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The most common disinfectants used to treat recreational water release chlorine, also known as hypochlorous acid. Often, these are used interchangeably. Hypochlorous acid is an effective killer of pathogens and algae while also oxidizing other materials. Because of its stability, it is also very good at leaving a residual concentration that can be maintained for hours or even days depending on the circumstances. Chemicals that chlorinate, or chlorinating agents, include sodium hypochlorite, calcium hypochlorite, lithium hypochlorite, chlorine gas, trichlor, and dichlor. These chemicals are divided into two categories; unstabilized and stabilized. Unstabilized chlorine can also be called inorganic, and stabilized organic because of the inclusion of a carbon atom. When any of those chemicals are added to water, the chemicals produces hypochlorous acid, the hypochlorite ion, the hydrogen ion, and a by-product specific to the type of agent used. For example, the by-product of sodium hypochlorite, commonly called liquid chlorine, and water is table salt. The hypochlorous acid is 60 to 100 times more effective than the hypochlorite ion at killing microorganisms. So how is it determined what is present? The pH of the water is the determining factor for the division. The relationship between pH and the hypochlorous acid and hypochlorite ion is shown on this chart. The molecules do not just choose what they are to be. The situation is fluid. Many times a second hydrogen ions are connecting and disconnecting, but the ratio always stays constant based on the water’s pH. As the water gets more acidic, there is more hydrogen present and therefore more hypochlorous acid. At a low pH, chlorine is very active and used up rather quickly. At a high pH, the hypochlorite ion is most prevalent and therefore there is not adequate disinfection. Notice the sweet spot created at a pH of 7.5. The hypochlorous acid and hypochlorite ion are balanced. Together, they are measured as free chlorine. Free chlorine is the active available disinfectant in the water and is written in shorthand as FC. As described on the last page, it is the sum of the hypochlorous acid and the hypochlorite ion. The common practice is to maintain the levels between 2 and 4 parts per million in pools. In the United States, the accepted minimum is 1 part per million and the maximum is under debate. From jurisdiction to jurisdiction, there are different maximums, and sometimes it is not defined by regulation. Product labels approved by the EPA set the limit at 4 parts per million, but many health codes set it at 5 parts per million or higher. The free chlorine is tested with a DPD test, which will be discussed in chapter 8. Combined chlorine is formed when free chlorine reacts with either ammonia or nitrogen compounds in the water. Chloramines, either inorganic or organic, are formed. A DPD test for combined chlorine does not distinguish between the two and a reading for total combined chlorine is produced. Combined chlorine is not an effective disinfectant and controlling it is a concern. Chloramines evaporate and cause the smell often experienced in indoor pools. The strong chlorine-like odor is not because there is too much chlorine in the water, but too many chloramines in the air. We will discuss mitigating combined chlorine in the pool and spa water problems chapter.

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wonderful comparison tool between different disinfectants. The first two rows, available chlorine and active chlorine percentages are important to differentiate. Available chlorine content, the first row, is developed using chlorine gas as the standard reference for 100%. ACC is a comparison to the relative amount of chlorine released into the water. Active chlorine percentage, the second row, is the percentage of weight of the technical grade active ingredient in the product. In some cases, other ingredients , or inert ingredients, are added to the disinfectant. The presence of these ingredients lowers the amount of active ingredient in the product. This chart also shows the effect of the pH when added to water and the physical appearance of the product. Keep this chart handy!