low TA, high pH

[geeman]

my pool was just plastered 4 days ago. my pH is high which i expected. I added muriatic acid to try to lower the pH but my TA is also dropping. ok, first i want to get my low TA up to about 100 by using Sodium Bicarb. Then i want to lower the pH with muriatic acid? Is this right? Won't the acid bring my TA back down too? Should i bring the TA up above 100 knowing that it will come back down when i try to lower my pH with the acid? Any help would be appreciated.

[Backglass]

Sodium Bicarbonate (Baking Soda) will raise your TA a lot, but only raise your ph some. I would add and then wait before adding acid and I will bet the ph settles down on it's own.

[pucklordofchoas]

Ta and PH are a delicate balencing act when you adjust one you adjust the other a little too, alot of it is hit and miss, use your best judgments or find some one really good at chemistry and math

[chem geek]

If you post your pool water volume and your starting pH, TA and CYA numbers and your target pH, TA and CYA numbers then I can tell you how much of each chemical you need to add. I have a spreadsheet that does all these calculations here, but it is not for novice users. Generally, you just put in your starting numbers and your target pH and then press "Calculate Acid/Base/TA" since that will tell you how much acid or base you need to add and will tell you the resulting TA. If this TA is far off from what you want, then you can change it to your target amount and press "Calculate Acid/Base/Buffer" and it will tell you how much Alkalinity Up / Baking Soda / Sodium Bicarbonate you need to add (to raise TA) or will tell you if you need to aerate instead (aeration raises the pH with no change in TA).

The Pool Calculator here is much easier to use, but it is only approximate with regard to pH and TA since those calculations are quite complex.

Finally, unless you are using Trichlor pucks/tabs as your source of chlorine (and these are very acidic), then you don't need your TA to be high -- 80 is fine, for example. Your pool will experience less of a tendency to rise in pH (when using bleach or chlorinating liquid or have an SWG pool) if the TA is lower. This is counterintuitive, but is true as a higher TA causes faster outgassing of carbon dioxide and that causes the pH to rise, especially when there is lots of aeration (an SWG pool has lots of aeration due to the hydrogen gas bubbles it produces).

A newly plastered pool will have concrete cure and that greatly increases pH and alkalinity both as well as increasing Calcium Hardness (CH). Curing essentially adds calcium hydroxide to the pool. So normally adding acid is all that is needed unless your TA was low to begin with. As noted above, you do not need a very high TA (80 is fine) as it will just make the pH rise even faster. If your CYA is low, you can use Trichlor pucks/tabs as your source of chlorine and its acidity will help compensate for the curing plaster.

Richard

[gmain]

I manage a 22,000 gallon pool and 2,000 gallon hot tub. Rarely does the pool give me trouble but the hot tub makes me want to pull my hair out (if I had any left) at times. According to our local health department, we should maintain TA between 80-120 and, of course, pH between 7.2-7.6. Every morning, TA is around 50 and pH is 7.2-7.4. I use Alkalinity Booster to raise TA 30-40 ppm. A few hours later, TA is fine but pH is well over 8.0. If I then use pH Down (sodium bisulfate), the TA also drops below acceptable range. Raising TA again only raises pH too high. I've tried raising TA with one fell swoop and also a little at a time and the results at the end of the day are always the same. . . ARRRGH! It's a never ending battle! I've tried baking soda hoping it wouldn't affect pH but it does the same as Alkalinity Booster (which the label says is also 100% sodium bicarbonate). I am open to anyone's 2 cents-worth on this matter. Thank you.

[chem geek]

The Total Alkalinity (TA) has nothing to do with health so for the health department to enforce an 80-120 ppm rule is misinformed at best. The reason for having TA is to provide a pH buffer (and in plaster pools in addition to calcium hardness, to provide calcium carbonate saturation to prevent dissolving plaster surfaces). TA is essentially over-carbonating a pool or spa, just like a carbonated beverage (though not that carbonated). In a spa with jets running, the water is aerated and carbon dioxide is driven out of the water into the air, just as if you blew bubbles into a carbonated beverage (or stirred it vigorously) which makes it go flat -- it removes the carbonation.

The problem is that when carbon dioxide leaves the water, this makes the pH rise with no change in TA. Technically, carbon dioxide in water is carbonic acid (and bicarbonate and carbonate) and when you force it out of the water you are removing an acid so the pH rises. The TA doesn't change for technical reasons I won't get into here.

When you add acid to lower the pH of the spa, that lowers both the pH and the TA so the net result of aeration plus acid is a lowering of TA. So while TA from carbonates acts as a pH buffer preventing swings in pH from EXTERNAL sources of acid or base, it also is a SOURCE of rising pH on its own, especially noticeable when there is lots of aeration. In fact, if you look at the TA lowering procedure in this post, it should look somewhat familiar as to what you have been doing unwittingly, except that you didn't lower the pH first (which makes the outgassing of carbon dioxide happen even faster -- the high aeration in your spa made that step unnecessary).

There are two solutions to this problem and both together are the best thing to do. One is to not maintain as high a TA level. 50 ppm is sufficient for a pH buffer unless you are using an acidic source of chlorine (i.e. Trichlor or Dichlor). If you are using bleach or chlorinating liquid as your source of chlorine, then these are pH neutral (when accounting for the usage of chlorine which is acidic). What is your source of chlorine for the spa?

The second solution is to provide an additional pH buffer that is NOT carbonate (carbon dioxide). You can add 30-50 ppm Borates to the water -- from 20 Mule Team Borax. It takes 23 ounces weight (about 1.5 pounds) of 20 Mule Team Borax combined with 15 ounces weight of dry acid (sodium bisulfate) (or 11 fluid ounces of 31.45% Muriatic Acid) to add 40 ppm Borates to 500 gallons of water, so you can scale this accordingly for the size of your hot tub. Borax is a base so makes the pH go up which is why you also need to add the dry acid. You can add them back and forth or just add the Borax and then the dry acid -- it's not so much as to cause a problem. The Borates will act as an additional pH buffer, but will not contribute to the rising pH because they are not carbonates (i.e. not dissolved carbon dioxide).

If your health department absolutely positively requires a minimum TA of 80, then you are stuck because any significant aeration will cause the pH to rise.

Another thing you can do in the hot tub (and in the pool, if it's indoors so you aren't already doing it), but the health codes may not allow it, is to use a small amount of Cyanuric Acid in the hot tub -- around 20 ppm -- and then keep your Free Chlorine level at around 4 ppm or higher depending on the kind of bather load you have (so that you don't run out). The CYA will reduce the disinfecting chlorine concentration by an order of magnitude, but will still be enough to kill pathogens. This will let swimsuits last longer and you'll lose less chlorine into the air (so should find improved air quality) and you can maintain a higher FC to not run out of chlorine while not overdosing in disinfecting chlorine. The easiest way to add CYA is to use Dichlor since for every 1 ppm FC from Dichlor it also adds 0.9 ppm to CYA. 2.5 ounces weight of Dichlor in 500 gallons will raise the CYA by 19 ppm so you can scale accordingly. If you want, you can start with half this amount for around 10 ppm CYA to see how things go. You don't want to use too much CYA or else you won't get sufficient disinfection.

Richard

[mr_clean]

What is your source of chlorine for the spa?

I think knowing this and looking at his chemical readings will help.
I wonder how high chlorine really is and if PH test might be getting incorrect reading?