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Chlorine and chlorine-release compounds are widely used for the disinfection of drinking water and swimming pools, for the control of micro-bioligical growth in cooling water, and in many other water treatment systems. Accurate measurement of the chlorine residual is as essential aspect of the control of these chlorination processes.

The chlorine level can be expressed in terms of the free chlorine, combined chlorine or total chlorine residuals. For the majority of aaplications measurement of the free chlorine residual is the most important. The YSI DPD chlorine method provides a simple means of measuring free, combined and total chlorine residuals over the range 0 - 5 mg/l.

It is recommended that if any shock treatment compounds are known to have been used in the treatment of the water to be tested, that a DPD Oxystop tablet to be included in the test procedure as outlined below.


- Can be used for 50 times of the free chlorine testing in the water

- Can be use on YSI Photometer either 9300 and 9500


1 Package

Product Application

1. Rinse test tube with sample leaving a few drops of sample in the tube

2. Add and then crush the DPD #1 tablet in the few drops of the water sample until the tablet is thoroughly crushed

3. Add the 10 ml test solution, mix and seal the tube with the cap.

4. Select "PHOT 7" on Photometer.

5. Take Photometer reading in usual manner - see Photometer instructions.

6. The result represents the free chlorine residual in milligrams per litre (mg/l).


This YSI chlorine test uses the DPD method developed by Dr. A T Palin and now internationally recognised as the standard method of testing for chlorine and other disinfectant residuals. In the YSI DPD method the reagents are provided in tablet form for maximum convenience and simplicity of use.

Free chlorine reacts with diethyl-p-phenylene diamine (DPD) in buffered solution to produce a pink coloration. The intensity of the color is proportional to the free chlorine concentration. Subsequent addition of excess potassium iodide induces a further reaction with any combined chlorine present. The color intensity is now proportional to the total chlorine concentration; the increase in intensity represents the combined chlorine concentration. In this way it is possible to differentiate between free and combined chlorine present in the sample. The color intensities are measured using a YSI Photometer.

The DPD Oxystop tablet is added after measurement for free chlorine but before the DPD No 3 tablet. It prevents the reaction between stock treatment chemicals and potassium iodide which would give a positive response.

Expiration date

10 years after the manufacturing date