Improvement of an old spectrophotometric method for the microdetermination of ascorbic acid by the use of a micellar medium

A long known indirect method for the spectrophotometric determination of the ascorbic acid (ASCA) has been improved. The initial method, as well as the improved one is based on the same coupled redox-complexation reaction between Fe(III) and ASCA, the resulting Fe(II) reacts with o-phenanthroline (o-Phen) to form the well-known orange-red coloured ferroin chelate complex [Fe(II)-(o-Phen)3]2+. Whereas the initial method is carried out in an aqueous solution, the improved one employs an aqueous micellar medium formed by the cationic surfactant cetylpyridinium bromide (CPBR). The λmax in both methods is the same, i.e. 510 nm. The mean apparent molar absorptivity (ε) and Sandell’s sensitivity (Ss) were calculated as ε = 2.10 × 104 Lmol−1 cm−1 and Ss = 8.37 ng cm−2 for the initial procedure, and ε = 2.62 × 104 Lmol−1 cm−1 and Ss = 6.72 ng cm−2 for the improved one. The regression line equation for the improved method was: A = 1.487 × l0−1C −1.415 × 10−2 (r = 0.9998). The accuracy and the precision of the improved method were investigated and the conclusions were satisfactory. The results obtained for ASCA by both the described method and an official one, were statistically compared by means of the Student’s ttest and by the variance ratio Ftest; and no significant difference was observed.