 Greenwood and Earnshawn (1997) refer to a pK value of 5.15, while a pK value of 3.80 is also reported in the Vogel's book.The formation of the complex (more exactly, in fact an ester) between one B(OH)3 molecule and two mannitol (C6H14O6) molecules (sometimes referred as mannitoborate, the conjugated base of the mannitoboric acid, p"K"a = 3.80), releases three water molecules and one proton in water as follows