Biomedical Engineering Reference
In-Depth Information
can be connected by a metal wire to a high input impedance voltmeter to measure the
potential difference. The schematic construction of an electrochemical cell, the way
electrochemical potentials are determined and the necessary definitions are given
into some detail in Appendix
C
. The equilibrium potential of a metal immersed in a
solution with
a
M
nC
¤ 1
can be calculated by the Nernst equation:
RT
nF
ln
E D E
0
C
.a
M
nC
=a
M
/
(3.2)
with
a
M
, the activity of the metal, accepted to equal one.
The potential
E
0
is an equilibrium potential determined under the standard condi-
and temperature 25
ı
C. No reaction will occur between two systems,
when both are in standard conditions and having hardly differing standard potentials.
Any
redox
(oxidation-reduction) system with an
tion
a
M
nC
D 1
E
0
more positive than another one
will shift the equilibrium of the latter toward oxidation (to the left as shown in the
reactions given in Electrochemical Series of Table C.1, Appendix
C.1
). In ordinary
life, these simple standard and equilibrium conditions are not likely to happen and
consequently, not suitable to predict whether one partner in a particular metal cou-
ple present in an implant will oxidize the other or will be oxidized by the other.
E
0
s,
however, remain a rough guide for a first approach.
An onset to more realistic potential values was given in the early 1950s by Clarke
and colleagues [
86
]. They determined what was called
anodic back EMF
on single
metals but also on alloys. The measurements were done in equine serum, because
they realized that corrosion investigations were carried out
in inorganic fluids [were]
far simpler than those pervading living tissue
!InTable
C.2
, a few of their interesting
data were collected. Although it was not a particularly well-written paper, it was in
the subsequent decades not given the attention it merited.
Another onset to a more realistic approach was given by Pourbaix.
7
3.2.2
Pourbaix Diagram
Equation (
3.2
) relates potential clearly to activity of the metal ion. The metal can be
oxidized and transformed directly to a hydroxide
M
(OH)
n
or oxide:
e
C n
H
C
M C n
H
2
O
! M.
OH
/
n
C n
(3.3)
OH
! M
O
n
n
e
M C 2n
C n
H
2
O
C n
(3.4)
with electron transfer and thus potential and clearly pH dependent. The reaction is
generally stimulated to progress to the right by purely chemical reactions:
7
Marcel Pourbaix (1904-1998) became famous in particular for his E-pH diagrams. He was
founder or co-founder of many international and national (Belgian) committees on electrochem-
istry and one of the important players on the international scene of corrosion science.
