Hanna Instruments HI 4008 - 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 4108 or HI 4108 cupric electrodes are potentiometric
devices used for the rapid determination of free cupric ions
in samples and as a detector for the titration of cupric with
EDTA.
Cupric 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 4008 requires a separate reference electrode to complete
its electrolytic circuit. The HI 4108 incorporates a reference
electrode. The mixed cupric sulfide/ silver sulfide mem-
brane produces a potential change due to changes in the
sample
’s cupric ion activity. When the ionic strength of the
sample is fixed by the addition of ISA, the voltage is pro-
portional to the concentration of cupric ions in solution 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 ion (2+)
A
i
=ion activity in sample
T= absolute temperature in K
F= Faraday constant (9.648 x 10 4 coulombs/mole)