What would cause a monochloramine measurement to be higher than total chlorine?

Document ID

Document ID TE3494

Published Date

Published Date 02/02/2022
What would cause a monochloramine measurement to be higher than total chlorine?
Monochloramine measurements are higher than total chlorine.
Total chlorine should always be equal to or greater than monochloramine measurements.

If monochloramine measurements are significantly greater than total chlorine (meaning the difference in values is not due to the precision of the test), the following should be considered:

High pH and alkalinity can cause low total chlorine values. If you check the pH of the sample after the DPD reagent is added, the pH should be approximately 6.3. If the pH is high, you should adjust the pH to somewhere between 6 and 7 and repeat the test. This is described in article titled, " What could cause free chlorine measurements to be higher than total chlorine?" One can also switch to a mid range total chlorine procedure as the mid range procedures are better with high pH and alkalinity.

Check to see if any turbidity forms during the test after reagent addition. If it does, the addition of the Hardness Treatement Reagent Powder Pillow, pk/50 (Product # 2882346) should help.

Check the temperature of the samples. Cold temperatures lead to longer reaction times.

Make sure chloramines are completely formed. Take a sample and let it sit for about 15 minutes and see if the values change from the initial test values.

A reagent blank can be done to reduce the monochloramine value.

If the sample cell is used for total chlorine and monochloramine methods, the sample cells need to be thoroughly rinsed between tests to avoid carryover.

Try inverting the sample cells in monochloramine test before inserting them into the instrument. Sometimes the powder is slow to dissolve and there is a layer of concentrated reagent in the lower part of the sample solution.

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