Page 376 - Physical chemistry understanding our chemical world

BATTERIES 343

Detail
Salt bridge

Electrolyte Electrolyte
from from bridge
solution
c 1 c 2

Figure 7.18 The two half-cells in a cell are joined with a salt bridge. Inset: more ions leave the
bridge ends than enter it; the relative sizes of the arrows indicate the relative extents of diffusion

Table 7.13 Values of junc-
tion potential in aqueous cells
as a function of the concentra-
tion of inert KCl within a salt
bridge
[KCl]/c O E j /mV
0.1 27
1.0 8.4
2.5 3.4
4.2 (sat’d) <1

Minimizing junction potentials with a swamping electrolyte

The second method of minimizing the junction potential is to employ a ‘swamping
electrolyte’ S. We saw in Section 4.1 how diffusion occurs in response to entropy
effects, themselves due to differences in activity. Diffusion may be minimized by
decreasing the differences in activity, achieved by adding a high concentration of
ionic electrolyte to both half-cells. Such an addition increases their ionic strengths I,
and decreases all activity coefficients γ ± to quite a small value.
If all values of γ ± are small, then the differences between activities also decrease.
Accordingly, after adding a swamping electrolyte, fewer ions diffuse and a smaller
junction potential forms.

7.7 Batteries

How does an electric eel produce a current?

Introduction to batteries

The electric eel (Electrophorus electricus) is a thin fish of length 3–5 feet; see
Figure 7.19. It is capable of delivering an electric shock of about 600 V as a means