3
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)
