Swimming pool water generally contains three types of chlorine commonly known as Free Chlorine, Combined Chlorine and Total Chlorine.
To understand the difference between the three types of chlorine, consider this simple formula:
FC + CC = TC
When you add chlorine to your pool, it reacts with the water to form hypochlorous acid and hypochlorite ion. These compounds together form what we call free chlorine. Free Chlorine is what we commonly test for to determine the proper chlorine levels in pool water and is the chlorine that is still available to sanitize your water.
Once this chlorine begins to react with the contaminants in the water, such as nitrogen and ammonia, it becomes combined chlorine also known as chloramines. Combined Chlorine is the chlorine that has already been “used up” sanitizing your water. While combined chlorine is in the water, its ability to sanitise is reduced. Combined chlorine can irritate mucous membranes, cause red and stinging eyes and irritate the respiratory system. The strong chlorine odour often smelled at poorly operated pools is caused by combined chlorine not free chlorine. If you notice your combined chlorine levels going up, you have to rebalance the sanitiser in the water by adding extra chlorine to your pool.
Total Chlorine is simply the sum of the free chlorine and combined chlorine.