Page 376 - Instrumentation Reference Book 3E

Electrode potentials 359
potential difference at the junction of the two solution, which can be shown to be constant at a
liquids may be reduced to a negligible value either given temperature.
by having present in the two solutions relatively
large and equal concentrations of an electrolyte,
such as potassium nitrate, which produces ions 17.5.2 Variation of electrode potential with ion
which diffuse with approximately equal velocities, activity (the Nernst equation)
or by inserting between the two solutions a "salt The most common measurement of electrode
bridge" consisting of a saturated solution of potential is in the measurement of pH, i.e., hydro-
potassium chloride or of ammonium or potas- gen ion activity, and selective ion activity, p(ion).
sium nitrate. These salts produce ions whose dif- The circuit involved is as shown in Figure 17.14.
fusion rates are approximately equal. The measured potential is the algebraic sum of
When salt bridges are used in pH work, the the potentials developed within the system, i.e.,
liquid junction potentials are reduced to less than
1 mV unless strong acids or alkalis are involved. E = Eint.ref. + Es + 4 - EExt ref.
If an excess of neutral salt is added to the acid or where E~~~.~ef. e.m.f. generated at the internal
the
is
alkali, the liquid junction potential will be reference inside the measuring electrode, E, is the
reduced. Thus the error involved is rarely measur- e.m.f. generated at the selective membrane, E, is
able on industrial instruments. the e.m.f. generated at the liquid junction, and
AI1 measurements of the emf. of cells give the
potential of one electrode with respect to another. &xt.ref. is the e.m.f. generated at the external refer-
ence electrode.
In the Daniel1 cell, all that can be said is that the At a fixed temperature, with the reference elec-
copper electrode is 1 volt positive with respect to trode potentials constant and the liquid junction
the zinc electrode. It is not possible to measure potentials zero: the equation reduces to
the potential of a single electrode. as it is impos-
sible to make a second contact with the solution E = E' + E,
without introducing a second metal-solution
interface. Practical measurement always yields where E' is a constant.
a difference between two individual electrode The electrode potential generated is related to
potentials. the activities of the reactants and products that
In order to assign particular values to the vari- are involved in the electrode reactions.
ous electrode potentials an arbitrary zero is For a general half cell reaction
adopted; all electrode potentials are measured oxidized form + n electrons + reduced form
relative to that of a standard hydrogen electrode
(potential taken as zero at all temperatures). By or
convention. the half cell reaction is written as a aA + bB + ... +ne- ---t xX + yY + ...
reduction and the potential designated positive if
the reduction proceeds spontaneously with the electrode potential generated can be expressed
respect to the standard hydrogen electrode; other- by the Nernst equation
wise the potential is negatike.
-1n-
The standard hydrogen electrode consists of a E = Eo + RT OXID volts
platinum electrode coated with platinum black, nF RED
half immersed in a solution of hydrogen ions at or
unit activity (1.228M HCI at 20°C) and half in
pure hydrogen gas at one atmosphere pressure. In
practice. however, it is neither easy nor convenient
to set up a hydrogen electrode, so subsidiary refer- where R is the moIar gas constant (8.314 jouIe.
ence electrodes are used, the potential of which mol-'K-'), Tis absolute temperature in Kelvins,
relative to the standard hydrogen electrode has F is the Faraday constant (96487 coulomb.
previously been accurately determined. Practical mol-'), and n is the number of electrons partici-
considerations limit the choice to electrodes con- pating in the reaction according to the equation
sisting of a metal in contact with a solution which defining the half cell reaction. The value of the
is saturated with a sparingly soluble salt of the term 2.303 RTlnFis dependent upon the variables
metal and which also contains an additional salt n and T and reduces to 0.059h volts at 25 "C and
with a common anion. Examples of these are the 0.058h volts at 20 "C.
silver/silver chloride electrode (Ag/AgCl(gKCl) An ion-selective electrode (say, selective to
and the mercurylmercurous chloride electrode sodium ions) is usually constructed so that the
(HglHg2C12,KCI) known as the calomel electrode. ion activity of the internal reference solution
In each case the potential of the reference electrode inside the electrode is constant, and the Nernst
is governed by the activity of the anion in the equation reduces at constant temperature to