mL with water. 1.00 mL of the solution was taken and its content of AA was determined by the method described above. For injection sample, 0.10 mL of the drug was taken and diluted to 250.0 mL with water. 1.00 mL of the solution was taken and analyzed in the same way. Test for recovery was made by addition of standard solutions of AA to substantial samples of VC tablets and injections, results of recovery found were 97.4% (for tablets) and 98.4% (for injections). Values of RSDs (n=6) found were 2.3% and 1.7% respectively. The proposed method was also used to determined AA in 3 fruit juice drinks, and the results obtained were found to check quite well with the results found by iodimetry.