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:

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.

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.

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.

The Palintest DPD Method

The DPD indicator, an abbreviation of diethyl-p-phenylene diamine, is available under different forms: powder, liquid and tablets. Dr Palin spent the initial part of his career at Palintest developing the DPD system, firstly as a liquid and then as convenient tablets numbered 1 to 4. Tablets are easier and safer to handle for convenient dosing and optimum storage.

All forms of DPD react with pool water containing chlorine, bromine or ozone, coloring the water sample in various shades of pink: the deeper the colour, the higher the concentration of sanitiser. The DPD method is based on measuring the intensity of colours produced by the reaction between reagents in the DPD tablets with the sanitisers to be measured in the water. Visual colorimetric techniques will help to translate the intensity of colors into values using calibrated colour charts. However, photometers can digitally analyse the colour generating results with more accuracy by referring to calibration data stored in the instrument memory.

DPD Tablets

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

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 KI
for monochloramine activation
Combined chlorine
DPD 3 (use with DPD 1) Stabilised KI for dichloramine Total chlorine
DPD 4 All reagents in a single tablet Total available chlorine


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) Result 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


Bromine
Bromine PLUS bromamine (DPD 1) No need to differentiate between both as both have disinfectant properties


Bromine and chlorine
Bromine only (glycine + DPD 1) A result in mg/L bromine
Bromine + fee chlorine (DPD 1) B result in 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
Ozone only (DPD 4) Result mg/L ozone


Ozone & Chlorine
Ozone + total chlorine (DPD 4) A result mg/L ozone
Ozone only (glycine + DPD 4) B result mg/L ozone
Total chlorine Total chlorine = A - B
DPD testing in pools and spas

DPD testing in pools and spas

Application Support Information - when and how to use the DPD method in pool and spa water quality monitoring.

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