The purpose of any sanitizer is to kill bacteria and algae in you swimming pool or spa water. Chlorine is used as a benchmark because it is the most common sanitizer.
The simplest form of chlorine, which is found in nature as chloride mineral salts, is as a gas. It is made by passing electricity through a saline solution, one by-product of which is sodium hydroxide (caustic soda). Liquid chlorine is manufactured by passing chlorine gas through this solution of caustic soda. Dry chlorine is subsequently made by removing the water from such a solution.
Water demands a lot. Chlorine demand can be defined as the amount of any chlorine product (in any form) needed to kill all the bacteria present in a body of water. Superchlorination is adding a lot of chlorine to a pool and is recommended by various experts as something to do monthly, three times a year, etc. Another time to superchlorinate is when the pool has been allowed to get exceptionally dirty or full of algae.
Below is an overview of some of the chlorine products and a summary of their properties:
|Product||pH||Available Chlorine||Common Form||Stability in Water|
|Cl2 gas||Low||100%||Gas||Very Unstable|
|Calcium Hypochlorite||11.5||65%||Dry Granular||Stable|
|Dichlor||6.8||60%||Dry Granular||Very Stable|
|Trichlor||3.8||90%||Tablet||Very, Very Stable|
Source: Tamminen, Terry. The Ultimate Pool Maintenance Manual