Constant PH climb suddenly stopped

[geordie]

No rain, no addition of fill water, only slightly less bleach added than normal...

For years my ph climbs steadily to 8.0 and beyond if unchecked. I don't let it go unchecked too often, and really hate using muriatic acid in the pool. I've developed a nice simple routine that uses tri-chlor pucks as a slow steady supplement to liquid bleach, which generally keeps my ph at 7.8 and doesn't drive my CYA through the roof before closing up in the fall. I like the ph at 7.8 because the water is much more comfortable on the eyes and skin at that level.

However, suddenly, my ph is at 7.4 all on its own, with no trichlor or muriatic acid added since the end of July. It has been a very dry, hot summer here. Our heat finally broke two weeks ago, and broke hard. The nights have been dipping into the 50s and so a week and a half ago I broke out the solar cover and slapped it on. It has only been off for about 6 hours one afternoon in 10 days. I have had to continue to add some bleach here and there, and yet the ph climb not only stopped, but started to drop. I've always known the high ph wasn't only because of the bleach routine. I've generally suspected something else was driving my ph up and the bleach was just aggravating it.

Last sunday the ph was 7.8-8.0 and there hasn't been any rain or anything added to the pool except 6.0% bleach since those numbers were taken.

It is just a matter of curiousity at this point, but any of our chemists have any ideas about what is behind this?

Jean

[chem geek]

Jean,

Could you post your test numbers, especially TA and whether it has dropped over time. Also, how old is your pool (when did it last have plaster/gunite/grout applied)? A high TA can outgas carbon dioxide which makes the pH rise. Also, a newer pool will have pH rise from the calcium hydroxide that is released during curing.

Now it does turn out that pH is a function of temperature of the pool water, but it takes quite an extreme change to show up and also goes the opposite direction that you saw. Going from 88F to 55F has pH rise by 0.23

Richard

[geordie]

Above ground vinyl liner- so nothing applied recently. Solar cover was clean and had not been used so far this year at all until 10 days ago.

24 foot round
water temp 80 (was 78 the when the ph was 7.8-8.0)
ph 7.4
TA 180 - right where it has been all year... and in the same neighborhood for about 5 years
CYA 40-50 depending on my eyes and the brightness of the light source
FC 6

[chem geek]

Solar cover was clean and had not been used so far this year at all until 10 days ago.

TA 180 - right where it has been all year... and in the same neighborhood for about 5 years

The data I quoted above is the answer to this "mystery". A TA of 180 is quite high and will lead to a lot of carbon dioxide outgassing and this causes the pH to rise. However, when you add acid to restore the pH, you should have seen the TA reduce as well as the pH, though it doesn't get reduced by very much. Perhaps your fill water is high in TA so whenever you fill up after evaporation you put back in more TA (that's the most likely scenario).

Now when you put a cover on your pool, you essentially stop the carbon dioxide outgassing and therefore stop the rise in pH as well as your need to add acid.

Fortunately, this is a case that is perfectly explained. If you want to reduce your need to add acid, you can either use the cover as you are now doing or you can lower your TA significantly using Ben's Procedure for Lowering Alkalinity though you may need to also apply this procedure to your fill water before adding it to the pool (if possible) or else your TA will rise again.

You should probably test your fill water to see what the TA level is. If it's high, then that would explain why it never seems to drop in spite of the apparent carbon dioxide outgassing.

Richard

[geordie]

Thanks Richard, makes perfect sense. Yes, the fill water has high TA. Our "city" water is well water and has high TA and high ph. Since you explained the outgassing problem I completely understand it. (not really a problem... just a new twist).

I really gave up a long time ago on trying to lower the alkalinity in the pool. To lower the ph and keep it low enough long enough for aeration to work, is way too much trouble given that it all immediately begins to rise again from the fill water. Rain water doesn't appear to be much better as an additive.

Thanks for the help.

[duraleigh]

Richard,

That was a very insightful post.....I learned from it and I imagine others did as well....Thanks!

[tphaggerty]

Ha! That explains why my PH was low after leaving the solar cover on for 4 or 5 days last weekend when we were out of town. I have an SWG and my PH stays rock steady at 7.6, except this weekend, when it was somewhere between 7.2 and 7.4 (I have trouble seeing the colors that well).

[chem geek]

Ha! That explains why my PH was low after leaving the solar cover on for 4 or 5 days last weekend when we were out of town. I have an SWG and my PH stays rock steady at 7.6, except this weekend, when it was somewhere between 7.2 and 7.4 (I have trouble seeing the colors that well).

Well, now you've introduced another mystery. Something introduced into your pool is apparently acidic so that when you have the cover off with carbon dioxide outgassing this compensates so you have no net gain in pH. But that still leaves the question of what is driving your pH DOWN when the cover is on. I have no answer for that one! Anything unusual about your pool that could explain this?

Richard

[tphaggerty]

Well, here in NY the rain is pretty acidic. We had about 1" (or possibly more) of rain during that same period (which on a 20x40 pool is about 500gals). Does that work?

(Plus we had one small, very icky decomposing frog, but I think he was on the cover and didn't drop in until we took off the cover).

[chem geek]

Well, here in NY the rain is pretty acidic. We had about 1" (or possibly more) of rain during that same period (which on a 20x40 pool is about 500gals). Does that work?

Yup, that could do it if the rain were particularly acidic. You would have needed the rough equivalent of 4 cups of Muriatic Acid to go from 7.6 to 7.3 in a 24,000 gallon pool (my rough estimate from your 500 gallon, 1" number, assuming average depth of 4 feet and assuming a TA of 100). 4 cups of Muriatic Acid in 500 gallons of pure water would make the pH of about 4.

Richard