Defined as a sanitizer, chlorine destroys micro-organisms, however, it carries out an additional function - as an ‘oxidiser’.   Oxidising is the burning up of organic contaminants introduced into the water by environment or swimmers.

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CHLORINE FACTS

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Studies have shown that only about 10% of chlorine is used in sanitizing, whilst the remainder is used for oxidation.

Sodium hypochlorite (liquid chlorine).
 Sodium hypochlorite is stable only in solution.   First produced in 1785 by adding chlorine gas to a solution of lye (sodium hydroxide) in a chemical plant in Javelle, France.   Used as laundry bleach in lower concentrations, the chemical used for pool sanitation has an available strength of 12.5%.
Advantages                                Disadvantages
- Completely soluble in water          - Contains 87.5% water, bulky and heavy
 - When sold as a bulk liquid, the        to handle, readily bleaches clothes and
   least costly source of chlorine       other materials.
 - Clean product, no residue in         - Most alkaline chlorine sanitizer, signifi-
   pool water.                                       cant acid demand.
                                                         - Significant decomposition in storage.

 Calcium hypochlorite (calcium based granular chlorine).
A white granule or powder with a chlorine-like odour, calcium hypochlorite was discovered by mixing chlorine gas with milk of lime (calcium hydroxide) then drying to a powder.   Calcium hypochlorite normally has an available strength of 65%.
Advantages                                Disadvantages
- Easily handled; compact               - Increases pH, high acid demand.
   source of chlorine                        - For hard water significantly raises
 - No significant storage                    calcium hardness.
   decomposition.                              - Can create turbidity from undissolved
 - For soft water, increases                calcium base that is 35% of the product.
   calcium hardness.

 Lithium hypochlorite (lithium based granular chlorine).
 Lithium hypochlorite is essentially a ‘Rolls Royce’ product at a ‘Rolls Royce’ price, and its cost has limited its use and acceptance as a pool sanitizer, although it is often used as a spa shock chemical.
Advantages                                Disadvantages
- Completely soluble in water          - Highest cost pool sanitizer.
 - Safest chlorine chemical              - High TDS chemical due to significant                                         - No premixing required; will             inert content (71%).                              -   
   not bleach vinyl liners at
   normal pool temperatures.
- Dissolves quickly at                 
   normal pool temperatures.

 Dichloro-isocyanurate  or trichloro-isocyanurate (stabilised granular chlorine).
 Stabilized chlorine contains cyanuric acid as part of its make-up and therfore could be regarded as self-stabilizing.
 After the chlorine portion of the stabilized chlorine has been used during sanitation, the stabilizer stays intact and remains dissolved in the pool water to accumulate.
 This offsets the loss of stabilizer from backwashing, splashing and carry-out.
Dichloro-isocyanurate is the only chlorine product with a near neutral pH and, for this reason, have found popularity in the spa market where small volumes of water make pH control a major concern.
Tricolor is often found in tablet form and is a very convenient way of chlorinating pools, through use of a floating dispenser.
Whenever a stabilized chlorine is used, care has to be taken to ensure that the stabilizer level in the water does not exceed the recommended levels, as this can result in chlorine lock, where chlorine is not released into the pool properly and