A researcher needs a buffer to maintain pH 7.4 during a cell-culture experiment. Which conjugate acid–base pair is most appropriate for this purpose?
a) imidazolium/imidazole (pKa=6.99)
b) HEPES acid/HEPES base (pKa=7.55)
c) carbonic acid/bicarbonate (pKa=6.37)
d) dihydrogen phosphate/monohydrogen phosphate (pKa=7.21)