It is well known that the build-up of cyanuric acid
stabilizer in swimming pool water will cause overstabilization, which usually results from the extended
use of stabilized sanitizers in conjunction with stabilized shock treatments.
It is also known that
overstabilization decreases the effectiveness of chlorine in killing bacteria and algae and will reduce
the oxidation reduction potential or the oxidizing power of chlorine.
Low levels of cyanuric acid do serve a purpose in protecting chlorine from sunlight degradation, however,
too much cyanuric acid will negate any benefit and cause problems. When used properly, the recommended
level of cyanuric acid is between 20 and 50 parts per million (ppm). Cyanuric acid use is not recommended
for indoor pools.
To determine the effect of cyanuric acid on sections of white pool plaster, Arch Chemicals conducted laboratory
studies in 2004. The levels of cyanuric acid tested were 200 ppm and 500 ppm. After five weeks, the cyanuric acid in the water
with the plaster 'coupons' - which are individual rectangles of plaster made using the same plaster composition that is used for
pools - had dropped considerably and surface analysis showed the accumulation of cyanuric acid on the plaster. Surface reaction was
observed at 250 ppm and 500 ppm, with the reaction of cyanuric acid being much faster in the 500 ppm sample, as shown in the graph
entitled 'Cyanuric Acid Reacts with Plaster'.
Based on these initial results, a six month tank test was conducted to better understand the effect of cyanuric
acid on plaster. The water in the tank tests was was adjusted to try and maintain pH between 7.2 and 7.8 and alkalinity between 60 ppm
to 100 ppm. Because the plaster coupons were new, the pH rose continuously and therefore needed to be adjusted daily. The free
available chlorine was maintained between 1 ppm and 4 ppm and cyanuric acid levels of 0 ppm, 25 ppm, 50 ppm, 100 ppm, 250 ppm, and 500 ppm
were tested. Images taken with a scanning electron microscope at a magnification level of 250X show degradation of the plaster surface
at increasing levels of cyanuric acid (see photo at bottom of page).
There was little plaster degradation at 100 ppm cyanuric acid. Due to the results of the tank tests, additional tests
were initiated in larger bodies of water where the water balance parameters could be maintained more easily.
Pool testing with plaster coupons was started in May, 2005, with the pools being operated at cyanuric acid levels
of 0 ppm - 50 ppm - 110 ppm - 125 ppm, and 200 ppm - 250 ppm. The water parameters were maintained at a pH of 7.2 - 7.6, alkalinity
at 80 ppm - 120 ppm, calcium hardness at 180 ppm - 250 ppm, and available chlorine at 1 ppm - 4 ppm.
After four months of operation, photographs show surfave degradation on the plaster
coupons in high, 200 ppm - 250 ppm cyanuric acid pools (see photos above).
Results reported in a July, 2004 study conducted by the National Pool
Industry Research Center (NPIRC) and the National Plasterer's Council (NPC) indicate
that low pH trichlor products can affect plaster surfaces. However, in the NPIRC
study, it was difficult to discern whether the effect on the plaster was a result
of the low pH or the trichlor sanitizer, or from some other chemical factor.
In the tank studies described above, the pH and alkalinity of the
tanks ran on the high side (pH~8, TA~90 ppm), while the pool were being maintained
as indicated above. Despite the high pH and alkalinity in the tank test, plaster
degradation was still seen. The pool tests further confirmed the effect of cyanuric
acid on plaster.
These results indicate that the presence of cyanuric acid
can affect plaster surfaces and that a minimum amount should be used only if
chlorine stabalization is necessary.
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Left: 14X magnification of plaster coupon in pool with 0 ppm cyanuric acid.
Right: 14X magnification of plaster coupon in pool with 200-250 ppm cyanuric acid. | |
