3
III.
III.
III.
III.
III. Theory of Operation
Theory of Operation
Theory of Operation
Theory 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)
