[gentoo]

Hi guys,

I'm having some embarrassing trouble creating buffers.  I need to make a 2, 7, and 12 pH phosphate buffers.  I understand that phosphoric acid has 3 pKa's that should make this possible and have had no trouble creating the pH 7 solution using the Henderson-Hasselbalch equation with monobasic and dibasic potassium phosphate.  The problem is with making the 2 and 12.  All of my solutions have been centered closely around pH7.  How do you "pick" the pKa value for phosphoric acid that you want to hone in on?

Sorry if this doesn't make much sense.  I appreciate the help.

[Borek]

When preparing a buffer you should select an acid with pKa as close to the required pH as possible.

[gentoo]

Yes, and Phosphoric acid has 3 pKa values.  One at 2.15, one at 7.2, and one at 12.38.  What I want to know is how to select the lower or higher pKa value when I'm trying to formulate a buffer near those pH values.

[sjb]

Yes, and Phosphoric acid has 3 pKa values.  One at 2.15, one at 7.2, and one at 12.38.  What I want to know is how to select the lower or higher pKa value when I'm trying to formulate a buffer near those pH values.

Perhaps read up on buffer capacity, e.g. http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity ?

[gentoo]

Perhaps if I rephrase the question:

I would like to make a phosphate buffer with a pH of 2.  How should I go about doing this?

[Borek]

Which of the three pKa values is closest to 2?

[gentoo]

The one at 2.15.

Plugging that into the HH equation gives that there should be a ratio of .71 base/acid ratio.  When I tested it using this ratio of moles of monobasic to dibasic potassium phosphate it resulted in a pH of around 7.

[Borek]

The one at 2.15.

Plugging that into the HH equation gives that there should be a ratio of .71 base/acid ratio.  When I tested it using this ratio of moles of monobasic to dibasic potassium phosphate it resulted in a pH of around 7.

Please write all three dissociation steps for phosphoric acid, and write formulas for all three dissociations constants (Ka1, Ka2, Ka3). Writing formulas for mono- and dibasic potassium phosphate would be also helpful.

[gentoo]

Ka1 is the dissociation between phosphoric acid and dihydrogen phosphate.

Ka2 is the dissociation between dihydrogen phosphate and hydrogen phosphate.

Ka3 is the dissociation between hydrogen phosphate and phosphate.

dibasic potassium phosphate is the potassium salt of dihydrogen phosphate.

monobasic potassium phosphate is the potassium salt of hydrogen phosphate.


Does this mean that in order to get a buffer centered around Ka1 I need to use .71 dibasic potassium phosphate to phosphoric acid?

[Jasim]

Maybe I'm misunderstanding, but buffers are meant to keep pH from moving around easily. Phosphates are great buffers because they have three spots per phosphate molecule to utilize in controlling the pH. If you want a phosphate buffer at 2 pH, I would make up the phosphate buffer and use phosphoric acid to lower the pH to what is needed. Phosphoric acid won't add anything new to the buffer, since the phosphoric acid will dissociate into phosphate ions. Likewise, you can use potassium hydroxide to basify if needed, since that won't add anything new into the buffer.


EDIT: My apologies, I misread above. I didn't really add anything to the discussion.

[Borek]

Does this mean that in order to get a buffer centered around Ka1 I need to use .71 dibasic potassium phosphate to phosphoric acid?

Yes. Obviously that's the mistake you were doing all the time. Actually it was pretty difficult to catch, as that's the first time I see someone misunderstanding buffer solutions this way, and I have seen a lot