User Guide

10
METHOD OF ANALYSIS
Amperometric titration involves measuring the electrical cur-
rent flow between two electrodes, usually platinum, immersed
in a known quantity of a sample solution which contains an
unknown concentration of the chemical to be measured. The
titration of the chlorine with the reducing compound
Phenylarsine Oxide (PAO) is an application of this technique.
When a small potential is applied across the two platinum
electrodes of the titrator probe immersed in the solution con-
taining Free Chlorine, a small electrical current will flow. The
reversible reaction Cl
2
+ 2e
-
2 Cl
-
occurs at both elec-
trodes as the reducible form is oxidized at the anode and the
oxidized form is reduced at the cathode.
FORWARD TITRATION
The gradual addition of the reductant PAO (titrant), in an
environment buffered at pH 7, irreversibly reduces the oxi-
dized form of the Chlorine present. The reaction it undergoes
is:
PhAsO + Cl
2
+ 2H
2
O PhAsO(OH)
2
+ 2Cl
-
+ 2H
+
(Ph =phenyl) (a)
The final removal of all oxidized Chlorine terminates the re-
versible reaction and the probe current goes to zero.
In the case of Chloramine determination, the pH is lowered
to 4 and potassium iodide is added to convert the chloram-
ine species to an equivalent amount of triiodide.
NH
2
Cl + 3I
-
+H
2
O + H
+
NH
4
OH + Cl
-
+ I
3
-
(monochloramine).
NHCl
2
+ 6I
-
+H
2
O + 2H
+
NH
4
OH + 2Cl
-
+ 2I
3
-
(dichloroamine).
The triiodide is titrated with PAO with the current change
measured amperometrically.
PhAsO + I
3
-
+ 2H
2
O 3I
-
+ PhAsO(OH)
2
+ 2H
+
(b)
By knowing the exact amount of the reductant added which
just extinguishes the probe current, the original concentra-
tion of Chlorine present in the sample may be calculated.
Required data for the calculation are: sample volume, re-