Solvent effects on mobile phases used in liquid chromatography: factor analysis applied to protonation equilibria and solvatochromic parameters

Abstract

Dissociation constant values of the acid components of the standard NIST scale buffer solutions and of acetate buffer in several hydroorganic mixtures (acetonitrile–water, tetrahydrofuran–water and methanol–water) were first subjected to factor analysis in order to obtain the number of factors that affect the variation of the whole data sets, and then to target factor analysis to identify these factors. The p K a values were correlated with the Kamlet and Taft solvatochromic parameters (α, β and π*) of the solvent mixtures according to the results obtained with these chemometric techniques. Two or three factors emerged, depending on the range of percentage of organic cosolvent in the mixture. Target factor analysis results showed that Kamlet–Taft equations were reduced in these mixtures: the independent term and π* solvatochromic parameter of polarity-polarizability appear up to 50% w/w of organic solvent, as does the hydrogen bond basicity β solvatochromic parameter when data up to 70% w/w were analyzed. Further, the quasi-lattice quasi-chemical theory has been applied to quantify the preferential solvation by water of electrolytes in acetonitrile–water, tetrahydrofuran–water and methanol–water mixtures. An overview of the effect of this preferential solvation on the values of the protonation constants in these media was also obtained.