Physical Chemistry of Foods

shielding the charge of the ion. Entropy is highest if all ions are completely
distributed at random, but the attractive electric energy between ions of
opposite charge tries to arrange the ions in a regular lattice. The attraction is
stronger if the ions are on average closer to each other, and consequently the
higher the ion concentration, the stronger the shielding (see Figure 2.10).
Here all ions contribute, and use is therefore made of the totalionic strength,
defined as

I:

1

2

X

i

miz^2 i ð 2 : 27 Þ*

wheremdenotes molarity andzthe valence of the ions. Note that the square
of the valence is needed. This implies that a 0.01 molar solution of CaCl 2 has
an ionic strength of ð 0 : 01? 22 þ 2? 0 : 01 Þ= 2 ¼ 0 :03 molar. Most aqueous
foods have an ionic strength between 1 and 100 millimolar (see also Figure
6.8b).
In the theory, the size of the ion also is involved; taking an average
value, the ion activity coefficient of a dilute salt solution is roughly given by

g+&exp



42? 105

ðeTÞ^3 =^2

jjzþz

ffiffiffi

I

p

ð 2 :28aÞ

whereeis the relative dielectric constant. For a higher ionic strength, higher
valences, and a lower dielectric constant, the activity coefficient is thus
smaller. (A lower temperature gives a higher dielectric constant, and this

FIGURE2.10 Electric shielding of ions by ions of opposite charge in a dilute and a

more concentrated solution. Highly schematic.