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278 CHAPTER 3

it follows by substitution of the expression (3.113) for and Eq. (3.44) for that

It will be recalled, however, that, in order to calculate the activity coefficient from
the expressions

and

i.e., from

it is necessary to know which is the potential at the surface of the ion due to the
surrounding ions, i.e., due to the cloud. Since, in the finite-ion-size model, the ion is
taken to have a size a, it means that is the value of at r= a,

The value of at r = a is obtained by setting r = a in Eq. (3.114). Hence,

By substitution of the expression (3.117) for in Eq. (3.115), one
obtains