Determination of Solvent Basicity Scale, β, of Mixed Solvents for Three Chromatographic Solvent Systems: 2-Propanol/Hexane, Ethyl Acetate/Hexane, and Methanol/Water

We have determined β 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 β 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 β vs. φ for the NPLC systems, a linear correlation of β with φ 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.