RP TLC of some newly synthesized azo-dye derivatives

Abstract

The chromatographic behavior of series of azo-dye derivatives has been studied by RP C-18 TLC with water-methanol, water-acetone, water-dioxane, and water-acetonitrile mobile phases. The mechanism of retention was investigated and the retention constants, R-M(0), were determined by extrapolation. Correlations between retention constants, R-M(0), and lipophificity, C-0 (C-0 = R-M(0)/m) and log P (ACD log P) were investigated. Better correlations were obtained between C-0 and log P than between R-M(0) and log P. These C-0 values may be use as a measure of the lipophilicity of the compounds investigated. The effect of mobile phase pH on retention of azo-dye derivatives was studied in RP C-18 TLC with diluted acetic acid-methanol, 20 + 30 (v/v) as mobile phase. Different methods were used to measure mobile phase pH. pK(a) values were determined on the basis of the relationships between analyte retention and mobile phase pH. Because the investigated azo dyes are newly synthesized compounds these pK(a) values have been determined for the first time. Correlations between pK(a) and log P are discussed.