Thermo Fisher Scientific Ion Selective Electrodes Chloride - Manual (Page 16)

Instruction Manual Chloride Electrode
ELECTRODE THEORY
Electrode Operation
A chloride ion electrode is composed of a glass or an epoxy body and a silver chloride/silver sulfide
membrane. When the membrane is in contact with a solution containing chloride ions, an electrode
potential develops across the membrane. This electrode potential is measured against a constant
reference potential, using a pH/mV meter or an ion meter. The level of chloride ions, corresponding
to the measured potential, is described by the Nernst equation:
E = Eo - S log X
where:
E = measured electrode potential
Eo = reference potential (a constant)
S = electrode slope (~56 mV/decade)
X = level of chloride ions in solution
The activity, X, represents the effective concentration of the ions in solution. The activity is related
to the free ion concentration, Cf, by the activity coefficient, γ , by:
X =
γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½
Σ CxZx 2
where:
Cx = concentration of ion X
Zx = charge of ion X
Σ = sum of all of the types of ions in the solution
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient,
γ , is constant and the activity, X, is directly proportional to the concentration.
To adjust the background ionic strength to a high and constant value, ionic strength adjuster (ISA)
is added to samples and standards. The recommended ISA for chloride is NaNO
3
. Solutions other
than this may be used as ionic strength adjusters as long as ions that they contain do not interfere
with the electrode's response to chloride ions. Samples with high ionic strength (greater than 0.1M)
do not need ISA added and standards for these solutions should be prepared with a composition
similar to the samples.
The reference electrode must also be considered. When two solutions of different composition are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur
from the inter-diffusion of ions in the two solutions. Electrode charge will be carried unequally
across the solution boundary resulting in a potential difference between the two solutions, since
ions diffuse at different rates. When making measurements, it is important to remember that this
potential be the same when the reference is in the standardizing solution as well as in the sample
solution or the change in liquid junction potential will appear as an error in the measured electrode
potential.
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