Algae, TA or what?

[chem geek]

In fact, I'll start lowering TA on my pool either tonight or this weekend--it's about 80, so getting it down to 50 should be a snap. Plus I've got my "splashers" to aerate it!

Carl,

Unless your pool experiences a regular rise in pH over time, then lowering the TA isn't going to show any noticeable difference. Also, there is a slow upward pressure on pH from chlorinating liquid and bleach due to the "excess lye" and this varies by brand with 6% Clorox Regular unscented having the lowest with a pH of 11.9. Some off-brand bleaches have a pH of 12.5 or higher. Chlorinating liquid also varies with better 12.5% brands having a pH of 12.5 while others have 13 or higher. If you tell me the concentration and pH of your chlorine product and how much FC you use per day on average, I can tell you the pH rise (with TA assumptions) and amount of acid you need to compensate. No matter how low you lower the TA, you won't get rid of the need to compensate for this "excess lye".

As for recommended TA levels, when using a hypochlorite source of chlorine you generally don't want to go above 80 ppm, but with covered pools a higher TA isn't going to be much of a problem. As for how low to go, that depends on how much aeration there is in the pool and isn't easily calculated. So one would just have a more generic rule for those pools experiencing a regular rise in pH of letting the TA drop as low as 50 ppm if needed and seeing if this improves the rate of pH rise and then compensate pH and/or CH. For most pools, 70-80 ppm will be OK. It's SWG pools and pools with water aeration features that have more of an issue, partly due to increased aeration from hydrogen gas bubbles (though calculations show this probably isn't the entire effect) but also, I believe, from undissolved chlorine gas outgassing. This latter effect can't be compensated by TA and is probably worse in pools with short pipe runs from the SWG to the pool.

As for the point one needs to get in the carbon dioxide equilibrium chart before pH becomes stable, that is only able to be determined empirically and will be different for every pool since the rate of outgassing depends on the amount of air-water surface interaction so on aeration, wind, etc. Very roughly speaking, nearly everyone (with no pool cover) experiences a problem when they get into the red region with 20 or more times as much carbon dioxide in the water than the equilibrium amount from air, many have a problem in the orange region between 10 and 20, and most seem to do OK with some pH rise that is tolerable in the green region less than 10, but there are definitely those highly aerated pools who see a rise in pH until they get down to nearly 2 in the chart. For spas, the hotter temperature and vastly increased aeration from spa jets almost always has to get below 4 and sometimes down to 2 or 1 before the pH becomes stable. The use of 50 ppm Borates provides additional pH buffering when the TA is low, but without the negative pH rising side effect of the carbonate buffer system.

Thanks,
Richard