The use of chemical probes for the characterization of solvent mixtures. Part 2. Aqueous mixtures

The question of whether chemical properties, such as polarity or hydrogen bond donation or acceptance, can be measured in aqueous solvent mixtures by means of indicator probes, or whether their use is obviated because of preferential solvation, is examined. In some cases, such as the Kamlet–Taft π* or β parameters, the use of several probes yielding convergent results provides acceptable values of the properties. In another case—i.e. the Kamlet–Taft α parameter—this question must remain open because of the relatively large spread of values obtained with different probes, which is not necessarily related to preferential solvation. A single probe such as the betaine used for the ET(30) polarity parameter, cannot provide an answer.