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ISE THEORY
An Ion Selective Electrode (ISE) is an electrochemical sensor that changes voltage with the activity or
concentration of ions in solutions. The change in voltage is a logarithmic relationship with concentration,
and is expressed by the Nernst equation:
where:
E
- the measured voltage;
E
o
- standard voltage and other standard system voltages;
a
- the activity of the ion being measured;
S
- the Nernst slope factor and is derived from thermodynamic principles:
R
- the universal gas constant (8.314 J/(K
⋅
mol));
T
- the temperature in degrees Kelvin;
F
- the Faraday’s constant (96,485 C/mol);
n
- the ion charge.
The slope may be positive or negative depending upon the Ion charge (n).
SPECIES
SLOPE
(mV/decade)
Monovalent cation
+59.16
Monovalent anion
–59.16
Divalent cation
+29.58
Divalent anion
–29.58
E
=
E
o
+
S
log(
a
)
S
= 2.303
RT
nF
Activity and concentration are related by an “activity coefficient”, expressed as:
where:
a
- the activity of the ion being measured;
γ
- the activity coefficient;
C
- the concentration of the ion being measured.
a
=
γ
·
C
ADDITIONAL INFORMA
TION
