Why do some standard formulas for PBS buffer preparation incorporate KH2PO4 and Na2HPO4 as acid-base pairs instead of KH2PO4 / K2HPO4 or NaH2PO4 / NaHPO4?
That's a good one, you have to realize PBS is not only a buffer but it's also a buffer that mimic the ion concentration of body fluid. So you can get a buffer from the composition that you,re suggesting but that will not necessarily mimic the ion concentration of the body fluid. Obviously that could change the behavior of cells and organisms