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 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 right), 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)
For the most effective monitoring of free and total chlorine, we recommend Kemio™ Disinfection. The latest multiparameter platform from Palintest, the Kemio™ Disinfection tests for bromine, free chlorine, total chlorine, chlorine dioxide, chlorite and peracetic acid (PAA).
Giving repeatable and reliable results for free chlorine in 60 seconds, Kemio™ guides the user and prompts action when concentration levels fall outside of your defined limits. To find out more about how Kemio™ Disinfection can help you, click here.
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15th November 2022
In summer, Product Manager for our Wagtech product range, Shaurya Sheoran, visited a WaterAid field site in India to discuss the current challenges on water quality faced by rural parts of India and how our kits are helping them achieve their mission of providing safe drinking water.
31st October 2022
Last month Product Manager for our Wagtech range Shaurya Sheoran and Technical Support Specialist Jessica McKenna provided some water quality training sessions as part of the RedR/ Cranfield University borehole drilling course. The event is a week-long training course covering well siting, drilling supervision and water quality sampling.
31st October 2022
Last month we closed our offices globally for one day to allow our employees some well-deserved rest. Our designated wellbeing day took place on 7th October 2022 and was day off work for all Palintest employees to relax and recharge in whatever way they felt appropriate. We have received some great stories of how our employees chose to spend this extra time away from the business to relax and focus on their physical, mental, and emotional state.