pH/mV Amp
39
A P P E N D I X D
D
Electrode Behavior
The Nernst Equation
Most pH and ion selective electrodes exhibit behavior that closely
follows the Nernst equation which describes the relationship between
potential, ion activity (≈ ionic concentration), and temperature.
where
•
E
is the observed potential (in volts)
•
E
0
is the potential when pY = 0 (in volts)
•
R
is the gas constant (8.314 J K
-1
mol
-1
)
•
T
is the temperature in Kelvin
•
F
is the Faraday constant (96487 C mol
-1
)
•
n
is the number of electrons transferred at the electrode (usually
the charge of the ion)
•
pY
= –log
10
[Y] where [Y] is the activity of the ionic species
Note that if
pY
is plotted against
E
then the Nernst equation predicts
that a straight line should be obtained of intercept
E
0
and a slope of
–(RT/nF)
. For pH measurements (
n
=+1) at 25°C, the expression
RT/nF
has a value of 0.05916 V. This is referred to as the Nernstian slope of
the electrode.
E
E
0
2.303 RT
nF
------
pY
–
=
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