Henderson-Hasselbalch equation for EDTA - (Sep/20/2007 )

I came across my friend's homework assignment and I had no idea how to answer it.

The question asked you to use the Henderson-Hasselbalch equation to make a 0.5M EDTA solution (pH 8.0)
If you're using the disodium dihydrate- which Pka is relevant?

Thanks.

-labrat612-

Henderson-Hasselbalch is when you use the law of mass action to calculate a ph value.

for example if the pka of the acid X is 5 it would be:

( [X-] * [H+] ) / [HX] = 10^5

[HX] is the concentration of the undissociated acid
[X-] is the concentration of the dissociated acid
[H+] is the concentration of H+-ions (log this to get pH)

EDTA is a little bit more complicated because it is a donor for up to 4 H+. so try to split it up in 4 equations first to solve this problem.

-coastal-

I did find the answer. The pH that is being used is highly relevant as to which pKa value is to be used.

-labrat612-

QUOTE (labrat612 @ Sep 20 2007, 12:26 PM)
I did find the answer. The pH that is being used is highly relevant as to which pKa value is to be used.

I'd use the pKa closest to pH 8. Is one of the pKa values within one of pH 8? If so, that ionization will be a reasonable buffer for a pH 8 solution.

-Jon Moulton-