Chlorine testing DPD method

To apply the correct chlorine dosage is a key factor when talking about pool water treatment. Among all existing measuring methods, the DPD is the most common one.

The DPD method was first introduced in 1957 by Dr. Palin, a German chemist and the acronym stands for N,N-diethyl-p-phenylenediamine. The methodology is quite simple: the reaction between the chlorine and the DPD reagent is measured in terms of the color depth.

Chlorine is a powerful disinfectant, and thus it is usually applied in swimming pool disinfection. It is so effective because it is able to kill bacteria and microorganisms in the water. A common task of pool managers is to keep its levels under control in oder to prevent infections. The delicate part is to minimise eye irritation without compromising a complete disinfection.

Swimming Pool Aqua and more

Swimming Pool Aqua and more

How do I keep the chlorine level as low as possible?

Let’s start from the beginning! There are three different types of chlorine in swimming pool water: free chlorine, combined chlorine and total chlorine. Total chlorine is the sum of free chlorine and combined chlorine.

Free chlorine refers to the amount of chlorine that is still available to disinfect your pool water. That’s to say, the chlorine that can effectively sanitize contaminants and thus get rid of bacteria and microorganisms.
When chlorine binds to any type of contaminants, it results in combined chlorine. It develops when the pool is being sanitized and is responsible for the characteristic odor of indoor pools.
The total chlorine is the sum of the both free chlorine and combined chlorine.

The level of total chlorine should always be greater than or equal to the level of free chlorine. By keeping the right amout of free chlorine, a clean and fully disinfected pool will have no combined chlorine at all.

How effective is a DPD method?

A DPD test kit will help you measure the pH, free chlorine and total chlorine levels by comparing the resulting pink color with a color scale. The DPD method is a very effective way of measuring the chlorine level in our pool. Nonetheless, it’s important to bear in mind that it does not only react with chlorine but also with other oxidizing agents used in pool water treatment such as ozone or bromine.

There are other reasons that could lead to reading wrong chlorine levels: e.g. unclear water can result in a darker color, the photometer could be wrongly calibrated or the cuvette could be dirty or even an expired test reagent.

In order to avoid misleading measurements, it is important to get at least 5 different samples by rejecting the highest and the lowest values and calculating the median of the other three.