Is it possible to use DPD Total Chlorine reagent to test for Free Chlorine? Is the opposite possible?

Document ID

Document ID TE5821

Published Date

Published Date 27/05/2017
Question
Is it possible to use DPD Total Chlorine reagent to test for Free Chlorine? Is the opposite possible?
Summary
Free chlorine with total reagent, or total chlorine with free reagent.
Answer
See also What is the difference between free chlorine and total chlorine? for background information.

Free chlorine reagent reacts immediately and directly with the free chlorine in the sample. If given enough time some combined chlorine forms will start to react with the indicator as well. This is why the procedure for free chlorine states to proceed to the next step immediately after the mixing step and that the reacted sample cell should be read within 60 seconds of reagent addition. If given enough time the free chlorine results will more closely resemble the total chlorine results as more and more combined chlorine forms react with the indicator, but no matter how much time is given not all combined forms will react with the indicator. So even if the sample is read hours after reagent addition, the results will always be lower than a true total chlorine result. Therefore it is not possible to get total chlorine results with free chlorine reagent.

Total Chlorine reagent is the free chlorine reagent with additional components. These additional components convert all combined chlorine forms to free chlorine, which then react with the indicator. The conversion of some combined forms to free chlorine take longer than others, which is why there is a 3 minute reaction time for the total chlorine test. But some combined forms will be converted to free chlorine immediately and then immediately after react with the indicator. So even if the sample is read immediately after reagent addition, the results will always be higher than a true free chlorine result. Therefore it is not possible to get a free chlorine result with total chlorine reagent.