Indirect spectrophotometric determination of ascorbic acid in pharmaceutical samples and fruit juices

An indirect, sensitive spectrophotometric method for the assay of ascorbic acid is described. The procedure is based on the oxidation of ascorbic acid by a known excess of iodate in the presence of acid. The unreacted iodate is utilized for hydroxylamine oxidation to generate nitrite. Sulfanilic acid is diazotized by the nitrite formed and the resulting diazonium ion is coupled with N-(1-naphthyl)ethylenediamine dihydrochloride to form an azo dye which shows an absorption maximum at 540 nm. The absorbance is found to decrease linearly with increasing concentration of ascorbic acid, which is corroborated by the calculated correlation coefficient value of -0.9994. The system obeys Beer's law for 0-40 ug of ascorbic acid in an over-all aqueous volume of 10 ml. The molar absorptivity and RSD were calculated to be 2.1 × 104 l mol-1 cm-1 and 2.5% (n = 10), respectively. The proposed method was applied successfully to the determination of ascorbic acid in pharmaceutical preparations and fresh fruit juices. The reliability of the assay was established by parallel determination by the standard iodimetric method and recovery studies.