User Guide

21
INTERFERENCES AND SOURCES OF ERRORS
Despite Standard Methods section 4500 Cl-A.3.b. states that
“the amperometric method is the method of choice because
it is not subject to interferences from color, turbidity, iron,
manganese, or nitrite nitrogen”, the amperometric method
will detect (as all of the common chlorine methods) disinfec-
tants such as bromine (Br
2
), Ozone (O
3
), Chlorine dioxide
(ClO
2
), and hydrogen peroxide (H
2
O
2
).
In general all oxidants which can be reduced by the strong
reducing agent PAO will interfere with the free chlorine deter-
mination. For the total chlorine determination, interference
can be caused by compounds that oxidize iodide to iodine
and those that can be reduced by PAO. For example, man-
ganese in the lower oxidation states +2, +3, or +4 can be
oxidized by the free chlorine. The oxidized formes of manga-
nese (+4 to +7) can be reduced by PAO in free chlorine
titration or manganese (+4 or +7) can oxidize iodide to
iodine during the total chlorine titration.
Hanna Instruments researchers found that nitrite interference
can cause either a positive or negative interference depend-
ing on the order of reagent addition.
Therefore the preferred procedure in the back titration for
Total Chlorine determination is buffering the solution to pH 4
before adding KI in order to minimize nitrite, manganese and
iron interference.
For both free and total chlorine determination, Hanna in-
struments has selected PAO as reducing agent because it
gives a sharper end point.
The potassium iodide used for the total chlorine determina-
tion can be oxidized with enough exposure to oxygen and
ultraviolet light. Therefore keep the bottle of HI 70468 tightly
closed and out of direct sunlight. Another possible error dur-
ing total chlorine determination is volatilization of free iodine.
Volatilization from the reaction mixture during the forward
titration is minimized because excess iodide is present, but
after adding the potassium iodide, start the titration as soon
as possible. Keep the standard iodine solution in a closed,
dark bottle to avoid volatilization of iodine.