Water treatment in pools. The DPD testing.

Dosing chlorine correctly is a key factor in water treatment in pools. Let’s have a look at the world’s most common method.

The DPD testing is one of the most common methods for measuring chlorine levels. The acronym DPD stands for N,N-diethyl-p-phenylenediamine. This method was first introduced by Dr. Palin, a German chemist, in 1957. It’s a simple procedure where we measure the colour depth of the reaction between the chlorine and the DPD reagent.

The use of chlorine in swimming pool disinfection is very usual and pool keepers need to control it very carefully in order to prevent infections while minimising eye irritation. How to make it possible? By keeping the combined chlorine residual as low as possible. No clue what I am talking about? Let’s take it step by step.

First of all, there are different types of chlorine present in pools. Swimming pool water generally contains three types of chlorine known as free chlorine, combined chlorine and total chlorine. You get the total chlorine by adding the free chlorine to the combined chlorine.

    • Free chlorine is the chlorine that is still available to disinfect your pool water.

    • Combined chlorine is the chlorine that has already been used to disinfect the water. It is thus combined with organic matter and responsible for the annoying smell of chlorine.

    • The total chlorine is the sum of the two.

Once the free chlorine has joined with microorganisms, its ability to disinfect decreases. Actually 25 parts of Combined Chlorine are needed to do the sanitising work of one part of Free Chlorine. The Total Chlorine level must be higher than the Free Chlorine level.

With the help of a DPD test kit in the form of tablets or drops, you can measure the pH, free chlorine and total chlorine levels by comparing the resulting pink colour with a colour scale.

Is the DPD method an effective way of controlling the water quality?

The DPD testing is indeed an effective method in order to measure the chlorine level in our pool. The only problem is that DPD not only reacts with chlorine but also with other oxidizing agents such as bromine, iodine or ozone. Its levels in private pools are though almost non-significant.

There are a few issues that you could come up with when reading the chlorine levels:

    • Unclear water can result in a darker colour

    • A dirty cuvette

    • A wrong calibration of the photometer

    • Expired test reagent

In order to avoid misleading measurements, it is important to get a large enough sample. We recommend to take at least 5 different samples, reject the highest and the lowest one and calculate the median of the 3 remaining values.

Follow these simple steps in order to keep your pool disinfection under control and enjoy your bathing season without eye irritation.