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Confused about pH and ionsplease help

  1. Nov 8, 2012 #1
    1. If the statement ph=-log([H+] is true, then how come a solution can be acidic when carbonate is present? I have been told that even if carbonate does not accept H+ the solution will still be basic just by the presence of CO3(2-)? The confusion is this: ph only depends on [H+] then how come the presence of negative ions change the acidity level, even if they don’t actually accept hydrons?

    I thought acidity levels change ONLY because CO3(-2) ----> H2CO3 H+ is taken away.
  2. jcsd
  3. Nov 8, 2012 #2


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    Carbonate anion is a base, but it can accept hydrogen ion and be protonated. If protonated, it is an acid, H2CO3, carbonic acid.

    A solution of carbonic acid contains CO3-2, HCO3-1, H2CO3, and solvated H+ .

    How does that work?
    A solution of carbonic acid will give this incomplete reaction causing an equilibrated concentration of each reactant and product:
    H2CO3 <------> H+ + HCO3-
    HCO3- <-------> H+ + CO3-2

    Carbonic acid is a weak acid, so the presence of reactant is favored in contrast to the products.
  4. Nov 8, 2012 #3
    I see so HCO3(-) is preferred and therefore CO3(-2) takes away H+ from the solution, BUT is it tru that the presence of a acid or base in its acid or base form (with out reacting) would cause a solution to change acidity? And is it true that negatively charged ions can make a solution more basic without sucking up H+ ?? Or is it ONLY the presence of H+ and OH- that determines how acidic or basic a solution is?
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