Chemical potential of KCI and its ion constituents in concentrated protein salt solutions

Abstract

In order to obtain information about the thermodynamic state of simple salts in living cells we measured the mean activity coefficients of KCl and the “apparent” single ion activity coefficients of K+ and Cl‐ in concentrated bovine serum albumin‐KCl solutions and bovine hemoglobin‐KCl solutions and in water soluble fractions of the cytoplasm of calf liver cells, using the EMF method with ion‐exchange membrane electrodes. The protein concentration range extended up to 45% by weight with neutral salt concentration of about 0.1 mole/kg H2O. The pH value varied from 3.5 to 9.8. Allowing for the effect of site binding and for nonsolvent water, the results show that the “electrostatic” mean activity coefficient of KCl decreases linearly with the equivalent concentration of the polyions. The effect is small, since, in a 0.1 M KCl solution with 20% protein, the activity coefficient of KCl is only 10 to 20% lower than in the pure 0.1M KCl solution. The meaning of the measured “apparent” single ion activity coefficients is discussed briefly. The results are compared with the predictions of the Marcus‐Katchalsky‐Alexandrowicz theory and of the Oosawa theory.