woodyp said:
Because TA acts to help buffer PH from rapid rises.............
If the pH is rising in a pool due to carbon dioxide outgassing, then increasing the TA NEVER helps and actually hurts by increasing the rate of pH rise. The higher TA the higher the amount of carbon dioxide in the water and the faster the rate it will outgas and that causes the pH to rise. The use of a non-carbonate buffer such as borates will normally slow down the rate of rise, but will not lower the total amount of acid that needs to be added. This is because it increases pH buffering, but does not fundamentally solve the core problem which is too much carbon dioxide in the water.
The simple thing to remember is that
though TA is a pH buffer, it is also a SOURCE of rising pH in its own right. So increasing TA usually only makes sense if the pH tends to fall or if the TA is quite low so has the saturation index be quite negative. A rule of thumb is that 60 ppm TA is the lowest without borates and 40 ppm with borates, though this will also depend somewhat on the CYA level (but as a rough rule of thumb it's a reasonable floor for TA). If needed, the calcium hardness (CH) and pH target can be higher to prevent the saturation index from becoming too negative.
pool_dufus said:
I add acid (1-2 pints) about once every 10 days or so. The pool is 25 years old but appears to have been replastered. The plaster is getting pretty badly pitted. I have a spa so there is some CO2 outgassing from the small fountain and the small waterfall. That would tend to lower the carbonic acid in the pool and raise the pH. That's my current thinking, at least. I read that bleach also raises the pH. I don't have a SWG.
I'm thinking of adding 10# of sodium bicarb to raise the alkalinity and 20# of either borax of a product of borax and acid that is balanced, do these things slowly in stages, and monitor the pH and bring it back down with muriatic acid. I am hoping this added buffer will stabilize the pH a bit. That's my plan. Comments or advice?
By the way, 1-2 pints every 10 days or so is not a lot of acid in a 18,500 gallon pool. If you wait until your pH is higher before you add acid, that may lessen the amount you need to add if the pH rise is due to carbon dioxide outgassing. This is because there is more carbon dioxide outgassing at lower pH. So if you were to only add your 3 cups of acid after the pH went to 7.8, then that would lower it to 7.5. This chart shows the amount of over-carbonation in a pool at various pH and TA levels.
I had a period of time in my pool, which is covered most of the time so should have minimal carbon dioxide outgassing, where the pH was rising faster than normal and I was adding a pint every week or two to 16,000 gallons. That has since stopped and I think it was during a period where the pool was going through bursting of calcium nodules so having more exposure to calcium hydroxide under improperly bonded plaster. Perhaps your pool is going through something similar, though actual pitting is not a good sign and usually indicates the saturation index is too low. Yours is currently at around -0.2, but if you add enough acid to get your pH down to 7.3 (3 cups would do that from 7.6), then that's a saturation index of -0.5 which with a "poor plaster" job might be too low, especially over a longer period of time.
The fountain and waterfall would indicate aeration may be at least part of the source of pH rise so if that is the case I would NOT raise your TA. Increasing the borates to 50 ppm may help slow down the rate of rise a little (though not the amount of acid you add over time), but quite frankly you'd notice more of a difference going from no borates to some and you already have some. Also try targeting 7.8 before you add acid and don't go below 7.5 after doing so. If you plan to keep the TA this low permanently, you might consider bumping up your CH to 400, though that's not critical.
