What would cause positive results using DPD when no chlorine is present in the sample?

Document ID

Document ID TE197

Published Date

Published Date 08/16/2018
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Question
What would cause positive results using DPD when no chlorine is present in the sample?
Summary
Positive interferences to Chlorine methods
Answer
Other oxidants such as bromine, iodine, ozone, chlorine dioxide, or hydrogen peroxide can react with DPD and cause false positives.

The most common interferent is oxidized manganese, which can be corrected for by treating the sample with potassium iodide and sodium arsenite.

Sunlight can react with the DPD indicator during the 3-minute reaction time for total chlorine; keep the sample covered during the reaction time if testing outdoors.

If the Ultra-Low Range (ULR) method is utilized on a colorimeter or a spectrophotometer, confirm thata reagent blank using deionized water has been determined and subtracted from the results of analyzing the sample.

It is also a good idea to use the same sample cell for zeroing the instrument and reading the sample concentration. This avoids any effects that might be due only to optical differences between the zero and read cell.

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