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Thread: Strong and weak acids

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    Strong and weak acids

    Technical discussion originally from this thread. JasonLion

    Lemon juice (citric acid) is not going to be effective at lowering pH. Some people have also suggested vinegar for this but it's not effective either. The reason why is that they are 'weak' acids chemically.
    Actually, any kind of acid should work at lowering the pH, although some work better. A "strong" acid just means that virtually 100% of the acid dissociates into solution to form H+ ions at any given time. Some of the common strong acids include HCl, HBr, HI, H2SO4, HNO3, and HClO4 (there are some other wacky ones that scientists have made too).

    This is not the same as the amount that an acid will lower pH. For example, if you take a fairly basic solution, and add equal quantities (on a NORMAL basis; this refers to how many H+ ions are added, so it's basically the number of molecules * the number of H+ that will dissociate per molecule... 1 Molar HCl is also 1 Normal as there is one H per HCl, but 1 Molar H2SO4 is 2 Normal since there are 2 H's) of a weak acid like acetic acid and a strong acid like hydrochloric acid, you will end up with the same change in pH (all the acid will be used up to neutralize base, as long as you don't exceed the amount of base present). Acetic acid exists in a partially dissociated state (determined by the equilibrium constant of the dissociation reaction). Acetic acid is a carboxylic acid (these are very important in organic reactions, and are highly oxidized) and has formula CH3COOH. The last H is the one that dissociates in a carboxylic acid, which is acidic because of the resonance stabilization of the negative charge between the two oxygens when the H+ dissociates. Note that while each molecule of acetic acid has 4 H's, only ONE is acidic to any reasonable degree, so 1 molar acetic acid is 1 normal as well. The others are methyl group hydrogens and are among the least acidic hydrogens that you can have. Carbon simply is not electronegative enough to stabilize the negative charge that would result from dissociation of an H+.

    The dissociation equation is:
    CH3COOH <-> CH3COO- + H+

    Equilibrium strongly favors the left side (hence why carboxylic acids are weak acids), and only about 0.5% of the acetic acid will exist in a dissociated form (the right side) at any given time.

    However, the reason that this acid will fully neutralize a strong base is LeChatelier's principle. If there is base present, the H+ from the acetic acid will act to neutralize it (assuming an Arrhenius acid/base definition here). When that H+ is no longer there, more acetic acid will dissociate (LeChatelier's principle states that in a chemical equilibrium, if you add or remove something from either side, equilibrium will shift to counter your change). Thus, more H+ will be produced. This in turn will be used to neutralize base, and so more acetic acid will dissociate, etc. until all of it has dissociated and you have only acetate ions left.

    That all being said, the basic idea stated above that using vinegar (acetic acid) or lemon juice (citric acid) to change your pool's pH is bad is absolutely correct. For one thing, you would need a lot of it, as the acids are fairly massive per molecule (high molar mass means fewer molecules to release H+ per unit mass) and not very concentrated in lemon juice or vinegar (lots of water and other stuff mixed in). Also, you don't want to be dumping all that other stuff in your pool. Vinegar and lemon juice both contain lots of complex chemicals, sugars, and other things that are not a healthy part of your pool. At best, you'll filter them out, at worst, they will serve as food for bacteria, make your water cloudy, or have some other bad effects on the pool. Putting sugar in your pool (contained in both vinegar and lemon juice) is like putting bacteria food in your pool.

    As the pH changes, the amount that an acid will dissociate also changes. So as you get more and more acidic, a weak acid will essentially exist in a completely undissociated form. For example, if you took an acidic solution of hydrochloric acid (pH < 1 or so) and added acetic acid (don't try this at home... the reaction can be violent in some circumstances), the acetic acid will not dissociate because the pH is too low. Alternately, adding hydrochloric acid to an acetic acid solution will cause all of the dissociated acetate ions to pair up with H+ ions (basically, the Cl- is more stable than the acetate ion, so HCl + Acetate will form Cl- + Acetic Acid). The weaker an acid is, the higher the minimum pH you can achieve with it is (for acetic acid I think it's somewhere around 4-5). This is a number lower than you'd be trying to get your pool to, so you *could* get it there with acetic acid, but it's not a good idea. Hydrochloric acid (muriatic acid is hydrochloric) is really ideal in my opinion because it adds no TDS to your pool (the solid acid adds sulfates). It only adds H+ and Cl-, which is in your pool anyways (water produces a certain amount of H+... it's pH is 7.0, and pH is the - log (base 10) of the H+ concentration).

    As a general rule, I'd suggest trying to keep anything organic out of your pool (this would include citric and acetic acids). They are much more likely to muck things up. The only carbon that should be added to a pool, imo, is carbonates for buffering (baking soda and soda ash).

  2. Back To Top    #2
    JasonLion's Avatar
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    I moved your post into The Deep End

    I moved your recent post into The Deep End using a subject of "Strong and weak acids".

    Your discussion of chemistry, while fascinating, is too technical for the spa care area.

    Thanks
    JasonLion
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3
    Guest
    Which is why I did not get into such concepts as dissociation constants, etc. Those kinds of discussions are for this area. I do have a chemistry background but I try and put it into layman's terms so more of the members can follow it, even here in the Deep End.

  4. Back To Top    #4

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    Matt, welcome to TFP!!

    It's great to have another chemist 'on board'! I must admit that I didn't follow much of what you wrote (I barely passed high school chemistry), but I do appreciate your trying to educate folks like me Even when I can't fully wrap my pea brain around a technical chemistry discussion, I like to read them - whatever small % that I can grasp makes me more confident in doing my job of servicing pools - and sometimes when a customer is asking just TOO many questions, I'll quote some of the advanced chemistry I've read to shut them up

    I hope that you are not discouraged by having your post moved here! It's not a 'slap in the face', Jason is just trying to keep the regular discussions 'user friendly'

    Again, welcome to TFP! I look forward to reading more of your posts - especially when it comes to the really technical part of chemistry!
    Luv& Luk
    -Ted

    Having done construction and service for 4 pool companies in 4 states starting in 1988, what I know about pools could fill a couple of books - what I don't know could fill a couple of libraries :-D

    POOL SCHOOL, TF Testkits, Jason's Pool Calculator, CYA vs. cl chart, (Just a few DARNED handy links!)

  5. Back To Top    #5
    Actually, I'm not discouraged. I think it's encouraging to have a forum dedicated to more in depth chemical discussions. I'd like to find some other people to help me work out more details of the chemistry... not just what to add and what effect it has, but reactions and mechanisms so I can really understand it. Real information on the details of pool chemistry is very hard to find on the web, which is understandable as most people really just want to know how to make their pools work well and don't care why as long as it works.

  6. Back To Top    #6
    Guest
    I think that if you take some time to read through the deep end section you will find that there is quite a bit of theoretical pool chemistry here. You might also want to check out www.poolforum.com in the china shop section. (they are no longer accepting new members which is one of the reasons this forum was started) That is where a lot of this started. There is even a very lengthy discussion on the dangers of dihydrogen monoxode (DHMO) in swimming pools that you will find very enlightening over there!

  7. Back To Top    #7
    Lol. I've read before about the hazards of DHMO. It's frightening to think that the government allows such high levels of DHMO in our food and water!!!

  8. Back To Top    #8
    Guest
    Quote Originally Posted by Matt
    Lol. I've read before about the hazards of DHMO. It's frightening to think that the government allows such high levels of DHMO in our food and water!!!
    not to mention that gallons of the stuff are found in every swimming pool in the world!!!!!!!! If just a small amount gets into your lungs it can kill you!

  9. Back To Top    #9

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    waterbear,



    This is funny stuff. But don't we run the risk that some of newer members might [edit]......[/edit]?

    Titanium
    24,000 gallon inground freeform pool/spa circa 1983 (113 ft perimeter, 625 sq ft) with 350 gallon attached spill-over spa
    2007 2 HP, three-phase Hayward TriStar pump which is powered by an Ikeric VS-200 variable speed drive system
    1983 Laars XE Pool/Spa Heater Type ES 400,000 BTU, 1998 Hayward Super Star-Clear C-4000 cartridge filter (400 sq ft, 4 separate cartridges)
    1998 Polaris 380 pressure-side cleaner w/ 3/4 HP booster pump
    One skimmer :( and one PoolSkim :), One Supervision Galaxy LED pool lamp, Second story solar panels
    Hayward/GoldLine AquaLogic PS4 (replaced 1983 vintage dual circuit Intermatic timer)

  10. Back To Top    #10
    Guest
    we're in the Deep End...just like with a pool if you can't swim you might drown!

    (anyway, you just gave away it's a joke!)

  11. Back To Top    #11

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    Oops. I fixed that boo-boo.

    Titanium
    24,000 gallon inground freeform pool/spa circa 1983 (113 ft perimeter, 625 sq ft) with 350 gallon attached spill-over spa
    2007 2 HP, three-phase Hayward TriStar pump which is powered by an Ikeric VS-200 variable speed drive system
    1983 Laars XE Pool/Spa Heater Type ES 400,000 BTU, 1998 Hayward Super Star-Clear C-4000 cartridge filter (400 sq ft, 4 separate cartridges)
    1998 Polaris 380 pressure-side cleaner w/ 3/4 HP booster pump
    One skimmer :( and one PoolSkim :), One Supervision Galaxy LED pool lamp, Second story solar panels
    Hayward/GoldLine AquaLogic PS4 (replaced 1983 vintage dual circuit Intermatic timer)

  12. Back To Top    #12
    I would strongly urge all pool owners to drain their pools if they contain DHMO and refill them. Also, algae and bacteria won't even grow in a pool without DHMO present.

  13. Back To Top    #13
    poolhound's Avatar
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    I'm pumping mine out now and will refill with heavy water. Should I keep my FC levels the same due to the absence of DHMO?
    24000gal IG, VL, 1.5 HP, Pentair 60/60D Sand 325#, BBB, Liquidator, Dolphin M400

  14. Back To Top    #14
    If you eliminate the DHMO from your pool, you shouldn't even need a sanitizer.

    I find it's hard to test anyways, since even my Taylor kit was contaminated with large amounts of DHMO!

  15. Back To Top    #15
    poolhound's Avatar
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    I heard Dave was working on a test kit with a DHMO neutralizing coating but it will probably cost a fortune! I'm paying a huge trucking fee to get my heavy water from Oak Ridge, TN. Cooper River was closer but they wouldn't sell me any.
    24000gal IG, VL, 1.5 HP, Pentair 60/60D Sand 325#, BBB, Liquidator, Dolphin M400

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