CYA Mystery

[Hszwill]

Hello....Can anyone tell me the various ways stability (CYA) disappears? A month ago I finally got my stability to about 45 and now today it is gone. I did just brush algae off the walls. Is it the algae or the lack of bleach or both that cause CYA to disappear?

Needing help with mystery,
Shannon in VA

[chem geek]

If your chlorine level got to zero, then soil bacteria that can get into the pool can grow and convert the CYA into ammonia. If this happened, then it might take an extraordinary amount of chlorine before you start registering FC. If this seems to be the case for you, then get an inexpensive ammonia test from a pet/fish/aquarium store. You would need to add an FC amount that is about 10x the amount of ammonia you measure.

Richard

[pedsrn]

Wow, that is interesting and great to know, chemgeek.

So, is the CYA forever lost in the reaction or once the FC is back up, will it convert back to CYA?

[chem geek]

It is forever lost. The reaction pathway is shown in this link. The reaction is essentially as follows:

CYA + 4H2O --> H+ + HCO3(-) + 3NH3 + 2CO2
Cyanuric Acid + Water --> Hydrogen Ion + Bicarbonate Ion + Ammonia + Carbon Dioxide

So the net result is a lowering of pH with some release of carbon dioxide and increase of carbonates in the water (the TA remains unchanged for technical reasons I won't get into here). For every 10 ppm CYA that is decomposed, it produces around 3.3 ppm ammonia (measured as ppm Nitrogen) and would require around 30 or so ppm FC to get rid of. Fortunately, ammonia is a little volatile so some of it may outgas.

Richard

[waste]

It is forever lost. The reaction pathway is shown in this link. The reaction is essentially as follows:

CYA + 4H2O --> H+ + HCO3(-) + 3NH3 + 2CO2
Cyanuric Acid + Water --> Hydrogen Ion + Bicarbonate Ion + Ammonia + Carbon Dioxide

So the net result is a lowering of pH with some release of carbon dioxide and increase of carbonates in the water (the TA remains unchanged for technical reasons I won't get into here). For every 10 ppm CYA that is decomposed, it produces around 3.3 ppm ammonia (measured as ppm Nitrogen) and would require around 30 or so ppm FC to get rid of. Fortunately, ammonia is a little volatile so some of it may outgas.

Richard

Richard - YOU ROCK!! - I have saved this post for my collection of pool factoids