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When chlorine is added to water, some of the chlorine reacts first with inorganic and organic materials and metals in the water and is not available for disinfection (this is called the chlorine demand of the water). After the chlorine demand is met, the remaining chlorine is called total chlorine. Total chlorine is further divided into: 1) combined chlorine, which is the amount of chlorine that has reacted with inorganic (nitrates, etc.) and organic nitrogen-containing molecules (urea, etc.) to make weak disinfectants that are unavailable for disinfection and, 2) Free chlorine, which is the chlorine that is left over and is available to inactivate disease-causing organisms; it is a measure of the potability of the water. Thus, total chlorine equals the sum of the combined chlorine and free chlorine measurements.

For example, if using completely clean water with no contaminants, the chlorine demand will be zero, and since there will be no inorganic or organic material present, no combined chlorine will be present. Thus, the free chlorine concentration will be equal to the concentration of chlorine initially added. In natural waters, especially surface water supplies such as rivers, organic material will exert a chlorine demand, and inorganic compounds like nitrates will form combined chlorine. Thus, the free chlorine concentration will be less than the concentration of chlorine initially added (Free chlorine = Total chlorine measurement – Combined chlorine measurement).

Residual chlorine should be maintained at or around 0.5 mg/l or less. This is the limit strived at by most uk water companies. Readings of around 0.2mgl and 0.4 are ideal at the user point for consumption.

The world health Organisation has set a level  5mg/l for drinking water as a safe limit for chlorination. However at these levels its noticable.