Swimming pool water requires some form of “purification” in order to kill bacteria and other micro-organisms that could be potentially harmful to the pool equipment or more importantly, those swimming in the pool itself. Adding chlorine (in the form of sodium hypochlorite) to pool water is the most common form of sanitation. Following, is an explanation as to what occurs when you add the appropriate amount of chlorine to pool water, which ultimately leads to “Breakpoint Chlorination”.
When chlorine is added to water for normal residual chlorination it combines (reacts) with ammonia and organic contaminants found in pool water (sweat, urine, skin cells, and other organic matter) forming “combined chlorine” compounds commonly called “chloramines”. The terms “combined chlorine” and “chloramines” are often used interchangeably and refer generically to a family of chlorinated compounds. It is the build-up of combined chlorine that swimmers will most often notice in a pool, complaining of a “chlorine smell” overhanging the pool surface as well as eye and nose irritation.
Besides normal residual chlorination, Chlorine is commonly used to “shock” treat or super-chlorinate pool water in order break down contaminating organic compounds, kill algae and other micro-organisms, destroy impurities and dissolved waste products, and break apart the chemical bonds forming combined chlorine. The point at which the chlorine concentration is high enough for these chemical bonds to be broken is called “Breakpoint”. Breakpoint chlorination eliminates the chloramines and other reductants which otherwise cause an increased chlorine demand in the water.
In order to achieve breakpoint, a ratio of 7.6:1 free chlorine to combined chlorine molecules is required. At or above this ratio combined chlorine molecules are broken down and destroyed. Reaching the breakpoint is an all-or-nothing reaction. Not adding enough chlorine to reach breakpoint will result in more combined chlorine and lower free chlorine residual. When completed, breakpoint chlorination destroys the chemical bonds with ammonia leaving free chlorine, nitrogen, water and inorganic chloride (salt).
Several chemical reactions take place before breakpoint is achieved:
- Free Chlorine (HOCl) reacts with Ammonia (NH3) to form Monochloramines (NH2Cl).
- HOCl also reacts with NH2Cl to form Dichloramines (NHCL2) and further with NHCL2 to form Trichloramines (NHCL3) or nitrogen trichloride (NCl3) and Nitrates (NO3).
- NCl3 forms when the HOCl to nitrogen molecular weight ratio is greater than 12:1. Oily, insoluble colloidal particles will appear, cloud the water, migrate toward the water surface, and may be released into the air.
Shocking or super-chlorinating pool water to breakpoint should be done as needed when the level of chloramines present reaches 0.2 ppm or greater. Chlorine in any form may be used to reach breakpoint except stabilized chlorine products or Isocyanurates such as Trichlor or Dichlor. These products should not be used for breakpoint chlorination since excess cyanuric acid would be added to the water solution concurrently inhibiting the full reactions from occurring.
Before attempting breakpoint chlorination make sure that the water is chemically balanced and that pH is 7.2 to 7.4 in order to maximize the percentage of hypochlorous acid formed. Shock treating a pool with unbalanced water, particularly with a high (basic) pH or high total alkalinity, will result in the formation of a white carbonate precipitate which will cloud the water. However, some operators prefer to raise the pH when using acidic chlorine products like elemental gas chlorine for super-chlorination since more offensive forms of chloramines develop rapidly at a very low pH.
To calculate breakpoint concentration necessary to super-chlorinate, use a DPD (N, N-Diethyl-P-Phenelynediamine) or FAS (Ferrous Ammonium Sulfate) test kit to find both the free and total available chlorine levels. Subtract the Free Available Chlorine (FAC) from the Total Available Chlorine (TAC) to find the Combined Available Chlorine (CAC) level. Multiply the CAC by the factor 10, although only 7.6 is actually needed, to find the dose of chlorine you must introduce into the pool in order to reach the breakpoint. Ten is used as a factor because most pool operators are not sure of the precise amount of water in their pools, or of the exact percentage of available chlorine in the chlorine compound being used. We use ten as a factor to err on the side of caution and so that enough chlorine is left over after breakpoint has been achieved to satisfy the chlorine demand and leave an adequate residual.
Some health department regulations may prohibit swimmers from using the pool when chlorine concentrations are elevated. It is best to super-chlorinate in the evening or during hours the pool is not in operation to avoid respiratory irritation to users from off gassing during the super-chlorination process, and to allow chlorine levels to drop back to normal levels. How fast breakpoint is reached depends on several factors, including: pH, pool water temperature, the ratio of free available chlorine to combined chlorine, and the concentration of ammonia/nitrogen and organic nitrogen compounds which place a demand on the chlorine. If the chemical reaction takes place and breakpoint is reached, the large amounts of chlorine added to the water will be used up in the process. Free chlorine will return to normal operating levels, and the combined chlorines will be eliminated.