Accumet XL 20, User Manual, Page: 187 / 199

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Appendix:
ISE Theory
XL25, 50 and 60 meters
Ion selective electrodes (ISE) respond to ionic activity or concentration in a liquid because of the
characteristics of the electrode’s sensing membrane. Ideally, the ISE develops an electrical
potential which is proportional to the activity or concentration of the ion for which the membrane is
selective. For example, the fluoride electrode can be used to directly sense fluoride ion in drinking
water. Analyses utilizing ion selective electrodes offer the advantages of speed, ease-of-
operation, low cost, and versatility over other analytical techniques. The ISE can measure the ion
concentration of samples at very high levels ( > 1000ppm ) to very low levels ( < 1ppm ).
When an ISE, the indicator electrode, and a reference electrode are placed in a solution, and
connected to a pH/mV/ion meter, they form a potentiometric cell. At equilibrium, the meter
measures the potential difference between the ISE and the reference electrode. This millivolt
potential is proportional to the activity of the ion of interest, and the relationship is defined by the
Nernst equation:
E = E* ±2.3 RT (log a) where
nF
E is the measured potential
E* is the sum of the standard potential and all other constant potentials of the system
R is the gas constant
T is the temperature in °K
F is Faraday’s constant
n is the valence of the ion being measured
Activity, a, is not the same as concentration. It is a thermodynamic function whose value is
influenced by the ionic environment in which the ion exists. Activity and concentration are the
same in very dilute, ideal solutions. In non-ideal solutions, activity and concentration are related
by an “activity coefficient“.
ISEs sense the activity of free, uncomplexed ions rather than concentration. However, by
calibrating the meter with standards with a similar ionic back-ground as the sample, the difference
between activity and concentration becomes negligible. Most often this is achieved by adding an
ionic strength adjustor to both samples and standards. The Nernst equation becomes:
E = E* ±2.3 RT (log c) where
nF
c is the concentration of the ion of interest.