Cu(II) complex formation with xylitol in alkaline solutions

Equilibria in alkaline solutions of xylitol and Cu(II) ions were investigated by means of 13C NMR, Raman spectroscopy, polarography and spectrophotometry. Composition, stability, diffusion and optical characteristics of the complex species formed were determined. The formation of four Cu(II)–xylitol complexes was observed in aqueous alkaline solutions (11.0 ⩽ pH ⩽ 14.0, I=1.0, 20 °C) by means of direct current polarography and VIS spectrophotometry. Mononuclear hydroxy complexes, CuXyl(OH) − (log β=17.7 ± 0.5), CuXyl(OH) 2 2− (log β=20.2 ± 0.3) and CuXyl 2(OH) 2 4− (log β=22.4 ± 0.3), are formed at high ligand-to-metal ratios (L:M ⩾ 10), whereas dinuclear complex Cu 2Xyl (log β=29.2 ± 0.3) is the predominant species at low ligand-to-metal ratio (L:M=0.5). Diffusion coefficients and molar absorptivities of the complex species were determined. pH variable 13C NMR suggested that p K a values of xylitol are rather similar and equal to 13.8 ± 0.2, 13.9 ± 0.1 and 13.9 ± 0.2 for OH-groups adjacent to (C-1,C-5), (C-3) and (C-2,C-4) carbon atoms, respectively.