When any type of chlorine is added to water, it forms hypochlorous acid (HOCl - the most powerful killing form of chlorine in water) and hypochlorite ion (OCl-), a relatively weak form of chlorine in water. The percentage of HOCl and OCl- is determined primarily by the pH of the water.

As pool water pH rises, less of the chlorine is in the killing form and more of the chlorine is in the weaker form. The combined total of HOCl and OCl- is the measure of free available chlorine. Free available chlorine is the active, killing type of chlorine that we want in the water.

Chlorine molecules can combine with ammonia and nitrogen in the water to form chloramines, sometimes also called Combined Chlorine. By combining with ammonia and nitrogen, a chloramine loses most of its sanitizing power. Chloramines are 60 to 80 times less effective than an uncombined free chlorine molecule. They also give off a strong chlorine odor, as they gas off the surface, whereas free chlorine do not.

Chloramines are formed continuously with contaminants in the water. Some of these contaminants or compounds are introduced into the water by swimmers and bathers in the form of skin and hair products, cosmetics, perspiration, urine, saliva, sputum and tiny amounts of fecal matter. An active swimmer sweats one pint per hour, while the average person sweats three pints per hour in a heated spa.

Ammonia, phosphate and nitrogen compounds are also introduced into the water by rain, especially acid rain, or in smoggy, foggy areas or pools near heavy car traffic or low flying overhead planes, near a major airport. Each drop of rain has some dissolved nitrogen from our atmosphere and of our earthly emissions. Many lawn and garden products contain nitrogen and ammonia and phosphates, all very bad for pools; can raise your chlorine demand, and encourage algae growth. These can blow into the pool during application, or may wash into the pool during a heavy rain storm.

Chloramines, or combined chlorine smell bad, they are eye and skin irritants, and they get in the way of free chlorine trying to do its job. When a pool smells strongly of Chlorine, what smells is not free available chlorine, but chloramines. The ironic solution to the problem of a strong chlorine smell is to add more chlorine to the pool, much more, to break apart the molecular bond of the chloramine. Shocking the pool to remove chloramines also has the side benefit of oxidizing every other pathogenic contaminants in the water, disinfecting and essentially sterilizing the pool water.

When testing for Free and Total Chlorine with a DPD or other capable pool test kit, the level of combined chlorine molecules in your pool can be detected. The formula is the difference between the Free Chlorine reading and the Total Chlorine reading = the Combined Chlorine level. Subtract FC from TC, and we have the formula: FC-TC=CC.

Anything over 0.3 ppm should be treated to bring the level down by removing the Chloramines from the pool (see below). For most DPD test kits, this is a slightly discernable darkening of the Free chlorine sample when you add DPD reagent #3. If you don’t see the sample darken at all when you add the third reagent, then your level is very low, and shock treatment to break apart combined chlorine is not needed.

A DPD test kit will test for all both Free and Total chlorine levels. OTO test kits, typically use a yellow reagent and will only calculate Free chlorine levels, and thus will not allow you to test for chloramines. Some pool test strips may test for Free and Total chlorine, but their interpretation is difficult and the results dubious.

The most accurate methods to test for chloramines is to use the DPD-FAS method in the Taylor K2006 test kit, which uses a titration method (counting dropwise), or advanced digital readers like ColorQ Pro, which will display Free, Total and Combined chlorine levels, calculated with the photometric accuracy.

Regular DPD Test Kits that measure both Free and Total chlorine starts with the Basic DPD test kit, the Taylor K1004. The industry standard DPD test kit however, is the Taylor K2005 which performs 9 tests and FC/TC from 0.5-5.0 ppm. Taylor K2105 does the same 9 tests, but measures Low Range chlorine readings, 0.25-2.5 ppm, for pools with supplemental sanitizers such as minerals, ozone or UV systems.

A simple way to test for chloramines is to simply follow your nose – it sounds silly, but if you pay attention to the smell of your pool water, by scooping up a handful and sniffing it each time you test the water; you may notice a stronger chlorine smell as chloramine levels increase, all else being constant.

Chloramines can be removed from pool water by the following four methods:

1. By adding a high dose of chlorine, liquid or granular chlorine. Raise the level of chlorine 10-20x the level of combined chlorine, and hold it there for 4 hours. This threshold of "breakpoint chlorination" must be reached for total oxidation to take place. If you don’t use enough chlorine to reach breakpoint, more chloramines can be produced. Swimmers should not enter the water until the level of chlorine has dropped to 5 ppm or less.
2. By adding a non-chlorine shock, aka MPS to the water. The most common chemical used for this is monopersulfate. This "shocking" requires the addition of one pound of shock for each 10,000 gallons of pool water. The same threshold of breakpoint oxidation must be reached when using non-chlorine shock. As such, add a little extra just to be sure. Non-chlorine oxidizers will break apart the chloramine bond, as well as remove other contaminants from the pool.
3. By adding ozone to the water. If an ozonator is installed on a pool or spa, then oxidation of the ammonia and nitrogen compounds will take place whenever the ozone system is operating. The longer the system operates, the more the ozone can destroy the ammonia and nitrogen. Although most ozone systems operate only when the pool or spa pump is operating, there are 24 -hour systems available which will continuously oxidize ammonia and nitrogen as they enter the water.
4. By adding Zeolite sand to a sand filter, to replace regular filter sand. ZeoSand contains the positively charged mineral zeolite which traps ammonia inside the filter, which is removed during backwashing. Less ammonia in the pool water means less chloramine formation.

Remember, when you smell a strong chlorine odor in a pool - and your eyes are red, it's not because there is too much [free] chlorine in the water, but too much [combined] Chlorine. Free chlorine by itself does not smell, nor sting the eyes. The solution to this problem is to add a whole lot more chlorine, to reach breakpoint chlorination, where the molecular combinations of chlorine and nitrogen [or ammonia] will be destroyed.

If you have continued problems with rapid build-up of chloramines in the pool, look for the source of the contamination. It could be bathers, human or animal, trees or mulch, rain or wind, bringing in ammonia or nitrates – assuming good filtration and water balance and constant sanitation, which is also very important to preventing chloramine formation.

Phosphates can reduce the amount of HOCL that is actively working, and allow the more susceptible OCl- to form compounds more easily. You can test for phosphates with a test strip, and treat with phosphate remover chemicals. Phosphates enter the pool from a variety of sources, and are naturally occurring in nature. Rapid increase in pool levels are usually from air pollution or contamination from garden, lawn or agricultural fertilizers.

Chlorine Shock

ColorQ

Test Kit

Basic DPD Test Kit

Complete DPD Test Kit

FAS-DPD Test Kit

Non-chlorine Shock

ZeoSand