Why would my TA crash?

[TwistyKat]

Yikes, I pulled open a corner of my pool cover off and took a sample to quickly test PH and TA from my 23K G gunite and was shocked to see my TA test go red after ONE DROP!

The PH was 7.0, which is the lowest I have ever seen it since the pool was replastered five years ago. I usually have problems with PH and TA being too high, but last season I was having some PH and TA bounce issues which I thought I had under control.

I'm in the process of bring the TA back up now, but can anyone speculate on why the TA would crash from a normal reading in the late fall after it was closed for two months (I forget exactly, but it was fine) to something below what the test kit could measure?

[chem geek]

I don't know how this could happen, but IF for some reason a lot of acid, or some acidic substance, got into the pool, then the pH could get so low as to drive off the TA (as carbon dioxide). As the TA gets lower, it takes less acid to make the pH go lower so it's a bit of a runaway effect, but it would take a LOT of acid to make this happen.

Richard

[waterbear]

Did youl, by any chance, have some trichlor tabs in the pool while it was closed for a chlorine source?

[TwistyKat]

No, although I do use the tabs during the regular season in the skimmer...and I shock with bleach as needed. Not sure if that is a clue or not.

Once I close the pool I usually only do spot checks on the PH and TA during the off-season and don't worry about anything else.

In this case I hadn't checked anything since December.

[JohnT]

Lots of rain could do it if you have a permeable cover. Or a leak in a (formerly) impermeable cover.

[CrisL]

I have a mesh cover and my TA goes to 30 or so every winter, with my ph dropping somewhat, going down to 7.0 to 7.2. Once I bring it up, the TA stays at about 90-100 all summer with no other changes.

[chem geek]

According to this website, in Shrewsbury, MA from December to April it rained about 13" or about a foot. According to this EPA website, normal rain has a pH of around 5.6 (due to carbon dioxide in the atmosphere) while the most acidic acid rain is around 4.3

If your pool has an average depth of 4.5 feet, then one foot of rain would replace 1/4.5 = 22% of your pool water through a process of continuous dilution which is equivalent to a drain/refill of 20%. If you started with a TA of 80 (and a CYA of 30) and a pH of 7.5, then replacing 20% of your pool water with "acid rain" water with a pH of 4.3 means adding 10^(-4.3) moles/liter of hydrogen ion and you would remove up to 20% of the carbonate buffer. So the pool's TA would drop to 64 (due to CYA going to 24) mostly from dilution (not acidity), but the pH would barely drop at all (about 0.02) because even acid rain isn't very acid when added to the pool's "buffered" water.

Rain can certainly cause the TA to go down and the pH to drop, but it would take a lot more rain than one foot. Even replacement of 80% of the water would only have the pH drop down to around 7.2, but after that the pH can drop much more rapidly.

So I still can't explain why you saw such a large drop in TA.

Richard

[Ohm_Boy]

can anyone speculate on why the TA would crash from a normal reading in the late fall

Old reagents?

[TwistyKat]

Thanks for the replies.

We have had a lot of rain (and snow) here over the off-season, but we had a lot last year also and I didn't see the same numbers.

I did replace my reagents but after I already hit the pool with about 16 lbs of baking soda. My TA was at 190 by the next day (with the new reagent) and the PH was 7.9.

I've been hitting the pool with small amounts of Muriatic Acid all week, and I have a small pump tied up in the corner of the pool doing some circulation. It's also breaking the surface of the water to hopefully provide some aeration.

Last night the TA was down to 140 and the PH was down to 7.2.

I guess I can't rule out the the reagents were faulty, but they were new last summer and I keep them in the house during the off-season.

My plan is to get the TA down to 100 then work on bringing the PH up to normal.

[JohnT]

I never thought I'd be posting about low TA, but I'll add my story to this post. Just got mine in operation last night due to construction and replumbing issues. Since my fill water has a TA of over 400, and the water I had hauled in to fill the pool is around 280, I've always struggled with high TA. Last year I managed to subdue it to the 180ppm area.

Since I have algae, the first tests I was interested in on opening were CYA and pH. pH was high, as is normal if I'm not hitting it with acid twice a week, and the CYA was right at 35 just like last fall. Seemed a little odd since I'd had to drain water out once over the winter, and the pool has filled with rain to the top of the skimmer now. Got the chlorine in along with a little acid, and then went to do the full suite of tests on the remaining water from the original sample. I was stunned to see my TA at 50. A 130ppm reduction over the winter with no circulation and a high pH. The real kicker is that if the CYA test is right, it wasn't dilution that caused the TA to drop. Any chance that the algae in the water is causing a problem with the CYA test due to extra crud in the water?