Evidence of two-step deprotonation of d-mannitol in aqueous solution

Deprotonation of d-mannitol was studied in aqueous basic solutions by means of potentiometry and 13C NMR spectroscopy. Two-step dissociation in the pH range from 12 to 13.8 was shown, and successive dissociation constants Ka1 and Ka2 were determined. In a solution with ionic strength I = 1.0 M (NaOH + NaNO3) pKa1 = 13.1 ± 0.1 and pKa2 = 13.8 ± 0.2. With increasing ionic strength from 0.75 to 3.0 M, both pKa1 and pKa2 values decrease. Deprotonation-induced chemical shifts in pH-variable 13C NMR spectra show that the OH-groups next to internal carbon atoms C-3 and C-4 dissociate to a greater extent compared to OH-groups next to external carbon atoms C-1 and C-6.