Thermo Fisher Scientific Ion Selective Electrodes Calcium - Manual (Page 14)

Calcium Electrode Instruction Manual
14
Electrode Storage
The calcium electrodes may be stored for short periods of time in 1.0X10
-2
M calcium standard. For
longer storage (longer than two weeks), rinse and dry the calcium membrane and cover the tip with
any protective cap shipped with the electrodes. 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
Eutech Calcium Electrode consists of an electrode body containing an ion exchanger in a sensing
module. This sensing module contains a liquid internal filling solution in contact with a gelled
organophilic membrane containing a calcium selective ion exchanger.
An electrode potential develops across the membrane when the membrane is in contact with a
calcium solution. Measurement of this potential against a constant reference potential with a digital
pH/mV meter or with a specific ion meter depends on the level of free calcium ion in solution. The
level of calcium 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 (
∼27 mV/decade)
X = level of calcium ions in solution
The activity, X, represents the effective concentration of the ions in solution. Total calcium
concentration, Ct, includes free calcium ions, Cf, plus bound or complexed calcium ions, Cb. Since
the calcium electrodes only respond to free ion, the free ion concentration is:
Cf = Ct - Cb
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 on 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 calcium is potassium chloride, KCl.