Thermo Fisher Scientific CyberScan pH 5000 - Manual (Page 138)

Instruction Manual CyberScan pH5500/ 5000
15.5 ISE Theory
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 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 ( < 1 ppm).
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
millivolts potential is proportional to the activity of the ion of interest,
and the relationship is defined by the Nernst equation:
E* ± (log a)
RT
nF
E = 2.3
where
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 background as the sample, the difference between
activity and concentration becomes negligible. Most often this is
achieved by adding an ionic strength adjustor to both sample and
standards. The Nernst equation becomes:
E* ± (log c)
RT
nF
E = 2.3
c is the concentration of the ion of interest.
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