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What's the method called of calculating pH changes in solutions?

  1. Sep 1, 2010 #1
    I'm not looking for an indepth howto, I can look that up..... I've only taken up to High School chemistry but I have intentions to research further into chemistry on my own before I get into university.

    Say I have a solution of a known quantity and known pH, and I want to add a currently unknown amount of another solution to it, which I know the pH.... How do I calculate the amount of the second solution to add to get the pH to a specific level....

    I'm not looking for a how-to if it's a long answer, just what this method is called so I can look it up more specifically.

    Also what's the method called when you calculate the pH of a diluted solution. Like say you have Acetic Acid(ph 2.4) in a %5 aqueous solution... Meaning only 5% of the solution is Acetic Acid.. How would the Ph of the solution be calculated?
     
  2. jcsd
  3. Sep 2, 2010 #2

    Borek

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    Staff: Mentor

    There is no a single method that can be used, as a lot depends on the solution composition. If you are looking for a single name of the method my guess is that you may be looking for ICE tables. However, this is just a mnemonic device that helps to organize data.

    CHeck pH calculation lectures for an excessive discussion of methods used for pH calculation. Basically they all start with the same set of equations, which are later simplified in different ways, depending on circumstances and solution composition.
     
    Last edited by a moderator: Aug 13, 2013
  4. Sep 2, 2010 #3
    So If I don't know the exact count of all the molecules in the original solution then I can't calculate the pH, other than guessing and checking?

    I have a digital pH meter so I guess I can just do that and adjust it slowly over a few hours.
     
  5. Sep 2, 2010 #4

    Borek

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    Staff: Mentor

    Something like that. And if you are not sure about composition, it is even worse. There are zillions of solutions having the same pH, but differing by buffering capacity (ie pH change after acid or base is added).

    That's the safest approach. You can try to take a small sample of the solution, titrate it to required pH, then calculate how much acid/base has to be added to the bulk solution.
     
    Last edited by a moderator: Aug 13, 2013
  6. Sep 2, 2010 #5

    epenguin

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    It ought to be called Physics Forums - other sciences - chemistry (sometimes biology) since that's the most monotonously Frequently Asked Question there. Always practically the same damned question! Usually answered patiently by Borek :approve: - how does he cope with the boredom? Sometimes by me.

    Borek has put up his guide. I tend to say well there are just these 3 or 4 principles. So write out the equation given by mass conservation/electroneutrality/equilibrium law principle as it applies to your case. Mostly they never come back with an attempt at answer so I don't know whether they then understand everything or understand nothing or I have a bad web body odour. :frown:

    You just have to use these principles

    "For all questions you are going to meet in this area and which do cause headscatchings to students just remember all proceed from just 3 principles, at a stretch 4, always the same

    1. Conservation of mass, i.e. of the total concentration of the amount of any one species summed over all its forms.

    2. Electroneutrality - equality between the sum of all the + charge (per litre let's say) and all the - charge.

    3. The equilibrium laws that apply.

    All 3 embodied in equations that correspond to the case. Just write out those equations for your case.

    4. In some problems you may need also to understand what approximations you can use (i.e. what concentrations in your equations are going to be negligible compared to others and that you can ignore to shortcut or simplify the maths.)"
     
  7. Sep 2, 2010 #6
    I understood exactly what he said and typed up a large reply, then didn't post it because I realized there was no reason for me to say any of that extra information lol :P.

    As for your principals I knew all of those, just since I'm not doing the experiment at the moment and don't have all the supplies, I wont know how much of what I will be using... so I can't really do any calculations. Reason I asked for a method in general is because I do not know every aspect of chemistry and was wondering if there was something that would help me so I can study it and when I get around to doing the experiment I can understand more thoroughly.

    Thank you both for the help
     
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