Quantitative Determination of Dyes in Teatile Fibers

Cationic dyes in polyester fiber are determined spectro-photometrically after extracting the dye from the fiber with N,N-dimethyl formamide (DMF) or N,N-dimethylacetamide (DMAC) containing a strong acid, a salt of a strong acid, and a free-radical inhibitor. The extraction involves ion exchange between the dye cation at the negatively charged dye-site and a proton or a metal ion in the solvent. The cationic dye is extracted rapidly and quantitatively from polyester fiber at 140°C with DMF containing 0.4M (54.5 g) ZnCl 2 , 0.8% (by vol) conc hydrochloric acid, and 20 g/l. 2,6-di- t- butyl-4-methyl-phenol (DBMP). The precision and accuracy of the method are very good. Satisfactory results can be obtained also with DMF containing LiCl, a sulfonic acid, and DBMP, although extracted dyes are less stable in this extractant than in DMF containing ZnCl 2 , HCl, and DBMP. The ZnCl 3 /HCl reagent is useful also for the determination of cationic dyes in acrylic fiber by the dissolution method. The reagent can be used to extract mono-cationic dyes from polyamide fiber. Most bis-cationic dyes are extracted in completely from polyamide fiber, although their extraction from polyester fiber is quantitative.