Muriatic Acid versus granular acid

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
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Muriatic Acid versus granular acid

Postby BilliBob » Tue 10 Mar, 2009 04:55

I thought I had ph problems. Wow!! :!: :!:

I vow to never complain about having to add acid again. I go through no more than 50kgs of the granule stuff a year.


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Muriatic Acid versus granular acid

Postby Me... » Tue 10 Mar, 2009 09:06

Larry, I can't help wondering about the Sand Filters. Their size in particular and if increasing their size to extend times to backwash would help. Cutting down on fill water would seem to be something to aim for. Even though I don't care for Cartridge filters on pools I might even consider them here. That fill water is awful!
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Muriatic Acid versus granular acid

Postby chem geek » Tue 10 Mar, 2009 10:28

Good point about the cartridge filter to eliminate having to use fill water to replace water lost from backwashing. Also, using a pool cover will nearly eliminate evaporation. As this map shows, evaporation rates can be high. Even a 50-60" annual evaporation rate means that a 4.5 foot average depth pool has all its water replaced, but since evaporation only removes the water and not its contents, everything in the fill water just gets added back to the water and that includes CH and TA. Also, a pool cover will nearly eliminate carbon dioxide outgassing as well.

Of course, these things are easier said than done. I'm so glad I have an automatic safety cover. Though its thinner so does not provide as much insulation as a bubble cover, it's very convenient so always gets used.

Richard
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Muriatic Acid versus granular acid

Postby Larry » Tue 10 Mar, 2009 12:38

Me... wrote:... increasing the filter size to extend times to backwash would help. Cutting down on fill water would seem to be something to aim for.

We backwash as a matter of procedure at least every 2 weeks, regardless of the filter pressure. If we leave the time between backwashing too long, we periodically run into problems of the filter sand clogging or caking up. The pump is then unable to do a thorough backwash and the result is a lot more work for us.

An average backwash/ rinse cycle lasts about 3 minutes. With a 2Hp pump that means about 300 gallons (1,200 liters) per backwash, which is not excessive in a 35k gal pool. It works out to almost 1% of the pool volume and I can't imagine this would make such a big difference on the overall body of water.

Covers are out of the question for these pools in the summer as the pools are used daily, and the overflow circulation system would pose some design (and monetary) constraints on an automated cover.

I'm going to set up a "ramp" for the overflow channel water, to ease it into the compensation tank. A couple of test pools will allow me to see if the pH remains more constant without the excessive aeration.

At least the acid demand of the water drops after the first year and we only have this headache with newly filled pools.

Larry
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Muriatic Acid versus granular acid

Postby bpratt » Fri 08 May, 2009 11:40

The reason this is correct is that TA (Alkalinity) acts as a pH buffer, prevent quick rise or fall. If you lower your pH with Muriatic Acid, the TA will drop dramatically where as the reaction is much slower with a solid acid leaving the TA alone. Dry acid is much safer, and can be used to lower your TA as well, instead of broadcasting in on the surface of the pool put it in 1 area letting it settle on the bottom. You need to be careful with this method on vinyl pools however because it can damage the liner if left too long. Anyways you let it sit for a few hours then brush it up to dissolve it. The other reason why your pH can consistently by higher is a high TA, the easiest way to lower this is with muriatic acid, you just have to be careful because if you add too much your pH will drop faster than your TA, if you add it too slow, your TA will bounce back.
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Muriatic Acid versus granular acid

Postby chem geek » Fri 08 May, 2009 12:42

This is NOT true. Muriatic Acid and Dry Acid will have the same effect on the pool water. The "acid column" or "slug method" were formally debunked here . There is a negligible effect on the way acid is added, but the most efficient way to lower the Total Alkalinity (TA) is to follow the procedure described in the post at the start of this thread. If the dry acid dissolves slowly, then it's effect on BOTH pH AND TA will be slower -- there is no separate effect that somehow the dry acid dissolves only affecting pH and not TA!!!

pH and TA move together when adding acids or bases. However, aeration of the water raises the pH with no change in TA as it drives carbon dioxide out of the pool which ultimately is the ONLY way that TA gets lowered at the same pH level. That is, you can drive yourself crazy adding acid to lower pH and TA and then adding base, or even worse, pH Up, to raise the pH back up and find that the TA is even higher than before!

Remember that TA is mostly a measure of bicarbonate and that this is proportional to the amount of dissolved carbon dioxide in the water (that proportion varying with pH). Pools are intentionally over-carbonated (like a carbonated beverage, though obviously not THAT carbonated) in order to provide a pH buffer and to saturate the water with calcium carbonate (the carbonate coming from TA, the calcium from CH).

The key to a faster lowering of TA is to lower the pH by adding acid, usually to 7.0, and aerate the water as much as possible. As the pH rises, add more acid to keep the pH low and continue to aerate. This process will drive off carbon dioxide and lower the TA level. When the TA level is where you want it, stop adding acid and just aerate to raise the pH to the desired level.

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Muriatic Acid versus granular acid

Postby Me... » Sat 09 May, 2009 08:36

Heehee A topic that will never go away. I am not sure about dry acid but I do know I will continue to use this so called slug method. Its what I have always done as it always seem to work the way I intend it to work. And it has never caused any damage and ......................... it makes me happy :)
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Muriatic Acid versus granular acid

Postby chem geek » Sat 09 May, 2009 12:21

Me... wrote:Heehee A topic that will never go away. I am not sure about dry acid but I do know I will continue to use this so called slug method. Its what I have always done as it always seem to work the way I intend it to work. And it has never caused any damage and ......................... it makes me happy :)

To lower TA, you've dumped acid in one spot in a vinyl pool with the pump off? Low pH is the worst thing for wearing on a vinyl pool.

TA will get lowered based on the amount of acid that is added, regardless of how it is added. If you then just wait for the pH to rise on its own from outgassing (that is, do NOT add any pH Up product), the TA stays at its same lowered value so there is a net reduction in TA. It won't matter how you've added the acid. If you add it slowly around the pool, you'll still get the same TA reduction. 7.2 cups of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 10,000 gallons lowers the TA by 10 ppm.

In other words, adding acid over time lowers the TA anyway so I'm not saying it doesn't work, just that it's not efficient if you want the TA lowered more quickly.

The only purpose of the "lower pH, aeration and acid" procedure is to accelerate the process by increasing aeration and having the pool's pH be lower as a whole so the entire surface of the pool participates in the carbon dioxide outgassing. You don't use less acid with any procedure -- you can only accelerate the process in a kind of "pay a lot now" vs. "pay later over time" tradeoff in terms of when the acid is added.

So are you saying that if you add the acid around the pool instead of in one spot that the pH will drop and the TA will not? It will be hard to see that unless you are dropping the pH a lot since at 100 ppm TA it takes dropping the pH from 7.5 to 7.07 or from 8.0 to 7.27 to see a 10 ppm drop in TA. At 150 ppm TA it takes dropping the pH from 7.5 to 7.17 or from 8.0 to 7.43 to see a 10 ppm drop in TA. Also, I presume with the slug method you are not just measuring the TA in the spot where you dumped the acid but are waiting after turning back on the circulation and then measuring the TA overall in the pool.

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Muriatic Acid versus granular acid

Postby Me... » Sun 10 May, 2009 08:24

Where I am, Alkalinity comes out of the tap at about 10-20ppm. I do a substantial number of auto feed systems using Liquid Chlorine and Muriatic Acid. I could have people fill their acid vat with straight Muriatic but instead I have them dilute it about 10-1. Put in 10 gallon of water for every gallon of acid. Their systems chug along nicely and the Alkalinity holds pretty steady with of course the occasional addition of Bi-Carb.

Several times either the owner or someone else has advised the owner to just use straight Muriatic. I know when this happens because all of a sudden their bi-carb consumption goes through the roof. And of course they now start to get nice big spikes in their pH readings. As we also know, if the pH is (spiked) low, the ORP reading will be falsely high and even if the pool is wanting some chlorine, the controller thinks it is good enough and won't feed.

When that false low spike is over you can imagine the chlorine demand is now higher and so it feeds more chlorine than in a normal cycle which of course creates more acid now. In the meantime all this acid feeding with straight Muriatic has lowered the TA and when the Bicarb goes in guess what, we have even more pH demand.

Anyways, feeding chemicals in stronger solution than needed can create havoc. The cycle described above is real and happens. Of course some fill water might deserve a stronger solution but this scenario has convinced me that adding it strong (in one place) will affect Alkalinity moreso than adding it weak (pouring it around the edge)

And no, I do not recommend ever adding any chemical to any pool with the pump off.
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Muriatic Acid versus granular acid

Postby chem geek » Sun 10 May, 2009 11:40

OK, that makes more sense. Concentrated chemicals combined with single-point measurements in a feedback system can create havoc so having slower diluted introduction with good mixing is far better. I was more commenting on the manual addition and not saying that there was no difference, but that it wasn't very much and that low pH with aeration was more efficient, especially for lowering the TA by larger amounts. Some people have used air compressors or special attachments to their returns or turn on waterfalls, fountains and spillovers that really aerate the water and it has the process go a whole lot faster (especially if the pH is lowered to 7.0).

One area where one does see a substantial noticeable difference in terms of how chemicals are added, especially in spas, is when adding baking soda to raise the TA. Theoretically, the rise in pH should be fairly low from such addition, but oftentimes one sees a larger pH rise. This is most likely due to immediate carbon dioxide outgassing when the baking soda concentrates the water near the surface where added to be very high in TA and then outgasses carbon dioxide raising the pH.

Richard

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