Hanna Instruments HI 4009 - 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 4009 or HI 4109 Cyanide electrodes are potentio-
metric devices used for the rapid determination of free Cya-
nide ions in effluent, for electroplating bath analysis and
as a detector for titration of cyanide with silver nitrate
.
The electrode functions as a sensor or ionic conductor. The
HI 4009 requires a separate reference electrode to complete
its electrolytic circuit. The HI 4109 incorporates a reference
electrode. The silver halide pellet is practically insoluble in
the test solutions being measured and produces a potential
change due to changes in the sample’s ion activity. When
the ionic strength of the sample is fixed by the addition of
ISA, the voltage is proportional to the concentration of Cya-
nide 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 (1-)
A
i
=ion activity in sample
T= absolute temperature in K
F= Faraday constant (9.648 x 10 4 coulomb/mole)