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.

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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.

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

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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.

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