[KBriggs]

I would like to buffer KCl a solution at pH 13, since the pH drops to about 11 after a few hours in air. Needless to say, finding a suitable buffer is not easy. I have forgotten a lot of my high school chemistry, so I have a pretty basic question.

I came across phosphoric acid, which has pKa values at 2.1, 7.2, 12.3. I know that something with a pKa of 12.3 would be a suitable buffer, but I am unsure what the effect of the other two values will be. Would it simply be a matter of having to use more KOH to overcome the first two point, after which time it would behave as a useful buffer, or is there more to it than that?

Thanks!

[Arkcon]

You could conceivably make a buffer at that pH using phosphate salts -- it has 3 pKa's depending on how pronated it is.  Its going to take lots of KOH however, to make phosphoric acid into a buffer at that pH, so you might as well buy the dibasic salt -- K₂HPO₄ and start adding KOH to it.  You will have to add lots, and you will be diluting your KCl, and adding phosphate, which may change how your solution behaves.  You might want to use another buffer -- glycine is a good choice for pH 13.

[KBriggs]

Thanks for the info! From the wiki page, however, glycine has a pKa at 9.6. Isn't that a bit low for buffering pH 13?

[AWK]

All buffers with high pH react with carbon dioxide from air, and pH soon be lower.

[408]

trisodium phosphate is available at hardware stores as a cleaner. it may be useful.