New pool acid usage

[dschlic1]

I have a new pool which was filled on Oct. 26. On Nov. 10 I had the water tested the results: :-D
TC 4.0 ppm
FC 4.0 ppm
CC 0.0 ppm
pH 7.6
TA 100 ppm
CH 250 ppm
CYA 60 ppm

During the following week I noticed that the pH was going up so I added about 19 oz of Muriatic acid to the pool. The weekend of Nov. 17 the pH was still up so I added another 32 oz of acid. On Nov. 24th I again had the water tested:
TC 3.0 ppm
FC 3.0 ppm
CC 0.0 ppm
pH 7.6
TA 125 ppm !
CH 325 ppm
CYA 50 ppm
Salt 3000 ppm

The store recommended adding 1.5 qt of acid to lower the TA. My question is, is it possible that a small pool (7,500 gal) will need over 1 qt of acid per week? Also the CH shot up at an alarming rate.

 

[Guest]

yes, this is entirely normal with new plaster. The acid demand will last for perhaps 6 months to a year and then tapers off as the plaster fully cures. Pebble tec and other aggregate finishes are plaster finishes.

 

[mas985]

I have a plaster pool and still use about 1/2 gallon per week. When it was first opened, I think I was using close to 1 gallon per week to keep the PH down.

After the plaster cured, I still needed quite a bit of acid because of high PH/Alk fill water and a SWG both of which drive PH up. I tried Borax but that didn't seem to help much for me probably because of the fill water.

 

[duraleigh]

Also the CH shot up at an alarming rate.

Probably a testing error. CH typically would not move unless you somehow added calcium to your pool.

The store recommended adding 1.5 qt of acid to lower the TA.

Think of the acid as lowering pH, not T/A. About the only way to permanently lower T/A is thru aeration. After all that, your T/A of 100 is fine and I wouldn't touch it. I would keep adding acid to keep the pH under control until your plaster cures. you should see a steady, slow decrease in the amount of acid you need to use.........as others have said, possibly taking a year, but normally less.

 

[Guest]

Also the CH shot up at an alarming rate.

Probably a testing error. CH typically would not move unless you somehow added calcium to your pool.

Actually, as the new plaster cures it is normal for the calcium level of the water to rise as calcium salts that form as a result of the curing dissolve. As long as the levels don't go too high it's not a problem. Keeping close tabs on your pH is the key to it all! Don't let the pH get too high. My rule of thumb is when it hit's 7.8 it's time to lower it back down to 7.6. If this means a daily addition of acid, so be it! Remember, the curing process won't last forever (even though it seems to!)

 

[chem geek]

The curing of plaster is mostly the following equation:

2Ca₃SiO₅ + 7H₂O --> 3CaO•2SiO₂•H₂O + 3Ca(OH)₂ + heat
Uncured Pool Plaster + Water --> Cured Pool Plaster + Calcium Hydroxide

where the uncured plaster (technically, anhydrous portland cement, tricalcium silicate) absorbs water into its crystal structure and releases calcium ions that raise Calcium Hardness (CH) as well as hydroxyl ions that raise the pH. If the above equation is what is going on, then for every 10 ppm rise in CH, it takes 25.5 fluid ounces of Muriatic Acid per 10,000 gallons to compensate for pH. Or equivalently, when the plaster is curing then the requirement to add 1 gallon of acid in 10,000 gallons roughly corresponds to an increase in CH of 50 ppm.

In your 7500 gallon pool, the increase in CH of 75 ppm would theoretically require 143 fluid ounces of acid to compensate for the rise in pH. You only added 51 fluid ounces of Muriatic acid so some of the calcium may be from dissolved plaster dust rather than just from the curing of the plaster (calcium oxide can combine with dissolved carbon dioxide to form calcium carbonate though I would expect that to raise the pH and TA as well). You did record an increase in TA so that makes some sense, but the amount of acid that would be needed to compensate for a CH rise of 75 ppm and a TA rise of 25 ppm would be about 98 fluid ounces which is still higher than what you added. Essentially, about 5 pounds of pool plaster cured and 10 pounds dissolved.

This link and this link are two of several that describe the process of curing of cement or concrete (pool plaster is mostly cement, not true plaster which is calcium sulfate hemihydrate).

Richard

 

[duraleigh]

Ooops! Sorry for the misinformation about the CH.

I should've remembered that so I'd like to cash in one of my "Senior moment" chips at this time.

I'm not sure how many I have left...I knew yesterday, but.........