A study of the metal binding capacity of saccharinic acids formed during the alkali catalysed decomposition of cellulosic materials: nickel complexation by glucoisosaccharinic acids and xyloisosaccharinic acids

摘要

The stoichiometry of the metal complexes formed between nickel and the ligand beta-glucoisosaccharinic acid (beta-GISA) and a racemic mixture of enantiomers of xyloisosaccharinic acid (XISA) has been determined at both neutral and alkaline pHs. Bjerrum plots, Job's plots and conductance measurements indicated that for each of the systems one to one Ni(ligand) complexes were formed at near neutral pHs (<7.5). At intermediate alkaline pHs (7.5-13) there is evidence to support the formation and precipitation of Ni-2(ligand)(OH)(3) complexes, finally, at high pH (>13) sparingly soluble Ni-2(ligand)(OH)(4) complexes were formed. The stability constants for the Ni(beta-GISA), Ni(alpha-GISA) and Ni(XISA) complexes formed at neutral pH were determined under identical conditions using polarographic studies. The measured stability constants for Ni(beta-GISA) (log(10) beta= 1.94 +/- 0.15) and for Ni(alpha-GISA)(log(10) beta = 2.07 +/- 0.13) are very similar; the value measured for the Ni(XISA) complex (log(10) beta = 0.83) was an order of magnitude smaller. The stability constants for the Ni-2(Ligand)(OH)(4) complexes formed at highly alkaline pHs were determined using the Schubert method. The measured stability constant for Ni-2(beta-GISA)(OH)(4) (log(10) beta = 30.6 +/- 0.5) was an order of magnitude bigger than the value for Ni-2(alpha-GISA)(OH)(4) (log(10) beta = 29.0 +/- 0.5) measured under identical conditions. Attempts to measure the stability constant for Ni-2(XISA)(OH)(4) were unsuccessful; Ni-2(XISA)(OH)(4) complexes were not present in significant amounts at high pH to allow the log(10)beta value to be determined by the Schubert method. (C) 2016 Elsevier Ltd. All rights reserved.