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The pKa of potassium Phosphate is 7.2. So if I want a buffer with pH 7.2 then according to the henderson-hasselbalch equation the recipe should call for equal parts of KH2PO4 and K2HPO4. However according to a chart I found the proportions depend on total concentration and it turns out the molar fraction of dibasic form is closer to 0.678 for a 0.05M concentration at pH 7.2. Obvious disagreement with H-H equation which would give molar fraction of 0.5.

So why does the henderson hasselbalch equation seems to fail and how do I correctly calculate the molar ratio of the two potassium phosphate forms?

Or maybe there is a better buffer for circular dichroism (i.e. low UV absorption) experiments?