A guide for DPD testing in pools and spas

Pool operators need to be confident that the disinfectant is correctly dosed and controlled, for both the health and comfort of the bathers.

Disinfection of swimming pools is usually achieved using chlorine. Adequate free chorine is required to prevent infections, while eye irritation is minimised by keeping the combined chlorine residual as low as possible. Read this article to understand the difference between free and total chlorine.

The well-established DPD method, developed by Dr A T Palin (originally called the Palin system), is the test procedure of choice for free, total and combined chlorine as well as bromine and ozone, due to its consistency and reliability.

DPD Reagents

Using DPD reagents in tablet form is the easiest way to test using DPD. There are four main DPD tablets which can be used when measuring chlorine or alternative disinfectants (such as bromine and ozone).

  • DPD 1 measures free available chlorine
  • DPD 2 (use with DPD 1) measures combined chlorine
  • DPD 3 (use with DPD 1) measures total chlorine
  • DPD 4 measures total available chlorine

Chlorine

Chlorine is used as a primary disinfectant because of its effectiveness in killing potentially harmful organisms. It forms hypochlorous acid in the pool water. The effectiveness of this is influenced by the other chemicals that are in the pool, primarily pH and the amount of bather waste.

  • Hypochlorous acid that has not reacted with bather waste is free chlorine and must be carefully monitored to guarantee a safe pool.
  • Hypochlorous acid that reacts with bather waste forms combined chlorine which is a less effective disinfectant than free chlorine. High levels of combined chlorine can also cause eye and nose irritation and an unpleasant smell. Combined chlorine results from the reaction between ammonia-based compounds from the bather waste and chlorine to form monochloramine, then dichloramine.

To measure combined chlorine only, you need to subtract the value for total available chlorine (using DPD 3) from the value for free chlorine (using DPD 1). Chlorine reagents can be viewed here.

Bromine

Bromine can be used as a disinfectant alternative for chlorine, especially in spas and hot tubs. The bromamines which are formed when bromine is added to pool or spa water are as effective as free chlorine in killing pathogenic microorganisms, therefore total bromine is the key parameter to monitor.

To test for free bromine and bromamine, you use DPD 1. There is no need to differentiate between both as both have disinfectant properties.

Ozone

Ozone is also used as a primary disinfectant and must be sufficiently dissolved into the pool water to carry out the required oxidation and disinfection. Although ozone is a powerful disinfectant, adequate residuals of ozone must be maintained in the pool water to ensure full control. It is very often used with chlorine or bromine as a secondary disinfectant due to the difficulty in maintaining constant residual.

To test for ozone using DPD, only DPD 4 is required. To view ozone reagents click here.

To find out more about pool and spa management download our Pool Chemistry Guide or our Spa Chemistry Guide. These guides include definitions of important parameters, a guide to the recommended levels and the consequences of poor water monitoring.

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