Thermo Fisher Scientific Ion Selective Electrodes Iodide - Manual (Page 12)

Instruction Manual Iodide Electrode
12
Electrode Storage
The iodide ion electrode may be stored for short periods of time in 1.0x10
-2
M iodide solution. For
longer storage (longer than two weeks), rinse and dry the sensing pellet and cover the membrane tip
with any protective cap shipped with the electrode. The reference portion of the combination
electrode (or the outer chamber of the reference electrode) should be drained of filling solution, if
refillable, and the rubber insert placed over the filling hole.
ELECTRODE THEORY
Electrode Operation
An iodide ion electrode consists of a sensing membrane bonded into a glass or an epoxy body. The
membrane is composed of silver iodide/silver sulfide. When an electrode potential develops across
the membrane, the electrode is in contact with solution containing iodide ions and is capable of
measuring free iodide ions. This electrode potential is measured against a constant reference
potential, using a standard pH/mV meter or an ion meter. The level of the iodide ions,
corresponding to the measured potential, is described by the Nernst equation:
E = Eo - S log X
where:
E = measured electrode potential
Eo= measured potential (a constant)
S = electrode slope (
∼57 mV/decade)
X = level of iodide 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 if all 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 is
added to samples and standards. The recommended ISA for iodide ions 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 the iodide 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.