Liquid chromatographic determination of Lake Red C amine and 2-naphthol in D&C Red No. 8

A method is described for the determination of the intermediates in D&C Red No. 8 by reverse-phase liquid chromatography (LC). The pigment is dissolved in boiling 95% ethanol-water (1 + 1) and then precipitated. The filtrate is chromatographed by gradient elution. Calibrations from peak areas at 254 nm for Lake Red C Amine sodium salt (LRCA-Na) and at 229 nm for 2-naphthol were linear, with prediction limits of 0.200 +/- 0.006% and 0.200 +/- 0.003%, respectively, for the maximum permitted levels. Calibration limits of determination were 0.01% for LRCA-Na and 0.006% for 2-naphthol. A 99% confidence level was used. Recoveries were 100.0-100.4% for LRCA-Na and 97.1-101.8% for 2-naphthol, each added at levels of 0.025-0.2%. Certified lots of D&C Red No. 8 that were analyzed by the LC method contained higher levels of LRCA-Na but the same levels of 2-naphthol when compared to results obtained previously by a cellulose column method, in which the pigment is not dissolved. The solubilities of D&C Red No. 8 in hot and room temperature solutions of 95% ethanol-water (1 + 1), water, and 95% ethanol were estimated.