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Stabilised Chlorine

Stabilised Chlorine - Friend or Foe?
Cyanuric acid (stabiliser) has been in use for almost 60 years and has been generally accepted as the most effective stabiliser of chlorine. The use of stabiliser is common in swimming pools around the world to reduce the loss of free chlorine by UV radiation. Up to 90% of chlorine residual may be lost within a few hours when exposed to sunlight. The addition of stabiliser may reduce this loss to only 10 to 15% of the total chlorine residual.
Many studies and investigations have been conducted to prove that stabiliser reduces the efficacy of chlorine. Studies have even been funded by manufacturers of competitive products.
In laboratory experiments, it was found that stabiliser resulted in the need for greater quantities of chlorine to achieve the same effect as lower chlorine concentrations without stabiliser (E. B. Robinton and E. W. Mood, American Public Health Association, 1965).However, other studies performed under swimming pool conditions, reported that stabiliser did not reduce the efficiency of chlorine, but in some instances actually improved the efficiency.
1. Sommerfeld and Adamson 1981.
Cyanuric acid, used as chlorine stabiliser in swimming pool water, it seems has a relatively minor effect on the algicidal efficiency of free available chlorine. The toxicity of free available chlorine to 3 typical types of swimming pool algae was reduced slightly by 25 ppm of stabiliser, but then less than ideal levels of chlorine were employed in the test. Higher stabiliser concentrations (up to 200ppm) generally resulted in no further reduction in the algicidal efficiency of free available chlorine.
2. Robinton and Mood 1967
found no significant difference in the bactericidal activity of calcium hypochlorite and trichloroisocyanurates when the concentration of these compounds was expressed as free available chlorine
3. Kowalski and Hilton 1966.
found that pools treated with stabilised chlorine have a better disinfection record than pools treated with chlorine gas or calcium hypochlorite.
4. Hodge 1959.
found no toxicity to rats and dogs from sodium dichloroisocyanurate, one of the forms most commonly used to disinfect swimming pool water.
5. Clayton & Clayton 1981-1982.
The immersion of the entire forearms of 10 individuals in a neutralized trichloroisocyanuric acid solution (100 ppm as available chlorine) 8 times daily for 7 days caused no irritation.
Results of research are often inconclusive or even contradictory thus cyanuric acid appears as both inhibitors and activators of chlorine. In practice and in the real world of outdoor swimming pool sanitising only benefits have been experienced with the use of stabiliser.
What is cyanuric acid?
Cyanuric acid is marketed as a "Stabiliser" for swimming pools. What does stabiliser do? Stabiliser forms a weak bond with free chlorine in the pool water, protecting it from the sun's ultraviolet rays to reduce chlorine loss. Properly managed, stabiliser has been shown to reduce the chlorine needed to maintain the ideal chlorine residual in an outdoor pool.
What are dichlor and trichlor?
Dichlor and trichlor, also known as chlorinated isocyanurates, are two solid chlorine compounds that are widely used in outdoor and indoor swimming pools. Dichlor and trichlor contain both chlorine and stabiliser so it is not necessary to add stabiliser to the pool water. Dichlor usually comes in a granular form and is marketed for the residential swimming pool market. Trichlor is often sold in a tablet form for use in an erosion feeder.
I have an indoor pool. Should I use stabiliser?
No there is no need. Remember that stabiliser is intended to reduce the loss of free chlorine caused by the sun's ultraviolet rays. Indoor pools are not exposed to direct sunlight and therefore, there is no benefit in adding stabiliser to the pool water. How much stabiliser should be used in a swimming pool? Most pool chemical suppliers recommend that the optimal range for cyanuric acid is around 40 ppm. At levels above 50 ppm, pools reach the point of diminishing returns where the cost of buying stabiliser outweighs the benefit.
How much is too much stabiliser?
Some international standards have set the maximum level at 100 ppm. However as previously stated no significant disadvantages are recorded even at much higher levels. How does one test for stabiliser? Any pool professional should be able to assist with these tests.
My pool has stabiliser levels above 100 ppm. Do I have to reduce them?
The question is; are you experiencing problems with the pool and have you eliminated other factors such as a high TDS, combined chlorine, phosphates and chlorine demand problems. The traditional solution to supposed high stabiliser levels ("chlorine lock") is to drain the pool. Clearly this solution will correct almost any problem that may exist. When this solution is suggested to you, make sure that it is your only course of action and that you are not unnecessarily wasting water. If you are not experiencing a problem, do not drain the pool - the naysayers will have it that the problem will occur but in the majority of cases this never happens, and when it does, it is usually related to other factors.

1 Stabiliser and stabilised chlorine (dichlor or trichlor) should be used in outdoor swimming pools to reduce the operating cost and maintain the hygiene effect by enhancing the longevity of chlorine in the presence of sunlight.

2. Both dichlor and trichlor also release stabiliser to the pool water. It is usually not necessary to add additional cyanuric acid into a pool that uses dichlor or trichlor.

3. Cyanuric acid should be tested at the beginning of each summer season and adjusted to at least 30 ppm for best effect during the season.

4. Pools that use stabiliser should, like all pools, maintain a free chlorine residual of 1- 3 ppm.

5. Most importantly, regularly have your pool water tested by a Pool Professional with a computerised water testing facility and ensure that your pool water remains chemically balanced for best results.