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ION–ION INTERACTIONS 263

and is simply because when the solute concentration (hence also activity)
tends to zero, its activity becomes equal to its concentration.
This solvent vapor pressure method for measuring the activity of electrolytes has
11
the advantage that the actual experiments one has to do are simple. The method can
be applied to any concentration (e.g., a 15 M solution!). The difficulty comes at low
concentrations when the difference of the vapor pressure between the solution and that
of the solvent becomes limitingly small. A huge amount of data (see Table 3.5) have
been determined by this method, particularly in the 1950s by a long-term Australian–
New Zealand collaboration between professors Stokes and Robinson.

3.4.8. A Second Method by Which One May Obtain Solute Activities:
From Data on Concentration Cells and Transport Numbers

Thermodynamics treats electrochemical cells in equilibrium and indeed such
hoary material, going back to the work of the great German physical chemist Nernst, 12
is a part of classical electrochemistry that is still being taught in universities to students
as if it were representative of modern electrochemistry! Consider then the chemical
potential of a metal as in the solid electrode.
The chemical potential of the ion is that of a solute in a solution and hence is given
by

11
There is often no need for an absolute determination of vapor pressure. The solvent vapor pressure can
be determined simply by setting up a closed system that contains a solution of large volume having an
already known solvent activity. The unknown solution will change its concentrations (and hence its
weight) until its solvent activity is the same as that of the reference system, which is known. Great accuracy
in the weighing is essential and one should use platinum vessels to minimize possible dissolution.
12
Walter Nernst was professor of chemistry in Berlin in the early years of the twentieth century. He
epitomized the professor as a “Great Man.” Among his many achievements was the work that led (via the
Nernst heat theorem) to the third law of thermodynamics. He was active not only in chemistry but also
made significant contributions to the theory of the expanding universe. Nernst was famous not only for
his real (many and great) contributions to physical electrochemistry but also for the cold and rigid
discipline he demanded from those who aspired to be his collaborators. Were one of these to arrive at his
workplace after the scheduled hour of 7:00 a.m., he might find a note from the professor reminding him
of the number of applicants who were waiting to occupy it.
One such collaborator (later himself a famous physical chemist) is known to have remarked that, in
making the mixture for Nernst, the Herr Creator had put in an extra dose of the intellectual but left out
the humanity.