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:
The principal tablets of the Palin system are numbered 1 to 4 as follows in Table 1 (below)
Additional tablets used in supplementary procedures are given below:
The DPD tablets listed 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|
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.
|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
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 PLUS bromamine
|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 + 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 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 & 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|
|Ozone only (DPD 4)||Result mg/L ozone|
24th October 2019
In 2017, the first ever Swimming Pool technician course was introduced at a school near Milan. During the course students were introduced to the Pooltest 10.
20th September 2019
With spa and hot tub use increasing, we’ve put together a handy spa chemistry guide with all the information you need to manage the water chemistry of your spa. Download it here.