What is Comined Chlorine in a Pool?
“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).
What are Chloramines in Pool Water?
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.
Why Do CCs Test in Drinking Water
Chloramines that appear as CCs are added to municipal water in some areas.
Chloramines (also known as secondary disinfection) are disinfectants used to treat drinking water and they:
- Are most commonly formed when ammonia is added to chlorine to treat drinking water.
- Provide longer-lasting disinfection as the as the water moves through pipes to consumers.
Chloramines have been used by water utilities since the 1930s. More than one in five Americans uses drinking water treated with chloramines.
For more information see US EPA - Chloramines in Drinking Water.
What causes that Chlorine Smell in Pools?
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 have absolutely no smell to it…the nose knows, as they say!
How Do You test For Combined Chlorines?
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.
The CC concentration in a clean swimming pool should be very low (< 1ppm and very often near 0). Many CCs are further oxidized and destroyed by UV light and are quite volatile compounds (they will naturally outgas from pool water),
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 bloom, or being treated to destroy algae, or water that has had a high bather load will often generate high CCs while the free chlorine is disinfecting pathogens and oxidizing bather waste. Eventually, though, the CC’s of a clean pool should be below 0.5 ppm most of the time.
What are Other Sources of Combined Chlorines in Pool Water?
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.
Free Chlorine testing after using Sulfamic Acid is discussed in Acid#FC_Testing_After_Using_Sulfamic_Acid.
Potassium Monopersulfate Compounds aka MPS or "non-chlorine shock' will interfere with the Combined Chlorine test. Taylor Technolgies has the R-0867 Deox Reagent that will neutralize MPS effects on the CC test. R-0867 Deox Reagent is available in the K‑2041 (.75 oz) and K‑2042 (2 oz). It’s also available as a stand-alone kit, the K‑1518 FAS-DPD chlorine/monopersulfate test.
Why Do I Have Algae With a CC of 0?
Plants, such as algae, create chemicals like glucose from carbon dioxide and water. Using light, the carbon in carbon dioxide oxidizes the oxide into oxygen.
6CO2 + 6H2O --> C6H12O6 + 6O2
Carbon dioxide + water--> glucose + oxygen.
This is how plants make oxygen and store energy in sugars. Most of the bulk of plants is carbon from carbon dioxide. Most of the weight of a tree comes from the air.
The carbon in carbon dioxide is in the +4 state.
The carbon in glucose is 4 at 0, 1 at -1 and 1 at +1. The carbon is "reduced", which is the opposite of oxidized. Reduced just means that the oxidation state is lower or reduced because the atom gained electrons which are negatively charged.
When chlorine oxidizes the carbon in glucose back to a +4 oxidation state, it reverts back to carbon dioxide.
This is a similar process to an animal using oxygen to burn sugars to release energy and exhale carbon dioxide.
So, chlorine reacting with algae is mostly an oxidation reaction and not a combination. Algae is mostly converted back into carbon dioxide and water.
Chlorine can combine with carbon compounds, such as methane (CH4) by replacing the hydrogen ions.
Carbon in methane is in the -4 oxidation state.
Algae mostly does not create CCs.
CCs are mostly created by compounds like ammonia where the nitrogen is in the -3 state. For ammonia, you get combination and oxidation. So, the CCs eventually go away, especially in sunlight where UV photons knock loose electrons from the nitrogen and make it easier for the chlorine to take them.
Active chlorine is +1, so it bonds with more negatively charged atoms, like nitrogen in the -3 state or carbon in the -4 state.
The carbon in algae has a zero net charge.
So, the reaction of chlorine with algae is mostly oxidation and not combination.