The pKa values of different nitrogenous bases, furan, and formic acid provide a qualitatively understanding of their interactions (see Table ). The pKa of formic acid and pyridine are 3.75 and 5.21 respectively leading to a ΔpKa = 1.46. Since, ΔpKa lies between 1-2, this co-crystal is expected to show a moderate propensity towards proton transfer from the acid to the base. In contrast, on the other extreme lies the furan…formic acid complex for which ΔpKa = -6.62 making proton-transfer from the acid to the base extremely difficult. Pyridazine…formic acid and pyrazine…formic acid with ΔpKa = -1.51 and -3.24 respectively lie the regime for which proton-transfer is unexpected at ambient conditions.