What Is the Difference in Chlorine & Free Chlorine?

Swimming pools require regular maintenance that includes scrubbing pool surfaces, maintaining pool filters and adjusting pool chemistry. Pool chemistry involves testing and adjusting the levels of various chemicals in the pool. Chlorine is used to kill bacteria and algae to keep the swimming pool water clean and healthy. To properly adjust the chemistry of your pool water, you must know the difference between the terms "free chlorine" and "combined chlorine."

Keeping your pool water sanitized requires an understanding of how chlorine works.

What Is Chlorine?

Chlorine, Cl, is a chemical that works as a water disinfectant to kill bacteria and algae in swimming pools. Roughly, swimming pools require about 2 to 3 cups of granular chlorine for every 50,000 gallons of pool water or half to three-fourths of a cup of liquid chlorine for every 50,000 gallons of pool water. These amounts depend on how heavily the pool is used, rainfall and other conditions. Free chlorine should be maintained at 1 to 3 parts per million according to the North Carolina Department of Environmental Health site. Combined chlorine should be under 0.2 parts per million.

Free Chlorine

Free chlorine is chlorine that is available to combine with contaminants in the water to disinfect and sanitize the water. Chlorine that is added to swimming pools is "free" chlorine in that it has not yet bonded with nitrates, ammonia or other compounds in the dirty pool water. Chlorine comes in a granular form called calcium hypochlorite or in liquid form called sodium hypochlorite. Either form is used to combine with impurities that contaminate the pool.

Combined Chlorine

Combined chlorine is the chlorine after it has bonded with contaminants. In this state, it is no longer able to continue its sanitizing effect. It has done its job and must wait for additional chlorine to come in to continue to disinfect the water. This combined chlorine, called chloramines, is what gives swimming pools their characteristic chlorine odor. Though people think this condition is caused by too much chlorine, it actually means there is not enough chlorine, according to PoolSpa writer David Dickman.

Total Chlorine

Total chlorine is the total amount of chlorine in the pool, that is, the sum of both the free chlorine and the combined chlorine that holds the dirt, oils and other compounds that make pool water dirty and foul-smelling. As the free chlorine is used up and combined with contaminants, more chlorine must be added. When water problems arise in swimming pools, such as algae growth or cloudiness, pool owners must "superchlorinate" the pool, which means adding 10 times the normal amount of chlorine generally used. This kills all the pool's bacteria, which is then extracted through the filter.