TA rising when using only dry acid to lower PH?

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4JawChuck

4JawChuck

0
Jun 13, 2010
223
Winnipeg, Manitoba, Canada
I am on the fifth or sixth week of my fresh liner install, here's the numbers.

FC=2.5
PH=7.4
TA=130
CH=60
CYA=30-40
No borates in the water

My question is I have stopped using MA to drop my PH and have switched to dry acid, my TA has jumped from 90-130 in the last four weeks and I have added nothing but sodium hypo to maintain my FC. My PH has been real steady and only rises about 0.1 every week, very happy with that! However my TA has jumped quite dramatically since the refill, going from 90 to 130 in only a few weeks, obviously MA has more effect on keeping it low than dry acid. I prefer the dry acid for the wife and kids to maintain the pool while I am away on business as its safer for them to handle, I currently use a 500 gram volume perforated container (baby food dish with a snap lid) for a dispenser to leave in the skimmer to dissolve the dry acid slowly.

Does dry acid raise TA? Is it the sodium hypo? Trying to figure out the chemical reason for the jump. I still have MA available here to use on an intermittent basis when I am home, but I really like the dry acid for its safety factor for young/inexperienced pool maintainers and would prefer to not use MA at all.

Thanks for advice in advance. :wink:
 
I'm not worried about the acid in the heater, there are only a few pinholes in the container and it takes a few days to dispense...I imagine it brings the PH close to neutral through the heater during that time.

The rising TA has me stumped, I had the exact opposite situation last year...I was adding baking soda to keep it up and now this year I need to keep it down. Is MA that much more effective at reducing TA than dry acid? I found that using MA gave me PH swings that the dry acid does not, probably the slow release of the powder container doing that I know.

Does dry acid reduce TA at all?

Trying to find the optimum balance of chemicals to use at appropriate times to achieve perfection, if MA is the only thing that works for TA reduction then I will use it every other week to maintain my PH.

Comments?
 
40ppm in a couple of weeks seems huge to me, according to this article (found on another pool forum) MA is more effective due to a stronger localized PH reaction.

Does this make sense?

Why you should not use muriatic acid to lower the pH of your pool.

Many customers come to us with the problem of their pool water pH constantly rising on them. While there are many reasons for the pH to continuously rise, one of the most common can be due to the use of muriatic acid to lower the pH. It is important that you understand what muriatic acid does when added to your pool water and why this can result in the pH constantly rising on you.

First, you need to understand what Total Alkalinity is and how it affects the pH of your pool water. By definition Total Alkalinity is a measurement of water's ability for the pH to change. In other words, the higher the Total Alkalinity, the harder it is for the pH to change. It is important that you maintain the Total Alkalinity between 80-120 PPM. (This can vary somewhat depending on local and source water.)


Muriatic acid is a dilute form of hydrochloric acid which is a very strong acid. Even diluted, muriatic acid is still a very strong acid. When you add muriatic acid to your pool (even if you have diluted it more in a bucket of water), not only do you lower the pH of your water but you also lower the Total Alkalinity of the water. With the Total Alkalinity lowered, it is now easier for the pH to rise again (this can be due to many reasons), so if you add more muriatic acid to lower the pH, you also lower the Total Alkalinity even more, thereby making it even easier for the pH to go up again. As you can see, it is a vicious circle you are in.


How do you lower the pH without lowering the Total Alkalinity? You should use a dry acid (pH lower, pH minus, etc.) mixed into a bucket of water first, then poured around the pool. Dry acid is a much milder acid than muriatic acid and when diluted in water will have almost no effect on the Total Alkalinity of the water while lowering the pH. This will allow you to maintain the Total Alkalinity level, which will make it more difficult for the pH to change, thereby reducing your pH fluctuation.


Why does the dry acid not reduce the Total Alkalinity like muriatic acid does? Because for the Total Alkalinity to go down, you must get the pH below 5.5 in the water. Adding muriatic acid (because it is so strong) lowers the pH of the water. When you pour it to below 5.5, the chemical reaction that lowers Total Alkalinity now takes place. Adding a dry acid (a much milder acid) especially when diluted in a bucket of water does not lower the pH in that area to below 5.5. Therefore the chemical reaction that lowers Total Alkalinity does not take place.


Now, when your pool professional tells you that it is cheaper in most cases to use a dry acid than to use muriatic acid, you will understand.


Note:If your source water (water you add to pool) has a very high Total Alkalinity, you may want to use muriatic acid to lower both the pH and the Total Alkalinity at the same time.
Click to expand...

EDIT: Never mind, Chemgeek I see you are active on this forum also and don't agree with the MA reaction theory
 
Yes, this is just a bunch of bunk and its not about agreement, but about science (chemistry). Adding a strong acid of any kind will lower the TA. Though it is true that hydrochloric acid in concentrated form as Muriatic Acid has a pH of -1, sodium bisulfate (dry acid) still has a pKa of 2 so will get WAY below the 4.5 pH transition of the TA test if that's what you really wanted to do. Of course, that isn't what you want since such low pH can harm/damage plaster and vinyl surfaces -- you just want to add acid to lower the pH and TA. You have to aerate the water to get the pH back up with no change in TA and then cycle.

The "slug" or "acid column" method was debunked in this report. You can test this for yourself in a bucket of pool water. 1/4 teaspoon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 2 gallons will lower the TA by 80 ppm. 1/4 teaspoon of dry acid (93.2% Sodium Bisulfate) will lower the TA by around 90 ppm (might be somewhat different that this since I'm assuming a specific density for the dry acid -- it's 0.14 ounces weight to lower the TA by 80 ppm). Go ahead and try this for yourself to convince yourself of what is really going on. You can use 1/8th teaspoon in 2 gallons to get around 40 ppm TA reductions so you don't go too far. Try adding it without stirring immediately (though mix it up later) and then repeat the experiment slowly adding it while stirring -- you'll get the same result. It is of course possible that you were sold some defective (weak/cut) sodium bisulfate dry acid so check the ingredients label.

Now in practice, adding a lot of acid to the pool won't lower the pH all the way down because as the pH gets lowered there will be outgassing to partially counteract that, but this happens regardless of the type of acid that is added so long as it isn't a weak acid that would contribute to TA itself (such as Cyanuric Acid).

The part about having to get the pH down to 5.5 in order to lower the TA is clearly ridiculous since there are thousands of people who have lowered the TA by only getting their overall pool pH to 7.0 or so and then aerating. As for their claim that dry acid cannot lower TA, that is also dis-proven by the many people at poolspaforum.com (and some here) who have lowered their TA to 50 ppm for the Dichlor-then-bleach method and used dry acid to do so in most cases.
 
I do not debate the "slug" versus "trickle" theory of acid addition, personally I thought it was BS when I first heard it anyway. However I am seeing a definite difference in using DA as opposed to MA to maintain PH. My CH has not budged from first fill and my CYA is right at the lower limit so this suddent rise in TA has me baffled.

The good thing is my PH is very stable the way it is and 10ppm above the recommended range for TA is not cause for concern to me. I'm going to monitor it over the next few months to see if it continues to rise or not, if it does I'm switching back to MA and will report any difference I see in this thread.
 
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