Biomedical Engineering Reference
In-Depth Information
Appendix C
Electrochemical Series
C.1
Equilibrium Electrochemical Series
The electrochemical series are set up by measuring series of metals and alloys with
respect to a second electrode, a standard
nonpolarizable electrode
,
in casu
the stan-
dard or normal hydrogen electrode (SHE or NHE) or, for practical purposes, the
standard calomel electrode (SCE). The characteristic of a nonpolarizable electrode
is that its potential is not perturbed by small currents imposed by the measurement
circuit itself (i.e., the power consumption, however small, of the voltmeter) or in
other words, that the internal impedance of the electrode tends to zero. The refer-
ence half-cell in Fig.
C.1
consists of a platina rod along which hydrogen gas at 1 atm
is allowed to bubble (the gas adsorbs on the surface of Pt) and which is immersed in
a solution of hydrogen ion with
activity
a
H
C
D 1
.
The potential of the couple H
C
=
1
2
H
2
, toward the solution is arbitrarily taken
to be zero when
(cf. reaction in Table
C.1
). In practice, an SCE is mostly
used. The electrode is always an integrated design containing the KCl bridge and the
mercury/calomel electrode (Hg
a
H
C
D 1
1
=
2
Hg
2
Cl
2
). Its potential with respect to the NHE is
C0:2681
V and this potential is to be added to the potential measured to obtain the
standard electrode potential. The other half cell, i.e., the left half of the corrosion
cell of Fig.
C.1
, consists of a rod of the metal
M
to be measured immersed in a
solution with
E
0
measurement, this activity should be 1.
The
activity
of an ion or
active concentration
at a given temperature is defined
as the concentration (normal, molar or molal) multiplied by an activity coefficient.
Although often taken to be the concentration, pH is equal to
a
M
n
C
D 1
.For
log
a
H
C
, the activity
of the hydrated hydrogen ion.
The activity coefficient corrects concentration for 'nonideal' behavior (compare
to pressure-volume relationship for ideal and real gases) and is a function of physical
interaction on the single hydrated ion by the solution; it can be calculated from the
ionic strength:
I D 1=2
P
i
D
1
c
i
z
i
with c the concentration (molar or molal) and
z the valence of the ion. The reference state for solutions is
infinite concentration
where f
1
. It is an integral part of the physical theory of solutions.
The total concentration of an ion in solution is also compromised by chemi-
cal reactions forming insoluble compounds or by complex-forming agents further
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