Hanna Instruments HI 4010 - 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 4010 or HI 4110 fluoride electrodes are potentio-
metric devices used for the rapid determination of free fluo-
ride ions in water, soft drinks, wine, emulsified foods, and
plating and pickling acids. The electrode functions as a
sensor or ionic conductor. The HI 4010 requires a separate
reference electrode to complete its electrolytic circuit. The
HI 4110 has a reference electrode incorporated in its de-
sign. The lanthanum fluoride crystalline pellet is practi-
cally 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,
the voltage is proportional to the concentration of fluoride
ions in solution and the electrode follows the Nernst equa-
tion.
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)