Predicting the precipitation of acid and direct dyes in natural waters

Abstract

A simple screening test was used to determine whether acid and direct dyes precipitate at calcium concentrations typical of hard waters in the Southeastern Piedmont region of the United States. Of 52 dyes tested, only three direct dyes (Direct Black 19, Direct Black 22, and Direct Blue 75) and seven acid dyes (Acid Red 88, Acid Red 114, Acid Red 151, Acid Brown 14, Acid Black 24, Acid Orange 8, and Acid Blue 113) precipitated. Structures of all hut three of the dyes, or ligands in the case of complexes, are in the literature. Conditional solubility-product constants (K sp) of calcium salts were also determined by titrimetric and batch-equilibration methods for five of the dyes (Acid Red 114, Acid Red 151, Acid Orange 8, Acid Blue 113, and Direct Yellow 28). The K sp values range from 3 × 10 −10 to 1·0 × 10 −7 M 2 and from 6× 10 −15 to 1 × 10 −9 M 3 for salts of divalent and monovalent dyes, respectively. Monitoring data show that maximum calcium concentrations in Southeastern Piedmont waters are approximately 2 × 10 −3 M and thus unlikely to precipitate most dyes of the types represented. However, it is not known whether these conclusions are valid for newer dyes that have structures not reported in the open literature.