User Guide

11
ductant concentration and the activity ratio of the reductant
to the measured substance.
BACK TITRATION
For waters which contain potential chemical interferences or
low concentration of Total Chlorine, a back-titration is rec-
ommended. In the back-titration procedure, a known excess
amount of PAO is added to the sample at pH 4 with an
excess of iodide. The PAO reacts with the free chlorine and
chloramines present. The amount of unreacted PAO is ti-
trated with an iodine solution. A blank back-titration is also
required. The total chlorine is then calculated, based on the
PAO left in the sample.
The back amperometric End Point is signaled when free io-
dine (triiodide ion) is present, which is indicated by a current
flow between the electrodes (see chemical reaction (a) and
(b)).
The back-titration method is popular in wastewater laborato-
ries because:
• the sample chlorine can be “fixed” at the sampling site
with the addition of excess reductant.
• Since the End Point is reversed, there is less interference
from iodine-demand substances in the sample.
END POINT DETERMINATION
At the point known as the End Point, abrupt change in the
slope of the current curve occurs and the titration is com-
plete. Typical titration plots for the forward and back
amperometric titration are shown in pictures below.
As the End Point is approached titrant has to be delivered in
small amounts, while microampere readings have to be re-