# The Naked Scientists Forum

### Author Topic: how do i calculate the pH of citric acid?  (Read 13812 times)

#### valeg96

• Jr. Member
• Posts: 47
##### how do i calculate the pH of citric acid?
« 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 [nofollow]
Quote
pKa1 = 3.14
pKa2 = 4.75
pKa3 = 6.39

What's the procedure?

#### chiralSPO

• Global Moderator
• Neilep Level Member
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##### Re: how do i calculate the pH of citric acid?
« Reply #1 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.

« Last Edit: 22/02/2014 17:14:03 by chiralSPO »

#### valeg96

• Jr. Member
• Posts: 47
##### Re: how do i calculate the pH of citric acid?
« Reply #2 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.
« Last Edit: 23/02/2014 08:31:06 by valeg96 »

#### The Naked Scientists Forum

##### Re: how do i calculate the pH of citric acid?
« Reply #2 on: 23/02/2014 08:24:07 »