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342 ELECTROCHEMISTRY

While it is easy to measure a value of emf, we do not know the magnitude of E j .
SAQ 7.21 illustrates why we need to minimize E j .

+
SAQ 7.22 The emf of the cell SHE |Ag |Ag, is 0.621 V. Use the Nernst
equation to show that a + = 10 −3 if E j = 0V, but only 4.6 × 10 −4 if
(Ag )
E j = 20 mV. E O = 0.799 V. [Hint: to compensate for E j in the sec-
+
Ag ,Ag
ond calculation, say that only 0.601 V of the emf derives from the Ag |Ag
+
half-cell, i.e. E Ag ,Ag = 0.601 V.]
+
What is a ‘salt bridge’?

Minimizing junction potentials

It’s called a bridge In normal electrochemical usage, the best defence against a junction
because it connects the potential E j is a salt bridge. In practice, the salt bridge is typically a
two half-cells, and salt thin strip of filter paper soaked in electrolyte, or a U-tube containing
because we saturate an electrolyte. The electrolyte is usually KCl or KNO 3 in relatively
it with a strong ionic high concentration; the U-tube contains the salt, perhaps dissolved
electrolyte. in a gelling agent such as agar or gelatine.
We connect the two half-cells by dipping either end of the salt
bridge in a half-cell solution. A typical cell might be written in schematic form as:

Zn (s) |Zn 2+ (aq) |S|Cu 2+ (aq) |Cu (s)

We write the salt bridge as ‘|S|’, where the S is the electrolyte within the salt bridge.
But how does the salt bridge minimize E j ? We recognize first how the electrolyte
in the bridge is viscous and gel-like, so ionic motion through the bridge is slow.
Secondly, the ionic diffusional processes of interest involve only the two ends of the
salt bridge. Thirdly, and more importantly, the concentration of the salt in the bridge
should greatly exceed the concentrations of electrolyte within either half-cell (exceed,
if possible, by a factor of between 10–100 times).
The experimental use of a salt bridge is depicted in Figure 7.18. The extent of
diffusion from the bridge, as represented by the large arrows in the diagram, is seen
to be much greater than diffusion into the bridge, as represented by the smaller of
the two arrows. A liquid junction forms at both ends of the bridge, each generating
its own value of E j . If the electrolyte in the bridge is concentrated, then the diffusion
of ions moving from the bridge will dominate both of these two E j . Furthermore,
these E j will be almost equal and opposite in magnitude, causing them to cancel each
other out.
Table 7.13 shows how the concentration of the salt in the bridge has a large effect
on E j : it is seen that we achieve a lower value of E j when the bridge is constructed
with larger concentrations of salt. A junction potential E j of as little as 1–2 mV can
be achieved with a salt bridge if the electrolyte is concentrated.