Pine Research MSR 636A, User Manual, Page: 134 / 176

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Each half reaction has an associated standard electrode potential (
E º
) which is
a thermodynamic quantity related to the free energy associated with the
equilibrium. Like many other standard thermodynamic quantities, the standard
electrode potential corresponds to a given standard state. The standard state
corresponds to a thermodynamic system where the activities of O and R are unity
(
i.e. , when all solution concentrations are 1.0 mol/L, all gases are present at 1.0
atm partial pressure, and other materials are present as pure phases with unity
activity).
To account for the (likely) possibility of non-unity activities, the Nernst equation
(see below) can be used to express the equilibrium electrode potential (
E
NERNSTIAN
)
in terms of the actual activities.
E
NERNSTIAN
= Eº + (R T / n F) ln [a
O
/ a
R
]
where F is the Faraday constant ( F = 96485 C / mol), R is the ideal gas constant
( R = 8.3145 J / mol K), and T is the temperature (K). Usually, the activities of
molecules or ions dissolved in solution are assumed to be the same as their molar
concentrations, so the Nernst Equation is often written as follows
E
NERNSTIAN
= Eº + (R T / n F) ln [C
O
/ C
R
]
where
C
O
and C
R
are the concentrations of the dissolved molecules or ions in the
oxidized and reduced forms, respectively, at the surface of the electrode . Note
that any liquid or solid phase materials at the electrode surface (such as the
solvent or the electrode itself) have unity activity and thus do not appear in the
Nernst equation.
This half reaction at an electrode can be driven in the cathodic (reducing)
direction by applying a potential to the electrode (
E
APPLIED
) which is more
negative than the equilibrium electrode potential (
E
APPLIED
< E
NERNSTIAN
).
Conversely, the half reaction can be driven in the oxidizing (anodic) direction by
applying a potential more positive than the equilibrium electrode potential
(
E
APPLIED
> E
NERNSTIAN
).
10.3 Voltammetry
The term voltammetry refers broadly to any method where the electrode
potential is varied while the current is measured.
[1-2]
The terminology associated
with voltammetry varies across different industries and academic disciplines, but
the underlying principles of all voltammetric techniques are very similar.
The most common form of voltammetry involves sweeping the electrode
potential from an initial value to a final value at a constant rate. When working
in the context of electroanalytical chemistry with a non-rotating electrode, this
technique is called linear sweep voltammetry (LSV). In the context of corrosion