A Review of Methods of Purifying and Analyzing Water-Soluble Dyes

Published methods of purification and determination of water-soluble dyes are criti cally surveyed, and some new experimental work is described. Very satisfactory puri fication can often be achieved by simple recrystallization from water or water-solvent mixtures, or by extraction with ethanol-water mixture, but where extreme purity is essential the sodium acetate salting-out method, perhaps followed by ion exchange, is sometimes preferable. Ion exchange by itself is often effective, but has disadvantages. The simplest analytical procedure for acid dyes is a recent method of titration with benzidine hydrochloride, but this has some limitations. More widely applicable, though requiring special apparatus, are the classical titanous chloride reduction method and new methods based on oxidation with potassium dichromate in acid solution. The latter are more rapid than the titanous chloride method, and the reagents are stable.