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Non Life Sciences => Chemistry => Topic started by: valeg96 on 22/02/2014 13:55:29

Title: how do i calculate the pH of citric acid?
Post by: valeg96 on 22/02/2014 13:55:29

how do I calculate the pH of a solution of citric acid? If i wanted to make it 2M, 1M, 0.5M and 0.1M what would their pH be? From http://en.wikipedia.org/wiki/Citric_acid (http://en.wikipedia.org/wiki/Citric_acid)

Quote

pKa1 = 3.14
pKa2 = 4.75
pKa3 = 6.39

What's the procedure?

Title: Re: how do i calculate the pH of citric acid?
Post by: chiralSPO on 22/02/2014 17:10:56

This sounds like homework, so I don't want to give too much away, but:

for the reaction H₃A <=> H₂A^– + H⁺

10^–pKa = Ka = [H₂A^–]*[H⁺]/[H₃A]

The values in square brackets are concentrations in molarity. Since one H⁺ and one H₂A^– is produced by the consumption of one H₃A, you can plug in the initial concentration of H₃A as whatever molarity you want (M), assume that there is no appreciable concentration of H⁺ or H₂A^–

so the calculation becomes

10^–pKa = x²/[M–x]

solve for x, which is then the concentration of H⁺, so pH = –log(x).

I have made the assumption that the citric acid is only deprotonated once. You can do the calculation taking all pKa values into account, but that is a LOT more work, and ends up giving a negligibly different answer.

Title: Re: how do i calculate the pH of citric acid?
Post by: valeg96 on 23/02/2014 08:24:07

It's not homework, I'm preparing various solutions to prepare a scale of a vegetal indicator I extracted some time ago. I calculated them long ago actually, and i got 1.57 for 1M, 2 for 0.5M and 2.57 for 0.1M, but I couldn't remember the procedure, and consequently if it was correct.

Edit: damn repetitions.