Hanna Instruments HI 4003 - Manual (Page 3)

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III. III. III. III. III. Theory of Operation Theory of Operation Theory of Operation Theory of Operation Theory of Operation:::::
The HI 4103 or HI 4103 cadmium electrodes are potentio-
metric devices used for the rapid determination of free cad-
mium ions in samples and as a detector for the titration of
cadmium with EDTA.
Cadmium sensors are “electrodes of
the third kind ” because they detect cations which also form
low-solubility salts with sulfide anions that also form low-
solubility salts with silver.
The electrode functions as a sensor or ionic conductor. The
HI 4003 requires a separate reference electrode to complete
its electrolytic circuit. The HI 4103 incorporates a reference
electrode. The mixed cadmium sulfide/ silver sulfide mem-
brane produces a potential change due to changes in the
sample
’s cadmium ion activity. When the ionic strength of
the sample is fixed by the addition of ISA, the voltage is
proportional to the concentration of cadmium ions in solu-
tion and the electrode follows the Nernst equation.
E= E
a
+ 2.3 RT/nF log A
ion
E= observed potential
E
a
= Reference and fixed internal voltages
R= gas constant (8.314 volt coulomb/K Mole)
n= Charge on cadmium ion (2+)
A
i
=ion activity in sample
T= absolute temperature in K
F= Faraday constant (9.648 x 10 4 coulomb/mole)