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PH of concentrated and dilute acids

  1. Oct 9, 2013 #1
    1. The problem statement, all variables and given/known data

    I understand that pH is a measure of the concentration of hydrogen ions in a solution.

    A more concentrated acid would have more hydrogen ions in a given volume of solution? Wouldn't that mean that it should have a much lower pH?

    Why then do we have the pH of dilute acids being lower?
  2. jcsd
  3. Oct 9, 2013 #2


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    Staff: Mentor

    Can you give examples? It is possible you refer to the way activity coefficients behave in high ionic strength solutions, but I would prefer to know what we are talking about before going into details.
  4. Oct 10, 2013 #3
    Thank you for the reply, Borek :smile:

    I am not too sure actually.

    My thoughts are based on the pH of dilute hydrochloric acid. It's 1.0, which is very low.

    Could it be that the pH of concentrated hydrochloric acid is lower than 1.0?

    In other words, maybe for a particular named acid, its concentrated version will always have a lower pH than its dilute solution? Sorry, I am confused and just jumping to conclusions. Hehe.
  5. Oct 11, 2013 #4


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    Staff: Mentor

    Yes, it can even get negative.

    In very concentrated solutions things get complicated. Our methods of pH calculation (and to some effect measurement) assume ions don't interact too strongly as they are separated by copious amounts of water. When there is not enough water molecules, these assumptions no longer work, and rather unexpected things start to happen.

    pH is a measurement of H+ activity in water solution. Think about pure sulfuric acid - simple logic tells us it should be highly acidic. Well, if it is pure, there is no water at all! What water solution, when there is no water?
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