Page 181 - Chemical equilibria Volume 4

Appendix 1 157
Hence, the constant K links the activity coefficients expressed, for a
iH
solute, in the two conventions: pure-substance reference and dilute-solution
reference. This constant does not depend on the composition of the solution,
but instead depends on the values of the intensive variables (pressure,
temperature, etc.), by way of the chemical potentials of the reference states,
amongst other things.
We have seen that, for the solvent, the two conventions are identical.

NOTE.– In a solution with more than two components, there is no reason not
to consider multiple solvents and multiple solutes.

A1.2.5. Convention (III)

Convention (III) again makes the distinction between the solvent and
solutes:
– for the solvent, the reference (III) convention is identical to
reference (I), the pure-substance reference, and thus the chemical potential
of the solvent is always given by relation [A1.9];
– for a solute, we choose to write that the activity coefficient of the solute
is equal to 1 for a solution containing 1 mole per liter of each solute. The
chemical potential of the reference state is that of the solute in that solution
which contains 1 mol/l of each of the solutes. The chemical potential of the
solute is therefore written:
μ s = μ s 0(III) + Rlnγ s (III) C s [A1.14]
T

NOTE.– It is not unfeasible, in certain cases of low solubility, that the
reference solution with 1 mol/l of each solute is perfectly imaginary, because
it is impossible to realize. This takes nothing away from the convention,
which requires only that the solvent and the solutes in the reference solution
be in the same state of aggregation as the solution, even if it is a fictitious
state.

Convention (III), the molar solution reference, is often used for ionic
aqueous solutions, although in the latter case, the chosen reference is often
the solution whose molarity tends toward zero, which does not change the
chemical potential of the reference state μ s 0(III) .