Page 364 - Physical chemistry understanding our chemical world

HALF-CELLS AND THE NERNST EQUATION 331

Lead to voltmeter

Glass tube

Platinum contact
Saturated KCl solution

Paste of Hg and Hg 2 Cl 2

Crystals of KCl
Porous sinter

Figure 7.13 Schematic representation of the saturated calomel electrode (SCE)

The potential of the SCE is 0.242 V at 298 K relative to the SHE.
Calomel is the old-
At the ‘heart’ of the SCE is a paste of liquid mercury and fashioned name for
mercurous chloride (Hg Cl 2 ), which has the old-fashioned name
2 mercurous chloride,
‘calomel’. Figure 7.13 depicts a simple representation of the SCE. Hg Cl 2 .Calomel wasa
2
The half-cell reaction in the SCE is vital commodity in the
Middle Ages because
−
−
Hg Cl 2 + 2e −−→ 2Cl + 2Hg (7.44) it yields elemental
2
mercury (‘quick sil-
So E (SCE) = E Hg 2 Cl 2 ,Hg . From this redox reaction, the Nernst equa- ver’) when roasted; the
tion for the SCE is mercury was required
by alchemists.

RT a (Hg 2 Cl 2 )
E Hg 2 Cl 2 ,Hg = E O + ln (7.45)
Hg 2 Cl 2 ,Hg 2 2
2F a a
−
(Hg) (Cl )
Oxidative currents
the square terms for mercury and the chloride ion are needed in
reverse the reaction
response to the stoichiometric numbers in Equation (7.44). Both in Equation (7.44).
mercury and calomel are pure substances, so their activities are Cl − and Hg are con-
unity. If the activity of the chloride ion is maintained at a constant sumed and Hg Cl 2
2
level, then E (SCE) will have a constant value, which explains why forms. The denomina-
the couple forms the basis of a reference electrode. tor of Equation (7.45)
Changing a (Cl ) decreases, and E (SCE)
− must alter E Hg 2 Cl 2 ,Hg , since these two variables
are interconnected. In practice, we maintain the activity of the chlo- increases.
ride ions by placing surplus KCl crystals at the foot of the tube.
The KCl solution is saturated – hence the ‘S’ in SCE. For this reason, we should
avoid any SCE not showing a crust of crystals at its bottom, because its potential will
be unknown. Also, currents must never be allowed to pass through an SCE, because
charge will cause a redox change in E (SCE) .
Table 7.9 lists the advantages and disadvantages of the SCE reference. Despite
these flaws, the SCE is the favourite secondary reference in most laboratories.