Concise Physical Chemistry

c14 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

THE CALOMEL ELECTRODE 229

whereaAgIof the solid precipitate has been set equal to 1.0 andKspis the solubility

product of the AgI precipitate. The potential of this cell is 0.950 volts and, solving

the Nernst equation, we find logKsp=− 0. 950 / 0. 0592 =− 16 .0, soKsp∼= 10 −^16.

Notice that we use 59.2 mV in the Nernst equation to calculate log 10 Ksp(Section

14.7). Stability constants of complexes, in which metal ion concentrations are reduced

to very low levels byligandslike EDTA, can be measured by a method analogous to

the one shown for AgI.

14.10 MEAN IONIC ACTIVITY COEFFICIENTS

Back calculating from the procedure of Section 14.8 producesγ±at molalities other

than zero. From

E=E◦−

2 RT

F

logγ±−

2 RT

F

logmHCl

E=E◦− 0 .1184 logmHCl− 0 .1184 logγ±

and

0 .1184 logγ±=−E+E◦− 0 .1184 logmHCl

logγ±=

−E+E◦− 0 .1184 logmHCl

0. 1184

At finite molality,Ecan be measured and everything on the right of the equal sign is

known becauseE◦is known.

14.11 THE CALOMEL ELECTRODE

The standard hydrogen half-cell has the advantage that its potential is 0.000 by

definition, but it is bulky and presents some safety hazards in the use of hydrogen.

Consequently, several half-cells that do not have these disadvantages are used as

reference electrodes. The most common one is the calomel electrode (strictly, half-

cell), which is easily set up, is light, and gives a constant voltage at a fixed temperature.

Calomel half-cells have liquid mercury in contact with a paste of Hg 2 Cl 2 (s) and solid

KCl in equilibrium with Cl−(aq). Connection to external circuitry is by a Pt wire

in contact with the mercury. Connection with any of a variety of other half-cells

is made through a saturated KCl solution called a salt bridge. The saturated KCl

solution does not change much upon drawing a small current, so the output voltage

is a constant 0.2444 volts at 298 K. The electrode is designated asaturated calomel

electrode(SCE) from the common namecalomelfor mercurous chloride (which was

once used as a medicine for children!!). The cell diagram

Pt; H 2 (1.0atm);H+(aq)||Hg 2 Cl 2 (s),KCl(aq sat′d); Hg; Pt