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Chlorine works as a:

Disinfectant - It kills water-borne bacteria, and prevents algae growth.

Oxidizer - It burns up small organic debris too small to be filtered out, and attacks dirt. Oxidation improves water clarity.
 
The chlorinating formula:
1. Chlorine dissolves in water to form FREE AVAILABLE CHLORINE.
2. FREE CHLORINE will bond with swimmer wastes to form COMBINED CHLORINE.
3. All dissolved chlorine in the pool is known as TOTAL CHLORINE.
4. FREE CHLORINE + COMBINED CHLORINE = TOTAL CHLORINE.
 
FREE AVAILABLE CHLORINE:
- This is the active, germ-killing form of chlorine.
- Has very little odor in water at proper levels.
- Is sensitive to changes in pH. High pH decreases the disinfecting power of free chlorine.
- Sunlight destroys free chlorine.
 
COMBINED CHLORINE:
- Unwanted by-product of chlorine disinfection.
- Very irritating to the skin and eyes.
- Partially used free chlorine, tied by wastes in water, i.e. perspiration, mucous, urine.
- Disinfection power is greatly reduced.
- Sometimes called chloramines.
- Must be burned off by massive oxidation, through: superchlorination, shock treatment, or break point chlorination.
 
TOTAL CHLORINE:
- A measure of all dissolved chlorine in water.
- Under ideal circumstances, Total Chlorine = Free Chlorine.
- Ideal Free Available Chlorine range: 1.0 - 3.0 ppm.
 
IDEAL CHLORINE LEVELS are based on the following:
- Swimmer Comfort: Moderately high levels of total chlorine can irritate skin and cause loss of body hair. High levels of combined chlorine irritates skin and eyes and has an unpleasant odor.
 
- Chlorine Effectiveness: Chlorine levels that are to low will not kill germs quickly enough to provide a healthful water.
 
- Equipment Life: Chlorine is an extremely corrosive material. High chlorine levels can significantly reduce the working life of pool equipment.
 
Factors That Affect Chlorine Demand:
- Amount of people using pool or spa.
- Amount of sunlight.
- Rain.
- Hours of daily filtration.
- Foliage (grass, leaves, pollen).
- Chemicals added.
- Algae spore - bacteria.
- Suntan oils, lotion and makeup.
 
SUPERCHLORINATION:
Superchlorination is the addition of a large amount of chlorine or of oxidizer. This process removes combined chlorine from the pool, leaving only free chlorine. Raising free chlorine levels to 10 - 15 ppm is usually enough.
 
The following ammonia compounds enter the water primarily from swimmer wastes:
- Perspiration.
- Mucous.
- Urine.
- Hair and body oil.
 
Combined Chlorine must be periodically "BURNED OFF" by Massive Oxidation. This process is called superchlorination, shock treatment, or break point chlorination.
 
HOW TO ADJUST CHLORINE LEVELS:
- To RAISE chlorine levels, add the correct amount of chlorine (see chart below).
- To DECREASE chlorine levels, Add Sodium Thiosulfate (a chlorine neutralizer), or dilute with fresh water.
 
Comparitive chlorine information chart
Chlorine Type
pH
Sun Stable
Amount to Raise Levels 1 ppm in 10,000 gal

Amount to Raise levels 1 ppm in 50,000 gal
Trichlor 2.9 Yes 1.5 oz. 7.5 oz.
Dichlor 6.0 Yes 2.3 oz. 11.3 oz.
Calcium Hypochlorite 11.7 No 2.0 oz. 10.0 oz.
Sodium Hypochlorite 13.0 No 1 1/3 cups. 6 2/3 cups.
Lithium Hypochlorite
 10.5 No 4.0 oz. 1 lb. 4 oz.
Elemental Gas 1.0 No 1.4 oz. 6.7 oz.
 
Required Chlorine Levels In Pools
Type pH Range Free Chlorine
Unstabilized Chlorine 7.2 - 7.49 1.0 - 6.0 ppm
7.5 - 7.79
 1.4 - 6.0 ppm
7.8 - 8.00 1.8 - 6.0 ppm
Stabilized Chlorine
 7.2 - 7.49 1.0 - 6.0 ppm
7.5 - 7.79 2.0 - 6.0 ppm
7.8 - 8.00 2.8 - 6.0 ppm
 
SUN AND CHLORINE
Cyanuric Acid: is a chlorine stabilizer, slows the loss of chlorine due to sunlight, and may inhibit the germ-killing action of chlorine.
 
Cyanuric Acid levels affect the following:
- No minimum level because chlorine levels can be maintained without it.
- The maximum was established to prevent a condition called "chlorine lock."
- There is some evidence to show that high levels of cyanuric acid can inhibit and possibly prevent the germ killing ability of free chlorine.
 
Required Cyanuric Acid levels:
- Minimum: none
- Ideal Range: 25 - 30 ppm.
- Maximum: 90 ppm.
 
To raise CYANURIC ACID levels, add cyanuric acid or use sun-stable chlorine products.
To lower CYANURIC ACID levels, drain and/or dilute with make-up water.
 
What causes cyanuric acid level changes?
- Using trichlor or dichlor.
- Splash-out.
- Dilution with make-up water.
 
Note: When unable to lower cyanuric acid level, drain and brush sides of pool at the same time.
 
CHLORINE INCREASE - SPA
ppm increase desired 100 gal. 150 gal. 250 gal. 500 gal. 750 gal. 1000 gal.
5 1/2 tb
 2 ts
 1 1/4 tb
 3 2/12 tb
 1/4 cp
 1/3 cp
10 1/4 tb
 1 ts
 3/4 tb
 1 1/4 tb
 2 tb
 2 1/2 tb
20 1/4 tb
 1 ts
 1/2 tb
 1 tb
 1 1/2 tb
 2 tb
30 1/4 ts
 1/2 ts
 3/4 ts
 1 1/4 ts
 2 ts
 2 1/2 ts
40 1/6 ts
 1/4 ts
 1/2 ts
 1 ts
 1 1/2 ts
 2 ts
50 1/8 ts
 1/4 ts
 3/8 ts
 3/4 ts
 1 1/4 ts
 1 1/2 ts
60 1/8 ts
 1/5 ts
 3/8 ts
 1/2 ts
 1 ts
 1 1/4 ts
65 1/10 ts
 1/6 ts
 1/4 ts
 1/2 ts
 3/4 ts
 1 ts
             
Note: ts = teaspoon, tb = tablespoon, cp = one cup (8 Fl. oz.)