Spectrophotometric determination of vitamin A in oily capsules using first derivative curves

The first derivative curve (D1) of the absorption spectrum of vitamin A acetate in cyclohexane possesses a trough (D11) at 348 nm and a maximum (D12) at 306 nm. D11, D12, and delta D1 (= D11 - D12) are linearly related to concentration over a range of 5-25 i.u. ml-1. When a tangent is drawn between D1 at 400 nm and D1, at 306 nm, the amplitude at 348 nm (D1 corr.) is linearly related to concentration. Ratios of magnitude of D11/magnitude of D12 and D1(corr.)/magnitude of D1 are independent of concentration, and have been used to reveal the presence of interferences in the D1 curves of vitamin A in oily capsules. Vitamin A in two oily preparations has been assayed using delta D1 and D1(corr.). The potency found was within +/- 2% from that obtained by using the B.P. method. The method is rapid, precise and accurate.