Isopotential solutions method

Let us briefly review which of the listed values and in what manner can be determined. It is possible to calculate the distribution of complex species in the solution, that is, bulk concentrations or activities, by means of the methods discussed in Chapter 1. Besides, the composition of solutions can be changed so that Eqs. (7.8)-(7.10) would become simpler. For example, one can use a series of solutions with constant concentration of free ligand for which i In l = 0- In the series of isopotential solutions (see Section 2.1), the condition dlna = 0 holds. By differentiation of the electrocapillary curve with respect to E for a solution of constant composition, one can obtain the value of the charge density e and use Eq. (7.6) for the calculation of total adsorptions Fj x nd Flx. Thus, electrocapillary measurements, as well as a number of other methods (radioactive indicator, surface stress, piezoelectric, extensometer methods), give information only about the total amounts of adsorbed metal and ligand. Consequently, for further solution of the problem posed, some model images are necessary. 

Arakawa and Timasheff [85A2, 86A1] developed a method for calculating the isopotential specific volume, 0, of proteins in concentrated salt, sugar, and amino acid solutions. The method is based on the finding that the preferential hydration of proteins in these solutions is relatively independent of the additive concentration and is proportional to the protein surface area. The calculations also demonstrate that 0 increases markedly with the concentration of the additive, particularly in the case of Na2S04, (NH4)2S04, sucrose, and for smaller proteins. 

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