3
III.
III.
III.
III.
III. Theory of Operation
Theory of Operation
Theory of Operation
Theory of Operation
Theory of Operation:::::
The HI 4012 or HI 4112 lead electrodes are potentiometric
devices used for the rapid determination of free lead ions in
plating baths and as a detector for the titration of sulfate or
thorium with lead perchlorate
.
Lead sensors are “elec-
trodes of the third kind” because they detect cations which
also form low-solubility salts with sulfide anions and forms
low-solubility salts with silver.
The electrode functions as a sensor or ionic conductor. The
HI 4012 requires a separate reference electrode to complete
its electrolytic circuit. The HI 4112 incorporates a reference
electrode. The mixed lead sulfide/ silver sulfide membrane
produces a potential change due to changes in the sample
’
s
lead ion activity. When the ionic strength of the sample is
fixed by the addition of ISA, the voltage is proportional to
the concentration of Lead ions in solution and the electrode
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 volt coulomb/K Mole)
n= Charge on ion (2+)
A
i
=ion activity in sample
T= absolute temperature in K
F= Faraday constant (9.648 x 10
4
coulombs/mole)
