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Cadmium Electrode
interference from hydroxide is given by a dashed line to the left of the dotted area on Figure 4.
Within this dotted area, cadmium combines with hydroxide to form Cd(OH)
cadmium concentration can be measured with the cadmium ion electrodes, a false reading results.
Electrode Life
A cadmium electrode will last six months in normal laboratory use. On-line measurements might
shorten operational lifetime to several months. In time, the response time will increase and the
calibration slope decreases to the point calibration is difficult and electrode replacement is required.
Electrode Storage
The cadmium electrode may be stored for short periods of time in 1.0x10
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
The electrode consists of sulfides of cadmium and other metals bonded into an epoxy or glass body.
When an electrode potential develops across the membrane, the membrane is in contact with a
solution containing cadmium ions. The electrode potential is measured against a constant reference
potential, using a standard pH/mV meter. The level of cadmium ions, corresponding to the
measured potential, is described by the Nernst equation:
E = E o + S log X
where: E = measured electrode potential
E o = reference potential (a constant)
S = electrode slope (∼27 mV/decade)
X = level of cadmium ions in solution
The activity, X, represents the effective concentration of free cadmium ions in solution. Both
bound, C b , and free, C f , cadmium ions are included in the total cadmium ion concentration, C t . The
cadmium ion electrode will only respond to free cadmium ions, the concentration of which is:
C f = C t + C b
The activity, X, represents the effective concentration of the ions in solution. The activity is related
to the free cadmium ion by the activity coefficient, γ , by:
X = γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½ Σ C x Z x
2
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Instruction Manual
. Since only free
2
-2
M cadmium solution. For
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