Interrelation and Application of Solvent Polarity Parameters

Using a different set of standard substances, i.e. substituting 1-butanol, pentan-2-one, and 1-nitropropane for the rather volatile ethanol, butan-2-one, and nitromethane, McReynolds developed an analogous approach [103]. Altogether, he characterized over 200 liquid stationary phases using a total of 10 probes. A statistical analysis of the McReynolds retention index matrix using the principal component analysis method has shown that only three components are necessary to reproduce the experimental data matrix [262]. The first component is related to the polarity of the liquid phase, the second depends almost solely on the solute, and the third is related to specific interactions with solute hydroxy groups [262]. 

Both of these approaches used in the characterization of stationary liquid-phase polarities by means of retention indices have been further explored and expanded [104, 259-261]. For a review on the characterization of solvent properties of phases used in gas-liquid chromatography by means of the retention index system, see reference [344]. Similar methods for the characterization of solvent polarity in liquid-liquid and liquid-solid chromatography can be found in references [105-107] cf also Section A-7 and Tables A-10 and A-11 in the Appendix. 

From the previous Sections, we can conclude that there are many empirical solvent scales, the most comprehensive of which are the solvatochromic ones cf. for example Table 7-3. Unfortunately, too many solvent scales have been proposed during the last decades. Around 35 different solvent scales are known. Only about ten of them have found wider application in the correlation analysis of solvent effects, i.e. Y, Z, t(30), a, 

DN and AN, as well as SPP, SA, and SB. The applieation of most solvent seales is restrieted by the faet that they are known only for an insuffieient number of solvents. The eatalogue of eommon organie solvents available to the ehemist numbers about 300, not to speak of the infinite number of solvent mixtures The extension of most solvent seales is restrieted by the inherent properties of the seleeted referenee proeess, whieh exelude the determination of solvent parameters for eertain, often important solvents e.g. chemieal reaetions between solute and solvent, solubihty problems, etc.). For this reason, the most eomprehensive solvent polarity seales are those derived from speetro-scopic reference processes, which are the most easily measured for a large set of organic solvents. 

In particular, there are good Hnear correlations between the Ej[ Q) values and some other empirical solvent polarity parameters according to Eq. (7-44), 

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