I need a chemist! pH question

Discussion in 'Equipment' started by sfsuphysics, Nov 3, 2009.

  1. sfsuphysics

    sfsuphysics Supporting Member

    Since I never took chemistry in college my understanding of pH is a tad weak so please fill in the gaps.

    The "number" of a pH basically has to do with a concentration of hydrogen ions, etc etc etc... basically pH 7 has 10^-7 where as a pH of 8.3 has 10^-8.3 concentration correct?

    So if I add limewater (pH 12.3?) of a certain quantity (lets just say 1 gallon worth to keep numbers simple) to 199 gallons of water at a pH 8.0 then it should go to reason (ignoring localized pH spikes of course), that the ending pH is

    log ((1* 10^-12.3 + 199 * 10^-8) / 200 ) will give me my pH (basically I'm taking a weighted average of the actual concentration)

    Which looks like 8.002 which kinda sorta makes sense from a mathematical sense since i'm really adding very little to a large volume (0.5%) however, the overall number makes me think that's actually quite low.

    The other side of this coin is that if I did the same thing with an acid (say vinegar) where the concentration of hydrogen ions swings more the other way, then my pH would crash to something like 4.7 (which equally seems wrong).

    So like I said... I need a chemist, who can tell me how to calculate the pH swings.

    Then as an aside, where exactly do the hydrogen ions go? Since even if I spiked my pH over time it would drop back down, is there any quick and easy formula to figure out the rate of ph drop?
  2. Gomer

    Gomer Honorary Member

    Neither the salt water nor the acitic acid are strong acid/bases, so you need to treat them as weak acid/bases. This means that you need to invoke Ka's.

    You'll need write out the full balanced equations using the Kas (or Kbs) for the system.

    I haven't done this stuff in....oh, 10 years, but if you want, I can try and remember how to do it all. It isn't extremely hard, but it isn't trivial either.

    EDIT: wikipedia has some info for you if you want to solve it. http://en.wikipedia.org/wiki/PH
  3. sfsuphysics

    sfsuphysics Supporting Member

    Alright, I do want to solve it, but any chance you can narrow it down what exactly I use to solve it? Ka? Remember I never took chemistry in college :D

    I see a quadradic equation, however quite a few things I just don't get.
  4. sfsuphysics

    sfsuphysics Supporting Member

    ok NM talked to Randy, and he says it's a bit impractical, and simply said 1.25% of the tank volume will raise the pH 0.6 to 0.7 units...
  5. Gomer

    Gomer Honorary Member

    It also gets much more complicated because you need to figure out the approximate buffering capacity and all that jazz. You basically have to solve a not so fun system of equations.
  6. robert4025

    robert4025 Sponsor

    This is why I hate Math. I think you guys are scaring the newbies with this... :D
  7. sfsuphysics

    sfsuphysics Supporting Member

    This is why us physicists created chemistry, so that beyond the hydrogen atom it's all in their hands :D
  8. JAR

    JAR Supporting Member

    this guy might be able to help.

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