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258 CHAPTER 3

[Eq. (3.60)] to the experimentally accessible mean ionic-activity coefficient so that
the Debye–Hückel model can be tested.
The procedure is to make use of the relation (3.72)

of which the general form for an electrolyte that dissolves to give -valent positive
ions and -valent negative ions can be shown to be (Appendix 3.3)

where and are the activity coefficients of the positive and negative ions, and

By taking logarithms of both sides of Eq. (3.75), the result is

At this stage, the Debye–Hückel expressions (3.60) for and can be introduced
into Eq. (3.77) to give

Since the solution as a whole is electroneutral, must be equal to and
therefore

Using this relation in Eq. (3.78), one obtains

Now, one can substitute for from Eq. (3.43)