Pool operators need to be confident that the disinfectants they use are 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.
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.
Understanding the difference between free and combined chlorine is crucial to understanding water quality after chlorination. Testing the water for both free and combined chlorine allows you to know whether the water you’ve chlorinated is of sufficient quality to drink or to disinfect your process, depending upon your application.
Pure chlorine exists as a molecule as a gas (Cl2 ) and when dissolved in water it will react with water to form mostly hypochlorous acid (HOCl):
Cl2 + H2O ↔ HCl + HOCl
Depending on the pH of the water (see image), the hypochlorous acid (HOCl) will partially dissociate to the hypochlorite ion (OCl–). Both hypochlorous acid and hypochlorite disinfect water but hypochlorous is acid is a more effective disinfectant.
By definition, free chlorine refers to all chlorine present in the water as Cl2(g), HOCl(aq) and OCl–(aq).
Portable water testing methods such as DPD do not distinguish between these three species, and they are all termed free chlorine, with the capability to disinfect microorganisms in the same way as Cl2. Free chlorine will often be denoted as Cl2 in water treatment applications and literature, but can refer to any of the three forms.
When free chlorine is initially added to water it can undergo a very quick reaction with other contaminants in the water, mostly ammonia, NH3. This will result in the formation of chloramines, and the ‘free’ chlorine is converted to ‘combined’ chlorine:
NH3 → NH2Cl → NHCl2 → NCl3
That is; ammonia → monochloramine → dichloramine → nitrogen trichloride (trichloramine)
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.
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.
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).
Bromine can be used as a disinfectant alternative to 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 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.