Solvent polarities have been related to retardation factor (R_f) of various nitro compounds as linear functions in binary mixtures of solvents. The average polarities of the two solvents in equal volumes have been taken as the first estimate. Such relations may prove useful in computing polarities of mixed systems to a first approximation and to evaluate single unknown polarities, if this value for the other solvent has been determined previously.

Solvent polarity has usually been measured by kinetics, thermodynamics, spectroscopy, and thin-layer chromatography (Drougard and Decroocq, 1969; Ahmad et al., 1996; Pirila, 1996; and Litvinova and Bel'nikevich, 2003). Various well-known polarity scales are known, of which E_T (30), Kasowers Z-scale, Gutamann Acceptor No. (AN), Gutamann Donor No. (DN), and Kamlet Tafts (Π^*) scales are most popular (Kosower, 1958; Meyer et al., 1975; Kamlet et al., 1977; Sindreu et al., 1996; and Matyushov et al., 1997). E_T (30) is a leading indicator of solvent polarity. A similar approach has been reported in the literature (Christopher et al., 1992; Nurok D et al., 1993; and Pirila-Honkanen, 1995). William Bentley and Sun Koo recently studied the role of hydroxyl concentrations in solvatochromic measures of solvent polarity of alcohols and alcohol-water mixtures—evidence that preferential solvation effects may be overestimated (William and Sun, 2004).

However, the need for a very simple method to evaluate solvent polarity compelled us to believe that thin-layer chromatography may be the technique of choice. In a previous manuscript (Ahmad et al., 1996), we gave the results of such a study using the R_f values of nitro compounds in various solvents. It was readily seen that the various linear functions obtained could be used to predict solvent polarities which are not known or can be used to evaluate solvent polarities of mixtures. In the present paper, we show that the polarity of a solvent mixture can roughly be designated as the average of polarities of the two solvents in various mixing ratios. This over simplification is nevertheless useful for a qualitative estimate of solvent polarity by thin-layer chromatography. In this paper, we report our results for different solvent systems in ratios of 1:1.

Silica gel G (E Merck India Ltd.) was used for thin-layer chromatography. The nitro compounds were from E Merck and Fluka. All solvents were analytical grade reagents from BDH and E Merck (India). The solvents were used without further purification, however, most of them were dried over 4 Å molecular sieves. Traces of water were not determined as it was felt that there would be no significant effect on R_f, a fact confirmed later by the linear relations obtained (Figures 1 and 2). For each chromatographic development the thin-layer chromatography tank was covered with a greased glass lid and the lid was removed only when the development was complete.

Chemistry Journal, Solvent Polarity, Thin-Layer Chromatography, Nitro Compounds, Chromatographic Process, Regression Analysis, Microscopic Slides, Chromatographic Development, Retardation Factor, Chemical Reactions, Thermodynamic Properties.