Determination of solvent basicity scale, beta, of mixed solvents for three chromatographic solvent systems: 2-Propanol hexane, ethyl acetate hexane, and methanol water

We have determined beta values of mixed solvents for the 2-propanol/hexane, ethyl acetate/hexane, and methanol/water systems using totally 5 indicator pairs (10 dyes) three of which were synthesized in this study. The variations of beta vs. solvent composition for the RPLC solvent system are in a striking constrast with those for the NPLC systems. While a definite maximum was recognized in the plots of beta vs. phi for the NPLC systems, a linear correlation of beta with phi was observed for the RPLC solvent system. The appearance of maximum is probably due to formation of aggregates of the more polar solvent when its composition gets higher.