INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 579
Volume of AgNO 3ConductanceConductanceConductanceConductanceVolume of HCLVolume of NaOH Volume of NH 3Titration of a salt of a weak acid Titration of the chloride ionTitration of a weak acid Titration of a weak acid(a) (b)(c) (d)FIGURE 33 Examples of conductometric titration curves. The first three curves a, b, and c represent acid/base titra-
tions and curve d a precipitation titration.conductance by developed empirical factors.^2 Conductivity is
also used to control plant processes, such as fluoride additions
to drinking water and coagulant dosage control.^97(e) Coulometric methods^98
Coulometry embraces three methods coulometric titra-
tions of constant-current (amperostatic) coulometry, constant-
potential (potentiostatic) coulometry, and electrogravimetry.
In these methods the oxidation or reduction of an analyte to a
new species or the reaction of an analyte with an electrolyti-
cally generated substance takes place. In the two coulometric
methods the number of coulombs, necessary to directly or
indirectly affect and electrochemical change of the analyte or
a species that reacts with the analyte, is determined. In elec-
trogravimetry the analyte undergoes an electrolytic reaction
and the new species is deposited on an electrode and measured
gravimetrically. The analyte concentration is calculated from
a knowledge of the chemical composition of the deposit.(1) Electrogravimetry
The analyte, metallic cations, halide ions, etc., are com-
pletely removed from the solution by plating on an inertelectrode, usually platinum, in electrogravimetry. The
increase in electrode weight is due to a species containing
the reduced or oxidized form of the analyte. Electrolytic
deposits can be pure metals on the cathode or compounds
such as lead peroxide, thallium oxide and manganese diox-
ide on an anode, and silver halide salts on a silver anode.
Complete removal of the analyte by electrolytic deposi-
tion from solution requires an unpolarized process where
large electrodes and vigorous stirring are used. Two condi-
tions are necessary concerning the electrolytic deposit: The
deposit must adhere well to the electrode so that an accurate
weight of the plated substance can be obtained, and a pre-
dictable, constant chemical composition of the deposit is
required. The composition of the electrolyte solution does
have, at times, a dramatic effect on the nature of the deposit.
Substances are added to the electrolysis solution to improve
the adherence of the plated substance to the electrode.
Electrolysis in electrogravimetry can be achieved under two
conditions—constant current through the cell or at a con-
trolled working electrode potential.
Constant-current electrolysis has the advantage of a con-
stant rate of reaction and is faster than the controlled-potentialC009_005_r03.indd 579C009_005_r03.indd 579 11/23/2005 11:12:28 AM11/23/2005 11:12:28