A guide for DPD testing in pools and spas

Pool operators need to be confident that the sanitiser concentration is correctly dosed and controlled, for both the health and comfort of bathers. Disinfection of swimming pools is usually achieved using chlorine. Adequate free chlorine is needed to prevent infections, while eye irritation is minimised by keeping the combined chlorine residual as low as possible. 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 but also bromine and ozone due to its consistency and reliability.

Disinfection treatments measured with DPD:

DPD Tablets

The principal tablets of the Palin system are numbered 1 to 4 as follows in Table 1 (to the right)

Additional tablets used in supplementary procedures are given below:

  • DPD Acidifying
  • DPD Neutralising
  • DPD Glycine

DPD tablets given in the Table 1 can also be used when measuring alternative disinfectants such as bromine or ozone. DPD methods can also be used to measure the different relevant sanitisers in pool water with the Palintest range of products.

Tablet Contents Parameter to Measure
DPD 1 DPD indicator plus buffer Free available chlorine
DPD 2 (use with DPD 1) DPD 2 (use with DPD 1) Stabilised Kl for monochloramine activation Combined chlorine
DPD 3 (use with DPD 1) Stabilised Kl for dichloramine Total chlorine
DPD 4 All reagents in a single tablet Total available chlorine

DPD Tablets

The principal tablets of the Palin system are numbered 1 to 4 as follows in Table 1 (to the right)

Additional tablets used in supplementary procedures are given below:

  • DPD Acidifying
  • DPD Neutralising
  • DPD Glycine

DPD tablets given in the Table 1 can also be used when measuring alternative disinfectants such as bromine or ozone. DPD methods can also be used to measure the different relevant sanitisers in pool water with the Palintest range of products.

Chlorine
Free chlorine (DPD 1) Results in mg/L chlorine
Total available chlorine (DPD 4) Result in mg/L chlorine (or 1 and 3 together)
Free chlorine only (DPD 1)

Total available chlorine (DPD 3)

Combined chlorine only

A Result in mg/L chlorine

B Result in mg/l chlorine

Combined chlorine = B – A

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 whose effectiveness at disinfecting 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 in order to guarantee a safe pool.
  • Hypochlorous acid that reacts with bather waste forms “combined chlorine” which reduces its power to disinfect and is also associated with the unpleasant smell and irritation of eyes and nose that can occur in pools. ‘Combined chlorine’ results from the reaction between ammonia-based compounds from bather waste and chlorine to form monochloramine, then dichloramine.

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 and do not give off odours. Therefore, total bromine is the key parameter to monitor.

Bromine
Bromine PLUS bromamine

(DPD 1)

No need to differentiate between both as both have disinfectant properties

 

Ozone

Ozone is also used as a primary disinfectant and must be efficiently and 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 disinfection method due to the difficulty in maintaining constant residual.

 

Ozone
Ozone only (DPD 4) Result mg/L ozone

 

Bromine and chlorine
Bromine only (glycine + DPD 1) A result in mg/L bromine
Bromine + free chlorine (DPD 1) B result mg/L = 0.44*(B – A)
Bromine + Total Available Chlorine (Glycine + DPD 3)(need 1) C result in mg/L bromine
Total available chlorine Total chlorine = 0.44*(C – A)
Combined chlorine only Combined chlorine = 0.44*(C – B)
Ozone & Chlorine
Ozone + total chlorine (DPD 4) A result mg/L ozone
Ozone only (glycine) B result mg/L ozone
Total chlorine Total chlorine = A – B

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