Using Muriatic Acid to Safely Lower Your Pool's pH

[BigTallGuy]

You can often buy 15% Muriatic Acid and if you can get it for roughly half the price of 31.45% Muriatic Acid then it's about as economical. I find that this half-strength fumes a LOT less. Nevertheless, my pool store sells the full-strength at the best price so I still use that and am just careful to be up-wind when pouring. I do measure it in a cup, but I do that pouring away from my face and over the pool.

The good news is that though the fumes are irritating, this is actually a good thing since it warns you well in advance of being too harmful. Nevertheless, it's the nastiest stuff most pool owners will experience with regard to chemicals.

It has been my experience here in Arizona that the full strength (31.45%) Muriatic acid is sold almost exclusively by pool stores, while the Grocery stores and Hardware stores sell the 15% strengths. It is also my experience that both strengths are not that far apart in price.

[MrTodd]

Wonderful explanation! I have a question, you said 1/4 gal. per 10,000 gal. for treatment. How much will that lower your PH? Is there a good "rule of thumb"? Mine is at 8.2 right now. How much should I add to get to 7.5? The following numbers are from the pool store's testing: CYA- 120, TC-10, FC-10, PH-8.2, Total Alk-178, Adj Total Alk-142, Total Hardness-261,TDS-400. I have a 26,500 gal IG with TA-60 sand filter, Hayward 3/4 Hp pump. Also should receive my K-2006 and K-1000 any day so I can do the tests myself. I went through several pages of posts to see if anyone has already answered this, but stopped after 6-7 pages. I am new to the site, and my search method needs help. Also, my water is dirty and slightly green. We vacuumed it, but I think I made a mistake and vacuumed to filter instead of to waste, and it just returned to the pool. While there for testing, the guy at the pool store talked me into buying some snake oil called flocculant. It is BioGuard Powerfloc. He did a cool demo, and it worked like a champ. I wanted to know what you guys thought before using it, or should I not? It seems everything has some sort of undesired ingredient in it. Thank you for your help!

[aylad]

It seems that everything has some sort of undesired ingredient in it. !

And THAT is why we're here--to help educate people about what they're actually putting into the pool--and that's why we encourage folks not to just blindly follow pool store adivce!!

We don't generally recommend flocculant--it has its place in pool care (some of them do, anyway), but if you don't have the right kind, and use it correctly, then it can create worse problems than you had to begin with. We would usually recommend it only as a last resort, and there are only particular kinds (I'm sure Pooldoc will have more to say on this).

How much acid it will take to lower pH is different in all pools--it's partially dependent on your TA, so that's a hard one to answer....

[PoolDoc]

Wonderful explanation! I have a question, you said 1/4 gal. per 10,000 gal. for treatment. How much will that lower your PH? Is there a good "rule of thumb"?

Short answer:

NO, it can't be predicted, but it can be measured!

Long answer:

How much effect a given dose of acid has on pool water CAN be calculated in theory. But in actual pool practice it cannot, for multiple reasons:

1. Incorrect pool volumes -- Most pool owner's estimates of their pool's gallons is INCORRECT by more than 15%. In many cases, the error exceeds 40%!
2. Testing errors -- in order to calculate the effect of a given dose of acid, you need ACCURATE measurements of the current pH, the carbonate alkalinity, the borate level, the cyanuric acid level, and possibly, other values. But, even with the best available test kit (the K2006) errors often exceed 10%, and exceed 20% with stabilizer values. If the pool owner is using other kits or 'guess strips', individual test value errors may exceed 80%!
3. Non-linearity -- because of complexities of buffer chemistry, pH change calculation in the presence of weak acids (boric acid / borates; carbonic acid / carbonates; cyanuric acid / cyanurates; etc) the change produced by a given dose does NOT follow a line, or even a smooth curve.

PoolCalculator.com attempts to provide the sort of calculation you ask for, but is notorious among the support team here for producing bizarre and misleading results, even assuming that valid data was provided. Of course, no calculator can out-perform the data it is supplied, and as noted above, pool owners CAN NOT provide the sort of data needed for accurate prediction.

HOWEVER, while pH change cannot be reliably or accurately predicted, it can be easily measured with the acid and base demand tests in the K2006.

Whether that has value or not is debated. My own opinion is that it has NO value for well operated pools, but does have value for a service guy encountering badly operated pools needed urgent correction.

What isn't really debatable is that the most practical approach, that will produce the most consistent results in pool care is using the test-small dose-retest-redose approach, instead of the all-at-once approach espoused by pool stores.

[chem geek]

The Pool Calculator uses a table-based approximation for a limited range of pH and TA though it does account for Borates (CYA turns out doesn't matter that much if one looks at TA, at least for typical pool pH ranges). The accurate calculation is complicated and is done in my Pool Equations spreadsheet. I didn't provide a simple enough formula to be used online in a timely manner. This post gives a formula where one can calculate the amount of acid or base needed for a given pH change without too much difficulty (but I found that after The Pool Calculator was already done), but the inverse calculation of predicting the pH change from a quantity of acid or base addition requires iterative searching (i.e. no closed-form formula).

[PoolDoc]

[The accurate calculation is complicated and is done in my Pool Equations spreadsheet. I didn't provide a simple enough formula to be used online in a timely manner.

Of course, even if the problems with the PoolCalculator *were* fixed, that would do nothing to resolve the issues with testing errors.

Like I said before, if you need to know how much acid or base to add, all at once, MEASURE it; don't calculate it. But, I strongly, strongly recommend against an all-in-one dose. If if you measure it perfectly, and even if you measure your doses perfectly, there's a 30% chance that your pool volume figure is off by 30% or more.

So -- at MOST -- add 1/2 of the dose calculated from an acid / base test, and then RE-TEST.

I have enormous respect for Richard's chemical knowledge and analytical acumen, but this is an area where we disagree rather dramatically. His approach, embedded in the PoolCalculator, is the SAME one followed by pool stores, and suffers some of the same problem: high precision calculation, based on LOW precision testing and pool volume data, used to determine ALL-AT-ONCE doses.

It's a bad idea in pool stores, and it's not a better one, simply because Richard's analytical formula are somewhat more accurate than the BioLab Alex program.

It works for Richard, both in theory and in practice, because he is preternaturally precise. I'm guessing he hasn't made a math error in 10 years! If you are the same . . . his approach may work for you.

But, for the other 99.99% of the world, trying to single shot pool chemical doses is a fast train to pool problems.

The ONLY times you should single-shot a large dose, is when you're adding chlorine under conditions (like heavy algae) where "too much" is almost certainly "not enough", or when you are adding something, like polyquat, where "too much" won't hurt anything but your wallet. All the rest of the time, you should NEVER add a complete single dose. Not with acid; not with bases; not with calcium; not alkalinity (baking soda).

If you have low calcium in your pool, and low carbonate alkalinity, AND you want to single shot your borax dosing (for 50 - 60 ppm borates) AND you have plenty of muriatic acid on hand, AND plenty of time, to stay with it till you get the pH back down -- if ALL those things are true, then if you want to, I suppose a single shot dose of borax to 60 ppm borates might be OK. I don't recommend it, but it will probably work out OK.

All the rest of the time, for your sake, and the sake of your pool, please leave the ALL-AT-ONCE dosing to Richard, and that tiny fraction of the human population who is as meticulous as he is.

[Kateyru]

Do you folks know the baume % in "safer muriatic acid?"
Thanks.

[PoolDoc]

Unless you are referring to a specific brand and product, that question has no definite answer.

However in general, if you dilute muriatic acid to about 12% (instead of 31%) the fuming stops. However, without special equipment, the fuming DURING dilution is so bad, it's easier to tell people how to handle it straight from the bottle.

Also, you really have to watch out for some of the "safer" products -- they are designed for surface cleaning, and have added detergents, etc. NOT what you want in your pool!

[YvonneSEU]

Thank you pool forum. I recently lowered my ALK level from 250 to 120 safely and slowly , using your recommendations, using muriatic acid, 24/7 pump and air compressor to aerate the pool. IT WORKS. Just have to be patient. I really appreciate all this great info everyone of you provide to this site.