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Chlorine Analysis in Water – All about DPD Tablets

Written by Janice M on . Posted in Water & Environmental

DPD Tablets measure Chlorine levels, the most common form of disinfectant in water. It is widely used in swimming pools, drinking water and industrial process water.

When using Chlorine to disinfect water, the aim is to kill off the germs (pathogens) and then to leave a small amount of active (free) Chlorine in the water. This remaining Chlorine is also known as the “residual Chlorine” and is important as it is available to disinfect further contamination of the water.

Combined (total) Chlorine is the sum of the free Chlorine available in the water to disinfect future germs and the Chlorine that has already been already used and bound together with pathogenic molecules.

It is vital to calculate the correct amount of Chlorine to be added – too much is negative to health and wastes money, too little can result in a high level of disease causing pathogens. The Chlorine demand can be determined by deducting the residual from the amount of Chlorine added.

Lovibond® DPD tablets in use

Lovibond® DPD tablets in use

DPD1 measures free (residual) Chlorine, the Chlorine available to kill bacteria.

DPD2 measures Monochloramine, a less effective and less widely used disinfectant than Chlorine.

DPD3 measures combined Chlorine, so add DPD3 to the same sample as DPD1 and you will be able to calculate the total Chlorine.

DPD4 is used to measure total Chlorine only.

DPD tablets can accurately measure Chlorine levels from 0.001 mg / l to 10 mg /l . When a sample has a chlorine level that is higher than 10, the colour is bleached out of the Chlorine tablet – there is a quick show of pink colour and then the sample returns to its original colour. Dilutions can be done to determine high range Chlorine levels.

DPD reagents are produced on the basis of international standard methods and the ISO standard method. The chemicals are of a crystalline white nature. If a reagent (tablets or powder) is not white but has turned grey/brownish or purple in colour, it has deteriorated. The use of deteriorated products must be avoided as they will give false results.

They are used with a comparator or a photometer to produce a measurable scientific result.
Should you use a comparator or a photometer?

Lovibond® DPD tablet reagents

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Historical comments (6)

    • Janice


      Thank you for your comment. Please note that DPD tablets are not used for purifying water, but for analysing the level of chlorine in the water. Many treatment facilities analyse chlorine levels before treating the water.

  • David McGaughey


    I’m a little confused on the what are safe drinking water measurement levels, for example I got .17 on my initial reading of free chlorine with DPD No 1 tablet then when I added the DPD No 3 tablet (waited 2 mins as recommended) it came up as zero.. 0.00. So what does this actually mean, sorry first time using this.

    • Janice


      DPD no 1 are used to measure Free Chlorine in Water and these tablets can only read Free Chlorine to a level of 10mg/l as the Chlorine in the water bleaches out the pink colour the DPD no 1 tablets make in the water. There is thus only a flash colour change. It could be that the instrument picked up this flash colour change – thus the 17mg/l
      Drinking water reading should be < 5 mg/l free Chlorine. The fact that you got 0.0 mg/l for Combined Chlorine may be due to the fact the Free Chlorine reading was erroneous.

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