Page 90 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNOAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION
In this scheme, a single vertical line represents a metal-electrolyte boundary at
which a potential difference is taken into account: the double vertical broken
lines represent a liquid junction at which the potential is to be disregarded or
is considered to be eliminated by a Salt bridge.
When reference is made to the electrode potential of a zinc electrode, it is
the e.m.f. of the cell:

or the e.m.f. of the half-ce11 Zn2+1Zn which is meant. The ce11 reaction is:

and the half-ceil reaction is written as:
ZnZ+ + 2e = Zn
The electrode potential of the Fe3+,FeZ+IPt electrode is the e.m.f. of the cell:
Pt,HzIH+(a = l)//Fe3+,FeZ+IPt
1
or the e.m.f. of the half-ce11 Fe3+,FeZ+ Pt. The ce11 reaction is:
$Hz + Fe3+ + H+(a = 1)+ FeZ+
and the half-ce11 reaction is written:
Fe3+ + e = Fe2+
The convention is adopted of writing al1 half-ce11 reactions as reductions:
Mn+ +ne + M
e.g. ZnZ + + 2e + Zn; Ee = - 0.76 volt
When the activity of the ion Mn+ is equal to unity (approximately true for a
1 M solution), the electrode potential E is equal to the standard potential Ee.
Some important standard electrode potentials referred to the standard hydrogen
electrode at 25 OC (in aqueous solution) are collected in Table 2.5.'

Table 2.5 Standard electrode potentials at 25 "C
Electrode reaction Ee (volts) Electrode reaction Ee (volts)
Li++e=Li TI++e=TI -0.336
Kt +e=K CoZ+ +2e=Co - 0.277
BaZ+ + 2e = Ba NiZ+ + 2e = Ni - 0.25
SrZ+ + 2e = Sr SnZ+ +2e=Sn -0.136
CaZ+ +2e=Ca PbZ+ + 2e = Pb -0.126
Naf +e=Na 2H++2e=Hz 0.000
MgZt +2e=Mg CuZ+ +2e=Cu f0.337
Al3+ + 3e = Al HgZ+ + 2e = Hg f0.789
MnZ+ + 2e = Mn Ag++e=Ag + 0.799
ZnZ+ +2e=Zn PdZ+ + 2e = Pd f0.987
FeZ+ +2e= Fe PtZ+ + 2e = Pt f1.2
CdZ+ +2e=Cd Au3+ +3e= Au f1.50

It may be noted that the standard hydrogen electrode is rather difficult to
manipulate. In practice, electrode potentials on the hydrogen scale are usually