EUTECH INSTRUMENTS CADMIUM EPOXY Instruction Manual page 15

Instruction Manual
where:
C x = concentration of ion X
Z x = charge of ion X
Σ = sum of 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 solution for the cadmium electrodes is
sodium nitrate, NaNO
ions that they contain do not interfere with the electrode's response to cadmium 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.
The composition of the liquid junction filling solution in the reference electrode is most important.
The speed with which the positive and negative ions in the filling solution diffuse into the sample
should be as nearly equal as possible, that is, the filling solution should be equitransferent. No
junction potential can result if the rate at which positive and negative charge carried into the sample
is equal.
Strongly acidic (pH = 0-2) and strongly basic (pH = 12-14) solutions are particularly troublesome to
measure. The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask
their effect on the junction potential with any concentration of an equitransferent salt. One must
either calibrate the electrode in the same pH range as the sample or use a known increment method
for ion measurement.
. Solutions other than this may be used as ionic strength adjusters as long as
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Cadmium Electrode