97
p r i n c i p l e s o f o p e r a t i o n / t h e o r y
r
The potential of each electrode is measured relative to a fixed, stable
voltage established by the double-junction silver/silver chloride refer-
ence electrode. An ion selective electrode develops a voltage that varies
with the concentration of the ion to which it responds. The relationship
between the voltage developed and the concentration of the sensed ion
is logarithmic, as expressed by the Nernst equation.
Nernst equation:
nF
E = E° + RT Log (γ C)
where:
E = The potential of the sensor in sample solution
E° = The potential developed under standard conditions
RT/nF = A temperature-dependent “constant” termed the slope, S
n = 1 for sodium and potassium; 2 for calcium
Log = Base ten logarithm function
γ = Activity coefficient of the measured ion in the solution
C = Concentration of the measured ion in the solution
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