Chlorine is added to swimming pools to kill algae and bacteria. Without it, the water would be green, cloudy and potentially unhealthy. Chlorine kills organisms by reacting with the lipids that make up their cell walls and membranes, rupturing them and killing the cell. To keep a pool clean, chlorine levels need to be above a certain level, but too much chlorine can irritate your skin and eyes. Sunlight and heat both play a part in determining how much chlorine is added.
Pool maintenance workers measure two types of chlorine -- free chlorine and combined chlorine. Free chlorine is the fraction of the total chlorine that is available to do the work of disinfecting. If free chlorine falls bellow one part per million, the pool is unsafe to swim in. Combined chlorine is the fraction of the total chlorine that has reacted with organic matter, such as algae and bacteria, and is, in essence, used up. High levels of combined chlorine can show that there are too many unwanted invaders in your pool, but free chlorine is the component that must be regularly replenished.
The Effects of Light
Free chlorine is lost when it reacts with bacteria and other organisms, but also when it is hit by sunlight. Chlorine forms hypochlorite ions in water, the molecule that is measured as free chlorine. When ultraviolet radiation from the sun hits hypochlorite, it breaks apart, releasing the chlorine as gas to the atmosphere. Sunlight is so effective in reducing chlorine that a bright sunny day can reduce the chlorination level of a pool by 90 percent in just two hours. Pool maintenance workers add chlorine daily and use chemical stabilizers to prevent this loss.
The Effects of Temperature
Temperature has an effect on chlorine breakdown, but the effect is indirect. Many bacteria species grow better in warmer water, and when bacteria proliferate, free chlorine is used up more quickly as it kills them. A general rule of thumb is that you need to add twice as much chlorine to the pool to maintain an adequate free chlorine level for every 6 degrees Celsius (10 degrees Fahrenheit) rise in temperature above 26 degrees Celsius (80 degrees Fahrenheit). This is less important for pools than it is for spas, which are regularly operated at higher temperatures.
Cyanuric acid is a chemical that stabilizes chlorine levels in the presence of sunlight to reduce the effects of ultraviolet rays. It reacts with free chlorine to form a compound that does not react in the presence of sunlight. The reaction of cyanuric acid can also go the other way and release free chlorine. As free chlorine gets used up, the cyanuric acid provides a reservoir of disinfection potential that is safe from the sun. This chemical is added to outdoor pools to maintain chlorination levels.