Hanna Instruments HI 4004 - 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 4004 and HI 4104 calcium electrodes are potentio-
metric devices used for the rapid determination of free cal-
cium ions in water, sea water and beverages. The electrode
functions as a sensor or ionic conductor. The HI 4004
requires a separate reference electrode to complete its elec-
trolytic circuit. The HI 4104 is a combination electrode with
a Ag/AgCl reference electrode with gel stabilized Cl
-
electro-
lyte in it’s inner chamber. The external reference chamber
is refillable. The PVC membrane used on the sensor is
impregnated with the organic ion exchanger.
This organic ion exchanger is considered a carrier ionophore
in that it is capable of shielding and carrying the charged
calcium ion in it’s polar cage freely through the apolar
regions of the membrane. A charge imbalance developes
between the test solution and internal cell of the sensor.
This voltage changes in response to the sampleís ion activ-
ity. When the ionic strength of the sample is fixed, the
voltage is proportional to the concentration of nitrate ions in
solution. The sensor 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 J/K Mol)
n = Charge on ion (2+)
A
ion
= ion activity in sample
T = absolute temperature in K
F = Faraday constant (9.648 x 10
4 C/equivalent)