Physical Chemistry of Foods

causes the temperature influence to be fairly small.) For water at 20 8 C,
e¼80, and the equation becomes

g+&expð
 1 : 17 jjzþz

ffiffiffi

I

p

Þð 2 :28bÞ

The valence has a large effect; comparing, for instance, a 0.01 molar solution
of NaCl with one of CaSO 4 , the activity constants are calculated as 0.89 and
0.31, respectively (assuming dissociation to be complete).
Figure 2.11 gives the total ion activity coefficients of a few salt
solutions, and the calculated values are given for comparison. It is seen that
the agreement is good up to an ionic strength of about 0.05. For higher
values, the equation gives activity coefficients that are too low. Note that
Eq. (2.28) does not discriminate between different ions. In practice, fairly

FIGURE2.11 Total ion activity coefficientsðTÞas a function of total ionic strength
ðITÞof NaCl, ZnCl 2 , and CuSO 4. The broken lines are according to Eq. (2.28) for
salts with ions of various valences (indicated).