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296 ELECTROCHEMISTRY

Edward Weston (1850–1936) was a giant in the history of electrical measuring
instruments. In the field of measurement, he developed three important components:
the standard cell, the manganin resistor and the electrical indicating instrument.
The main advantage of Weston’s cell was its insensitivity to temperature, and the
◦
emf of almost 1 V: to be precise, 1.0183 V at 20 C. It is usually constructed in an H-
shaped glass vessel. One arm contains a cadmium amalgam electrode beneath a paste
of hydrated cadmium sulphate (3CdSO 4 · 5H 2 O) cadmium sulphate and mercury(I) sul-
phate. The other arm contains elemental mercury. Its schematic is Cd(Hg)|CdSO 4(aq) ,
Hg SO 4 |Hg.
2
The Weston saturated cadmium cell became the international
The Clark cell was standard for emf in 1911. Weston waived his patent rights shortly
patented by Latimer afterward to ensure that anyone was allowed to manufacture it.
Clark in the 1880s, and Weston’s cell was much less temperature sensitive than the previ-
was the first standard ous standard, the Clark cell. We recall how the value of G changes
cell.
with temperature according to Equation (4.38). In a similar way, the
value of G (cell) for a cell relates to the entropy change S (cell) such
that the change of emf with temperature follows
Remember:the small
subscripted ‘p’indi- d(emf )
cates that the quantity S (cell) = nF (7.18)
is measured at con- dT p
stant pressure.It does
not mean ‘multiplied the value of (d(emf )/dT) p is virtually zero for the Weston cell.
by p’. If we assume the differential (d (emf )/dT ) is a constant, then
Equation (7.18) has the form of a straight line, y = mx, and a
graph of emf (as ‘y’) against T (as ‘x’) should be linear. Figure 7.5
shows such a graph for the Clark cell, Hg|HgSO , ZnSO 4 (sat’d)|
The temperature volt- 4
age coefficient has Zn. Its gradient represents the extent to which the cell emf varies
with temperature, and is called the temperature voltage coefficient.
several names: ‘tem-
perature coefficient’, The gradient may be either positive or negative depending on the
−5
−4
‘voltage coefficient’ or cell, and typically has a magnitude in the range 10 to 10
−1
‘temperature coef- VK . We want a smaller value of (d (emf )/dT )if the emf is
ficient of voltage’. to be insensitive to temperature.
Table 7.3 contains a Having determined the temperature dependence of emf as the
few values of gradient of a graph of emf against temperature, we obtain the value
(d (emf)/dT). of S (cell) as ‘gradient × n × F’.

Worked Example 7.7 The temperature voltage coefficient for a sim-
−1
That this value of ple alkaline torch battery is −6.0 × 10 −4 VK . What is the entropy
d(emf)/dT is neg- change associated with battery discharge? The number of electrons
ative tells us that transferred in the cell reaction n = 2.
the emf DEcreases
when the temperature Inserting values into Equation (7.18):
INcreases.
S (cell) = 2 × 96 485 C mol −1 ×−6.0 × 10 −4 VK −1
S (cell) =−116 J K −1 mol −1