So I want to make a Potassium Phosphate Buffer from monobasic and dibasic forms of potassium phosphate because it was recommended for doing circular dichroism scans due to its low absorption in the UV region. However I don't understand the proportions of mono basic and dibasic forms of potassium phosphates used. 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?