CYA disappears over the winter

[chem geek]

Hey Richard,
How would the contact time vs. concnetration equations work within the cell of a SWC system? Would you expect more rapid CYA degredation?

nater,

Sorry I didn't respond right away. This thread kind of slipped by so I didn't notice your post. Anyway, in an salt-water chlorine generation cell, the pH at the chlorine generating plate is very low (acidic) and very far away from the pH of 9-12 (optimal 9-10) that the patent described was when CYA broke down. So where the chlorine level is high in the salt cell, the pH is low. At some point in the middle as the water flows out of the cell, the pH is higher and the chlorine level is higher, but neither is in the range where the patent says that CYA will break down. Also, the contact time of water through the cell is measured in seconds so again wouldn't be likely to breakdown CYA (and even flowing through the pipe to your pool doesn't take very long).

So in theory, the salt cell is less likely to breakdown CYA than the addition of chlorinating liquid or bleach and the latter isn't very likely to breakdown much CYA either. Basically, the CYA is pretty darn stable under normal operating conditions of any pool. It's just this wintertime loss that we speculate is due to anaerobic soil bacteria (that got in the pool) that seems to be the culprit. I just don't like the fact that such bacteria seems to survive in chlorinated water -- this is what we're getting more info on (there were several users who reported CYA drops with pools over the winter that had constantly maintained FC levels, so I'm not disputing that -- I'm just trying to understand how that could be).

Richard

[chem geek]

FYI to those interested in obscure questions and answers. I had seen on several websites that the manufacturers of melamine cyanurate claim it has a solubility of 0.002g/100ml which would work out to 20 ppm. The Taylor K-1720 kit measures CYA down to 20 ppm so I thought "how is this possible?" and wrote to Taylor to find out. After all, if CYA were soluble at 20 ppm, how could you the black dot get obscured so you could get a 20 ppm measurement?

Taylor is a great company who answer any ridiculous question I ask (apparently) and they came back with the response that the standard solubility measurements are done in distilled water so they start out at a pH of 7. The CYA reagent used in the CYA test (which contains melamine) is buffered at a low (acidic) pH and apparently the solubility of the melamine cyanurate complex is less soluble at low pH and is essentially insoluble at the pH of the test so they can measure rather low levels of CYA in the test.

Richard