High PH - Spa spillover raising PH in SW Pools?

[chem geek]

If your fill water is high in TA or if you have a lot of water evaporation, then the evaporation and refill can increase the TA level as well (everything in the fill water gets added to the pool). Since your water is very hard, the TA may be high in the fill water as well.

Anyway, it is a known fact that increased aeration will result in faster pH rise. The solution is to get the TA as low as possible since that reduces the rate of this carbon dioxide outgassing. However, an SWG has other ways of raising the pH such as having chlorine gas that doesn't fully dissolve in the water so outgasses from the (usually closest) return instead. And I already mentioned how you are adding way too much acid if it's full-strength since it would lower the pH too much. That just causes faster see-sawing. Also, half a gallon of full-strength Muriatic Acid in 13,000 gallons should lower the TA by 19 ppm while half-strength would lower the TA by about 9 ppm. I'm telling you, something just isn't right about your situation and I suspect some information (maybe test results) are off.

Look on the label of the Leslie's acid that you have and tell us what is lists for the ingredients. Is it this stuff? If so, then that should be 31.45% hydrochloric acid (i.e. full-strength Muriatic Acid).

What kind of test kit are you using? You aren't using test strips by any chance, are you? Or are the numbers from a pool store?

[jaimslaw]

Yes, it is Leslie 31.45% acid.

I eliminated the fill water component as no fill water was added for much of the spring season. Test kit is the Home Depot off the shelf chem test tube type - I stay away from test strips. I replace it each year with a new kit. I would like to get the TA lower - is this just a function of gradually adding acid each day? I've seen some confusing posts about this process.

[chem geek]

Lowering the TA is ultimately through adding acid, though the process is accelerated by aeration (which you already have from the spillover) and having the pH be lower during the process, but in your case what is strange is that you are adding LOTS of acid but not having the TA drop. I simply do not understand this. Since you've got a lot of pH rise and you add acid frequently, your TA should be dropping. If you had new plaster or a recent replaster, then this would make sense since plaster curing increases pH, TA and CH, but you never mentioned anything about any sort of new or replaster job.

There can be some pH rise from undissolved chlorine gas outgassing and that will also raise the pH, but to have the TA be stable when adding half a gallon of full-strength Muriatic Acid every week, that would require almost 14 ppm FC to completely outgas as chlorine gas from the pool which seems inconceivable.

[waterbear]

Sandstone can be formed from either a silica or calcium carbonate (limestone) matrix with either silica or feldspar particles. The second type (limestone) can potentially raise pH and is soluble in acidic solutions.

[jaimslaw]

RE: Sanstone raising PH (or being a contributing factor): I took the test tube, filled it with pool water when the PH was in normal limits, and added some sandstone particles that I scraped of the sandstone facing. I shook it up and then tested the PH - not really too much difference; but I left it in the tube for a few hours and came back to test it, and found a significant increase in the PH. Not saying this explains everything but am still thinking that I have a combination of factors creating this situation, especially that dual spill over spa.

Still hoping someone reading this with a SWG and a dual spill over spa can weigh as to whether any PH problems they may or may not have. Anyone out their with this set up?