Field partial molal

Let us now ask how this value could be used as a basis from which to measure the local disturbance of the water structure that will be caused by each ionic field. The electrostriction round each ion may lead to a local increase in the density of the solvent. As an example, let us first consider the following imaginary case let us suppose that in the neighborhood of each ion the density is such that 101 water molecules occupy the volume initially occupied by 100 molecules and that more distant molecules are not appreciably affected. In this case the local increase in density would exactly compensate for the 36.0 cm1 increment in volume per mole of KF. The volume of the solution would be the same as that of the initial pure solvent, and the partial molal volume of KF at infinite dilution would be zero. Moreover, if we had supposed that in the vicinity of each ion 101 molecules occupy rather less than the volume initially occupied by 100 molecules, the partial molal volume of the solute would in this case have a negative value. 

To transform Equation 6.3 into a more useful form, we need to incorporate expressions for the chemical potentials of the species involved. The chemical potential of species j was presented in Chapter 2 (Section 2.2B), where Xj is a linear combination of various terms fij = fi + RT In cij +VjP -f ZjFE 4- mjgh (Eq. 2.4 fi is a constant, cij is the activity of species /, Vj is its partial molal volume, P is the pressure in excess of atmospheric, Zj is its charge number, Fis Faraday s constant, E is the electrical potential, m is its mass per mole, and h is the vertical position in the gravitational field). 

For convenience, we generalize the concept of the partial molal quantity defined in Eq. (2-5) to include the specification of constant electric field. Thus, we write... 

We note that the chemical potential of component i, /t, is not the partial molal free energy in the presence of an electric field. 

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