How you you calculate/estimate the pH of a solution with 3 components all of different pHs?

Is there a paper describing this?

For example: 0.1M Buffer PCTP at pH 9.5 1.6M Sodium Dihydrogen Phosphate at pH 3.39 0.2M Sodium Acetate at pH 9.0255

The measured pH is ~5.5.

If i take a weighted average i.e (0.1 * 9.5 + 1.6*3.39 + 0.2 * 9.0255)/ 1.9 the pH is estimated at 4.311.

I assume the buffer "drags" up the pH but how to estimate this?

  • 1
    $\begingroup$ What have you tried to solve this problem? Like, writing the definition of pH, expressing the equilibrium constants, ...? $\endgroup$
    – ssavec
    Oct 29 '13 at 19:21
  • 3
    $\begingroup$ Could you describe more of what you want to do? An example of the problem you are trying to solve would be very helpful. For example, do you want to be able to answer questions of the type "What is the pH of a solution that is 0.0234 M in HCl, 0.212 M in acetic acid and 0.0216 M in potassium acetate?" $\endgroup$
    – Ben Norris
    Oct 29 '13 at 21:03
  • $\begingroup$ Sorry for the delay, I've amended the question. $\endgroup$ Oct 31 '13 at 9:36
  • 1
    $\begingroup$ As I stated in my previous comments: take equilibrium constants of all the chemicals inside, calculate the equilibrium concentration of $\ce{H^+}$ and from that easily pH. There is no way around it. $\endgroup$
    – ssavec
    Oct 31 '13 at 18:16

You can't just take an average of pHs- supposing it was a simple dissolution question rather than one with chemical reactions. pH=-log(H). H=10^(-pH) First convert all the pH values to concentrations of hydrogen ions and then take an average of those values and then convert to pH- that will at least get you close the correct value.


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