Determination of the dissociation constants of sulfonated azo dyes by capillary zone electrophoresis and spectrophotometry methods

The dissociation constants of 10 sulfonated azo dyes, six of the most commo n food colours used as additives (Food Yellow 4, Food Yellow 3, Food Red 9, Food Red 7, Food Red 17 and Food Blue 5), and four commonly used as textil e dyes (Acid Orange 7, Acid Orange 12, Acid Red 26 and Acid Red 88), have b een determined by two different systems, one by using capillary electrophor esis (CE) with diode array detection and the other by using UV-visible abso rption spectrophotometry, which has been used as reference method to obtain the pK(a) values. The pK(a) values obtained by CE were determined in two w ays, first on the basis of the electrophoretic mobilities (calculated from the migration times), and after we propose a new methodology, in which the dissociation constants are determined from the spectra corresponding to the maxima of electrophoretic peaks. The py values obtained by using these CE methods have been compared with those obtained by using the spectrophotomet ric method. The results show that the pK(a) values obtained by the CE propo sed method are in general closer to the reference values than those obtaine d from the electrophoretic mobilities. Moreover, the proposed method retain s the advantages of CE, as the possibility of working with small amounts of sample, despite its purity. (C) 2001 Elsevier Science B.V. All rights rese rved.