3. Water & Environmental / Rapid Tests

Measuring high levels of free chlorine in water

Measuring high levels of free chlorine can be problematic;

Colorimeters used to test free chlorine are based on a colour change test; The DPD 1 (N,N-diethyl-p-phenylenediamine) method for residual chlorine was first introduced by Palin in 1957. Over the years it has become the most widely used method for determining free chlorine in water and wastewater. Hach introduced its first Chlorine test based on DPD chemistry in 1973 . A DPD 1 tablet is added to the water sample and this causes a pink colour change. The intensity of the colour is then measured by the meter to give a reading in mg/L or ppm free chlorine.

High levels of combined chlorine (chloramines) can cause false positives with these kits. Low to moderate levels of combined chlorine are neutralized in most DPD  kits, but combined chlorine can occasionally build up to such a point that it seems to indicate a level of free chlorine is present when, in fact, there may be none.

chlorine standards

Another problem can be caused by high sanitizer levels, when the DPD indicator is “bleached out.” A flash of pink colour occurs when the indicator is added and then quickly becomes colourless. This can happen when testing is performed very soon after dosing, (the high levels of free chlorine are exhausting the levels of reagents added and the test does not reach a steady state).Clean water


Hach’s DPD powder reagents are quite stable when protected from moisture, light and temperature extremes. Excellent reagent stability is achieved by sealing the reagent in unit-dose foil pouches. AccuVac® DPD reagent ampoules are air-evacuated and hence are protected from oxidation and moisture. It is recommended that all DPD reagents, both liquids and powders, be stored between 10 to 25 °C (50 to 77° F).


To measure free and total Chlorine (total Chlorine is free and combined Chlorine) in water the Hach Pocket Colorimeter MKII is a simple handheld device in routine disinfectant applications. Small, lightweight and battery operated this meter is a true ‘go-anywhere’ instrument. It can accurately measure free Chlorine between 0.02-2.0 mg/L. To read a higher concentration of chlorine with the same pocket colorimeter you can use this sample cell 1-cm/10-ml, this changes the way the meter can measure the colour concentration and allows you to measure chlorine levels up to 8.0 mg/L/. This option is detailed on the Hach method sheet.



If high levels of free chlorine are suspected and you need to measure above 8.0 mg/L, there are other chlorine meters that use tablet reagents that can be chosen.

If you want a meter that can measure both high and low chlorine ranges the compact chlorometer duo will do the job; this tests for Chlorine (DPD) 0.01 – 5 mg/l, Chlorine HR, 1 – 250 mg/l), and is a compact fully waterproof meter that’s both simple to use and highly accurate. If you just need to measure chlorine at a high range the MD100 pocket colorimeter will measure in the range of 5-200mg/l.

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