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PH, Alkalinity, and total carbonate concentration

  1. Oct 13, 2012 #1
    1. The problem statement, all variables and given/known data
    If the pH of a natural water is 7.8 and the alkalinity was measured to be 125 meq/L, determine the following:
    -Alkalinity as mg/L CaCO3: Answer = 6250 mg/l CaCO3
    -Approximate the total carbonate concentration CT, in moles per liter: Answer = 0.125 moles per liter

    2. Relevant equations
    Not certain

    3. The attempt at a solution
    I was able to determine the alkalinity by finding the equivalent weight of CaCO3, 50 g/eq. I then converted 125 meq/L to eq/L. Then I multiplied (50 g/eq)(0.125 eq/L)(1000 mg/g) and got an answer of 6250 mg/L of CaCO3. I am having trouble approximating the total carbonate concentration. Could someone help please?
     
  2. jcsd
  3. Oct 14, 2012 #2

    Borek

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    Staff: Mentor

    What is the definition of alkalinity? How is it related to the carbonate concentration?
     
  4. Oct 14, 2012 #3
    Well, alkalinity is the buffering capacity of the liquid. The resistance of the liquid to pH change. The greater the carbonate concentration the greater the buffering capacity.
     
  5. Oct 14, 2012 #4

    Borek

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    Staff: Mentor

    Let me put it differently then. How is carbonate alkalinity measured and how - in result of the way it is measured - does it depend on the carbonate concentration?

    I am not asking about qualitative description, but about a simple quantitative one.
     
  6. Oct 14, 2012 #5
    I realize it is going to be something like this:

    Alkalinity = (1 eq/mol)([HCO3]) + (2 eq/mol)([CO3]) + (1 eq/mol)([OH]) - (1 eq/mol)([H])

    The influence of the OH and H are minimal and easy to calculate. [H+] = 10^(-7.8), [OH-] = (10^(-7.8))/(1 x 10^(-14))
     
    Last edited: Oct 14, 2012
  7. Oct 14, 2012 #6
    I also know that ultimately we will get something like this:

    Alkalinity = f1Ct + f2Ct + [OH-] - [H+]

    f1 and f2 need to be calculated in order to ascertain Ct.
     
  8. Oct 14, 2012 #7

    Borek

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    Staff: Mentor

    You start with pH of 7.8. What do you have to neutralize?

    Note: I don't like the answer given, IMHO it is slightly off.
     
  9. Oct 14, 2012 #8
    We have to neutralize an H+ donating acid.
     
  10. Oct 14, 2012 #9

    Borek

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    Staff: Mentor

    Which is?

    You are giving the simplest possible answer, one that doesn't require any insight, and you wait for another hint. Try harder.
     
  11. Oct 14, 2012 #10
    Carbonic Acid (H2CO3) I believe.
     
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