Hanna Instruments HI 4221 User Manual
Page 54
54
ISE THEORY
ISE THEORY
ISE THEORY
ISE THEORY
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
-1
*mol
-1
);
T
- the temperature in degrees Kelvin;
F
- the Faraday’s constant (96,485 C*mol
-1
);
n
- the ion charge.
The slope may be positive or negative depending upon the ion charge (n).
Activity and concentration are related by an “activity coefficient”, expressed as:
where: a - the activity of the ion being measured;
- the activity coefficient;
C
- the concentration of the ion being measured.
In very dilute solutions approaches 1 so activity and concentration are the same.
Actual samples that are more concentrated have much smaller activity coefficients ( <1). The addition of an
inert background salt to standards and samples stabilizes the activity coefficient so that concentration measurements
may be made directly. Hanna’s Ionic Strength Adjustment Buffer (ISAB) formulations also may optimize pH, and
complex interferences in addition to standardizing the ionic strength.
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