3
III. III.
III. III.
III.
Theory of OperationTheory of Operation
Theory of OperationTheory of Operation
Theory of Operation
::
::
:
The HI 4013 and HI 4113 nitrate electrodes are potentio-
metric devices used for the rapid determination of free ni-
trate ions in water, emulsified foods and plant samples.
The electrode functions as a sensor or ionic conductor. The
HI 4013 requires a separate reference electrode to complete
its electrolytic circuit. The HI 4113 is a combination elec-
trode with a Ag/AgCl reference electrode with gel stabilized
Cl
-
electrolyte 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
nitrate 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 sample’s 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 (-1)
A
ion
= ion activity in sample
T = absolute temperature in K
F = Faraday constant (9.648 x 10
4
C/equivalent)
