TA high, crazy scaling in just 1 day

[chem geek]

I personally like the Taylor kits (and the tftestkits TF100 is a more logical repackaging with better reagent sizes), but regardless I think something else is going on with regard to your varying testing, even over time. I suspect that your varying results are due to the amount of shaking of the test vial between drops, though technically speaking the outgassing of carbon dioxide raises the pH and there should be no change in TA from that (later adding acid is what causes both pH and TA to drop). So though it's a bit of a mystery, you might try adding, say, 20 drops before doing the mixing and swirling.

Also, read this link from Taylor regarding a buildup of static electricity that can affect drop size. This would apply to all brands of tests, not just Taylor's. I wouldn't worry too much about the variation at this point. If it's due to the number of drops (either from extra mixing or from static electricity), this error will reduce as you get to lower TA levels (and therefore use fewer drops in the test).

As for the amount of acid to lower pH, as the TA goes down it will take less acid to lower the pH by the same amount. That's why I gave a range from 59 fluid ounces of acid at high TA to 32 fluid ounces of acid at low TA. This is the sort of lowering to be expected -- each to drop the pH from 7.2 to 7.0. Obviously, what you read on your pH test trumps whatever I tell you (i.e. you don't want to go too low). Fortunately, there is a bit of a spring effect in lowering pH in a carbonate buffer system (that the pool has) -- it takes more acid to go from 7.2 to 7.0 than it does to go from 7.6 to 7.4, for example. So, fortunately, it gets harder to overshoot as you go lower in pH (though it can still be done -- many Trichlor users that don't watch their pH levels and do not compensate accordingly can end up with acidic pools at a pH near 4 and zero measured TA -- an obviously very, very bad thing).

Richard

[justjabbin not logged in.]

How much is too much acid to add to a 19000 gal pool in one day? I seem to get answers all over the place from a few cups to a few gallons? How much at each time?

[fatybabe]

10 ounces at most.

[chem geek]

10 ounces of Muriatic Acid (31.45% Hydrochloric Acid) in 19000 gallons with a TA of 100 would lower the pH from 7.5 to 7.38 (a drop of 0.12) so is a decent limited adjustment to do at one time. But this is more for safety since the calculations are complex for arbitrary TA and starting/ending pH. For example, with a TA of 200, the pH would drop from 7.5 to 7.43 (a drop of 0.07) while at a TA of 100 but starting at a pH of 7.2 the pH would only drop down to 7.13 (a drop of 0.07).

I've safely added more as have others when using my spreadsheet to calculate the actual amount needed (for the TA lowering procedure, for example), but one needs to also consider measurement errors. Fortunately, as the pool gets lower in pH, it resists further lowering that much more so this prevents overshooting by too much unless one really mis-estimates by a large amount.

Regardless of how much acid is added, it should be added very slowly over the flow from a return (with the pump running, obviously). Do not ever pour it into the skimmer -- not even slowly. I've calculated that the acid is so strong that it completely overwhelms the carbonate pH buffer in the pool water where it is poured (even with skimmer flow rates) so even a slow pouring in the skimmer results in a pH hitting the pump, filter and heater of around 1-2 which is very, very acidic. Pouring slowly over a return flow at the deep end of the pool has the largest volume of water dilute the acid and minimize the low pH exposure.

Richard

[Guest]

WOW...you do not just add a gallon of acid in the pool all in one day if you are a 20k pool! i would add like 4pt(half a gallon) max!!! and when you add the acid you add it all in one spot away from jets that brings the alk down if you boardcast it then it bring down your ph more than the alk...You want to do acid treatments ONLY when you ph is 7.6 or higher...sometimes you can put in so much acid your ph will NEVER come back up, plus if the water is really low in ph that is bad on the salt generator!!! for the scaling you can put in some scale inhibitor or metal control same thing and that should stay in your pool for a couple week then at like a cup or so a week to keep it from scaling again

[chem geek]

Dumping acid in one place in an attempt to lower the Total Alkalinity (TA) is known as the slug method or the acid column method and is debunked in this link. Lowering the pH via addition of acid will ALWAYS lower the TA no matter how you add the acid. pH and TA move together when acid or base is added. It's not that dumping acid in won't remove any carbon dioxide at all -- it's just that it is far more efficient to combine aeration with the approach and to lower the entire body of pool water to the lower pH so that the entire surface of the pool participates in the outgassing.

The only way to lower TA is to drive carbon dioxide out of the pool and that only happens to any significant degree through aeration. It also happens faster at lower pH and higher TA levels as shown in this chart showing relative outgas rates as a function of pH and TA. This post describes the fastest way to lower TA by the combination of aeration at low pH with frequent acid addition to keep the pH low. One can, of course, just let the aeration make the pH go up higher and add the acid less frequently, but the process will take a little longer as the fastest outgassing (removal) of carbon dioxide (and carbonates) from the pool occurs at lower pH.

On the other hand, I agree with you that if you dump a gallon of acid into a 15,000 gallon pool that starts out even at a pH of 7.8 with a TA of 120 (and CYA of 30), you will end up with a pH of 6.67 and a TA of 87 and will much more rapidly outgas carbon dioxide which will make the pH rise with no change in TA. So you may end up with a pH of 7.2 to 7.5 with a TA of 87 after such a large acid addition and giving it some time for the outgassing (a pool cover will inhibit that pH rise, for example). So in that respect, I agree with you, that a large acid addition will end up lowering the TA (and be too low in pH, especially for vinyl, as well), but I disagree with the mechanism. It has little to do with adding it in one place or distributing it around the pool. It mostly has to do with lowering the pool's overall pH so low as to drive off carbon dioxide.

Also, in pools with very high TA, they are already outgassing a lot of carbon dioxide and having the pH rise even at moderate pH, so in that situation just adding acid to get the pH down to around 7.2 or so and then letting it rise back up again will work to lower TA. When the TA is lower, however, then getting it even lower is much easier to do by forcing the pH lower to 7.0 and, most especially, through adding additional aeration. An air compressor works really well as do waterfalls, fountains and other aerating water features, but even pointing up the returns helps. Those pools with an SWG already have lots of aeration due to the hydrogen gas bubbles produces in the SWG cell (you can see them at night if you run the pump and SWG with a pool light on) and is the reason most SWG pools tend to rise a lot in pH. Having a lower TA helps reduce this problem in such pools as does using 30-50 ppm Borates as an additional pH buffer and algaecide (which lets one lower the SWG output since less chlorine is consumed preventing algae growth).

Richard