Cyanuric Acid or Stabiliser (M77)

This factsheet offers Members some information regarding Cyanuric Acid (CYA) and its usage and effects.

Cyanuric acid (CYA) also known as stabiliser, can be usefully added to an open air pool being disinfected by an inorganic chlorine donor, such as calcium hypochlorite or sodium hypochlorite. This CYA treatment reduces the loss of chlorine from the open air pool water due to sunlight (ultra violet light) and so the pool water retains its chlorine residual for longer.

The loss can be so rapid that hand dosing is particularly haphazard leading to green pools and to increased running costs to the pool owner.

Generally the recommended initial dose should be 60mg/l (ppm). This should suffice for the season for an outdoor pool.

Organic chlorine donors such as sodium dichloroisocyanurate dihydrate (troclosene sodium, dihydrate) – usually supplied in the form of granules - or trichloroisocyanuric acid (symclosene) usually supplied in the form of tablets or sticks, already contains cyanuric acid and therefore no additional cyanuric acid should be added separately to the pool water. Indeed for every part per million of chlorine added to the water as dichlor granules, nearly 1 part per million (0.91ppm) of cyanuric acid is inevitably added to the pool water.

As the Cyanuric acid level rises, so a higher chlorine residual is required, as can be seen from the table on page 70 of PWTAG’s , Swimming Pool Water Book (reproduced below).

Cyanuric acid concentration (mg/l) Minimum free chlorine (mg/l)

25 1.5

50 2.0

100 2.5

200 3.0 – 5.0

If the Cyanuric acid (CYA) rises to exceed 200 ppm (mg/l) then “chlorine lock” can occur. The only way to reduce CYA in the water is by dilution with fresh water.

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At 200mg/l, most of the water should be replaced, while at 100mg/l, perhaps half of it should be replaced. Alternatively, dosing can be changed to Hypochlorite (Calcium or Sodium) which will stop increasing the CYA. Please note that inexpensive and simple Cyanuric acid test kits are available.

There is no need to add Cyanuric acid to an indoor pool unless it has a big glazed area.

Footnote on leak detection for pools with an undercroft

A second result of using CYA is that unlike pH, free and combined chlorine, CYA is not easily changed and by testing water found outside the pool shell (such as in an undercroft) for CYA, its presence (or absence) can determine whether it is pool water, or not. Obviously the pool water in question must have CYA present, due to the use of dichlor or trichlor. If cyanurate based chemicals have not been used for disinfection, the pool can be dosed with 30mg/l CYA, left for seven days to thoroughly disperse and then the suspect water can be checked. At the same time as dosing the pool the water outside the pool should be mopped up.

SPATA reminds its members that no responsibility can be taken by SPATA, its employees or agents in respect of any errors or omissions from this factsheet.

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