A Note on the Changes in the Nature of Yellow RFS after Wool Dyeing

(County Afialyst’s Departme t, Matlock, Derhyshirc) Yellow RFS changes after wool dyeing to another colour which is not on the permitted Iist. The general procedure recommended by the Association of public Analystsr for the extraction ol acidic colouring matters from lbod is by dyeing on to wool from a boililg solution acidified with dilute acetic acid, followed by removal of the colour from the wool with dilute ammonia solution. The colour so extracted is identilied by paper chromato- graphy and spectrophotometry. Using this method, one of the colours extracted from a sample of custard powder did not a$ee in chromatographic and spectrophotometric characteristics with any of the permitted colouls. The colour extracted was an azo dye. The difference in the absorption curve between acid and alkaline solutions resembled that given by Yellow RFS, and since Cooke, has previously shown a change in the Rr values ofYellou RFS after wool dyeing from a dilute hydrochloric acid solution, it was suspected that Yellow RFS was present in the original sample.