adding muriatic acid

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adding muriatic acid

Postby confused » Mon 12 May, 2008 18:32

I went to the pool store today to buy muriatic acid since they had it the cheapest. I had them run a test on my water and they said my TA is now 209 and ph is 7.6. However I asked 2 different ones how they would add muriatic acid and I got 2 diff. answers. One said to shut the pump off and "tunnel" it around the pool. Start somewhere and dump like 4 oz. then move a foot over and dump another 4 oz. (envenly dispense this around the pool). Then let it sit overnight and then turn the pump on in the morning. She said this would lower TA without affecting ph. The other place said no, that will bleach and really damage the liner but walk around the pool and pour it in with the pump running. Then to take a brush around the edges to make sure it is stired up. I thought I read somewhere on here (I have done so much reading now that I can't remember what I read) where to do it by the return. What do you recommend?


Thanks a lot.
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Postby chem geek » Mon 12 May, 2008 19:15

They are both wrong. The proper way to lower the TA is through a combination of lowering the pool's overall pH combined with aeration of the water and further addition of acid to keep the pH low. The procedure is described in this post. The pool stores and many pool service and installation professionals mistakenly think that the "slug" or "acid column" method is the way to lower the TA. This is debunked technically and with field data in this document.

You should always add acid VERY slowly over a return flow at the deep end with the pump running and then lightly brush the side and bottom of the pool in the area where you have added the acid. That way, you thoroughly mix the acid -- it's VERY strong and concentrated and you don't want it to settle (it's denser than water unless mixed). The same approach should be used for adding other chemicals such as chlorinating liquid or bleach to the pool (or Dichlor, for that matter -- Cal-Hypo is best mixed in a bucket beforehand and then poured as with other chemicals over a return flow). The only exceptions are with PolyQuat algaecide (and concentrated clarifiers) which is thick so needs to be dispersed around the pool and with Cyanuric Acid (CYA) which is slow to dissolve and can be added either slowly in the skimmer to get caught in the filter (and take up to a week to fully dissolve) or be put in a sock or panty hose and hung over a return flow (takes a couple of days to dissolve). Obviously, slow-dissolving products like Trichlor are used in floating feeders or inline chlorinators or sometimes with special versions (BioGuard Smart Stick) in the skimmer.

To aerate the water, turn on any water features you may have such as waterfalls, fountains, aerating jets, spillovers, etc. You can turn up your return "eyeballs" to point upward so water agitates the surface. You can also use this fountain device or this air injector device. If you have an air compressor and have a nozzle with many small holes, then putting that hose end into the deep end and running the compressor will aerate the water a lot and make the process go much faster.

Pools are intentionally over-carbonated (like a carbonated tasty beverage! though obviously not that carbonated) in order to provide a pH buffer and to protect pool plaster by saturating the water with calcium carbonate. The carbonates in the water are just that -- carbonation -- the result of dissolving carbon dioxide in the water. Adding baking soda (sodium bicarbonate; sold as Alkalinity Up in pool stores) adds bicarbonate to the pool which is in essence a form of dissolved carbon dioxide. The ONLY way for TA to get lowered permanently (rather than via the see-saw with pH) is to remove some of the carbonates from the water and that is done by aerating the water to drive out some of the carbon dioxide since it's at a much higher concentration in pool water than in the air. This process of carbon dioxide outgassing goes much faster at lower pH with relative rates as shown in this chart.

The slug or acid column method only partially works by lowering the overall pH, but if you don't keep the pH low and add more aeration, the process takes much longer. Technically, you could just not do anything but add acid to keep your pH in the normal range and very slowly over time your TA will get lower. It takes the same total amount of acid to lower the TA regardless of method -- the only difference is in the speed of TA drop. Of course, once the TA is lower, the rate of pH rise is also lower which is one reason to have a lower TA so that you don't have to add acid as frequently to maintain a proper pH.

It takes a LOT of acid to lower the TA -- much more than the initial amount that is added to lower the pH to 7.0 or 7.2 when you being your aeration. I can tell you how much acid you will need if you give me more info such as pool size in gallons, current pH and TA levels. Also tell me if the lowest reading on your accurate pH test (using phenol red indicator) is 6.8 or 7.0.

Richard
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TA and muriatic acid

Postby confused » Tue 13 May, 2008 15:20

Well, I tried posting a reply yesterday and I don't know what happened so I thought I would try again and almost got done and my 1 yr. old came in and deleted it. ahhhh. So I will type this for the 3rd time.

My pool readings yesterday was TA 209, PH 7.6, CH 275, CYA 10. The TA is down from 390 (my test kit). We did the aeriation last time with an air hose conncected to a garden hose with a sprinkler on it and it also rained over 2 inches and I also pointed the return jet up. That did seem to work great. The pool store told me to add 2 more gal. of muriatic acid. What do you think? The CH was 360 and yesterday it was 275. Did the rain water help dilute it?

These pool stores are a hoot. My friend gave me some dichlor left over from her little pool so I went ahead and added it thinking why not I'll raise the CYA and add chlorine at the same time. The pool store told me I need to buy 4 lbs. of stabilizer to raise the CYA from 10 to 25. I asked why can't I add more dichlor since that is doing 2 things at once and she said I was reading into it way too much. Maybe so or she is just scared I know to much and am using my brain and won't buy chemicals from them. Maybe I am stupid but that made sense to me. It obviously worked a little since my CYA is at 10. I didn't even add very much.

A different pool lady also told me not to be adjusting the levels til the water was 72 degrees. I wanted to get the levels close so we could put the solar cover on so it can warm up and be ready to swim.

I can't imagine people who call their pool store and say come get it ready and don't know anything about chemicals. I am not good at chemistry but I like to know a little about what is going on. I guess I tend to do a lot of research before I do something.

I do appreciate all of your help. I guess you can't really trust the pool stores. I don't know whether to believe their test or if mine is ok to use either. My does not test FC and I think I will have to get one that does. I know you recommend the Taylor one but it is so expensive. I am afriad my honey will croak from all the expenses getting this thing up and going. I tell him it should get better but we did start out with some nasty water.

Thanks
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Re: TA and muriatic acid

Postby chem geek » Tue 13 May, 2008 20:56

The rain may have diluted the CH but would mean that 24% of your water got replaced (overflowed) and that sounds high. If your average pool depth is 4.5 feet, then that's about 13" of rain. I suspect some of the difference is test error.

It will take 4.6 cups of Muriatic Acid (31.45% Hydrochloric Acid) in 10,000 gallons to get down to a pH of 7.2. I don't know your pool size so you'll have to scale accordingly. To lower the TA to 100 ppm will take a cumulative 2.2 gallons of acid in 10,000 gallons. So the 2 gallons from the pool store may be in the ballpark but not added all at once. Again, if you tell me the size of your pool, I can tell you how much you need to add. Or you can scale from what I just gave you. Anyway, you are on the right track and it appears that your aeration methods work well. Rain also helps aerate the water (as well as diluting it if it overflows).

You can get the Taylor K-2006 kit at a good online price here. It'll be the best investment you've made for your pool. Otherwise, you are at the mercy of pool stores and inaccurate tests, depending on what kit you now have.

As for pool stores, they don't generally pay a lot for the people at the counter and they don't get unbiased training from the industry. I can't even get the CPO training changed to show the true graph of chlorine in the presence of CYA instead of the traditional graph that is only true when CYA is not present.
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pool size

Postby confused » Wed 14 May, 2008 10:13

So sorry, My pool size is 25,000 gal. According to your post I think I would need aroun 5 1/2 gal. of muriatic acid (2.2 for 10,000) The pool store told me 2 and I wonder how they come up with that. I guess I need to write down your calculations to keep on hand.

I also found a Taylor 2005 test ket for 38.00 so I will ask my honey if I can purchase one. Thanks a lot!
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Postby chem geek » Wed 14 May, 2008 11:24

The Taylor K-2005 uses a DPD chlorine test so will only measure chlorine levels up to 5 ppm and is approximate since you compare the intensity of color to a standard. It's certainly better than nothing and has all the other tests you need (pH, TA, CH, CYA), but the K-2006 I recommended uses a FAS-DPD chlorine test where you count the drops, has a precision of 0.2 ppm (or 0.5 ppm depending on sample size), and can measure up to 50 ppm. If you can spring for the extra bucks ($49 plus shipping from the online source) you won't regret it.

The calculations for adjusting pH and for lowering TA are complicated. I have the full pool water chemistry in a spreadsheet, but it's not for novice users. The Pool Calculator is good for a lot of calculations, but again the pH calculations are approximate. The pool store was just giving you their best estimate, but they really don't know.
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how much muriatic acid

Postby confused » Wed 14 May, 2008 14:57

Thanks for the info. So, was I right on the 5 1/2 gal. of muriatic acid. You said if I told you the pool size then you could figure it for me. sorry to be a bother but I didn't see it posted in your last reply. TA was 209, PH was 7.6 (2 days ago) Thanks
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Postby chem geek » Wed 14 May, 2008 16:31

Yes, you calculated that correctly by multiplying my numbers by 25,000/10,000 = 2.5.

2.2 gallons * 2.5 = 5.5 gallons for the total cumulative addition (not all at once, of course). The initial add to lower the pH will be 4.6 * 2.5 = 11.5 cups.
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Thank you

Postby confused » Wed 14 May, 2008 22:05

Thank you!!!!!!
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stabilizer/dichlor

Postby confused » Wed 14 May, 2008 22:21

The pool store told me I need to buy 4 lbs. of stabilizer to raise the CYA from 10 to 25. I asked why can't I add more dichlor since that is doing 2 things at once and she said no. What do you think? Should I try the dichlor or add the stabilizer and then the bleach? Also, I just used that awesome pool calculator website and it said a little over 3 lbs. not 4. What is your suggestion.

Thank you!
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