“Combined Chorine” is a generic chemical term for chemical compounds formed from the reactions of chlorine with organic and biological contaminants in pool water. Chlorine, and specifically the active chlorine compound hypochlorous acid (HOCl), is a very powerful oxidizer. Oxidizers, in chemical terms, take electrons away from the molecules that they oxidize and, in the process, break those molecules down into different compounds. The most common forms of combined chlorine found in swimming pools are those compounds where chlorine has reacted with nitrogen-containing chemicals in human bather waste (sweat, urine, etc). These chlorine-nitrogen compounds are called “chloramines” and there are three primary types - monochloramine, dichloramine and nitrogen trichloride. Chlorine can also react with urea and form monochlorourea and related compounds. There are many other types of chlorinated organic compounds that can form and not all of them show up on a combined chlorine test. Combined chlorine compounds are extremely irritating to mucous membranes and have fairly low odor thresholds. Combined chlorines are responsible for that “gross chlorine pool smell” and that smell is a good sign that the pool water it is coming from is not being properly maintained. Properly chlorinated pool water with the recommended levels of free chlorine and low to no combined chlorine has absolutely no smell to it…the nose knows, as they say!
High quality pool water test kits test for both FC and CC. Because of the nature of the CC test, it is mostly sensitive to the simple chloramines but not all chlorine-containing organic compounds. Because many CCs are further oxidized and destroyed by UV light and because they are quite volatile compounds (they will naturally outgas from pool water), the CC concentration in a clean swimming pool should be very low (< 1ppm and very often near 0). It is possible to detect high levels of CCs temporarily in pool water and the CC level will fluctuate from time-to-time but any prolonged measure of CCs above 1ppm is indicative of a problem with the pool water being tested. Water that is suffering from an algae and being treated to destroy algae or water that has had a high bather load will often generate lots of CCs while the free chlorine is disinfecting pathogens and oxidizing bather waste. Eventually, though, the CC’s of a clean pool should be below 0.5ppm most of the time.
Some chemical compounds, such as potassium monopersulfate (a common non-chlorine shock used in hot tubs) or sulfamic acid (an organic acid used to remove calcium and metal scale) will show up on the chlorine tests as CCs.