Behavior and kinetic of hydrolysis of amine boranes in acid media employed in chemical vapor generation

Lucia D'Ulivo; Roberto Spiniello; Massimo Onor; Beatrice Campanella; Zoltan Mester; Alessandro D'Ulivo

doi:10.1016/j.aca.2017.10.034

Behavior and kinetic of hydrolysis of amine boranes in acid media employed in chemical vapor generation

Anal Chim Acta. 2018 Jan 15:998:28-36. doi: 10.1016/j.aca.2017.10.034. Epub 2017 Nov 3.

Authors

Lucia D'Ulivo¹, Roberto Spiniello², Massimo Onor², Beatrice Campanella², Zoltan Mester¹, Alessandro D'Ulivo³

Affiliations

¹ National Research Council Canada, Measurement Science and Standards, Ottawa, Canada.
² C.N.R, Institute of Chemistry of Organometallic Compounds, S.S. of Pisa, Via. G Moruzzi, 1, 56124 Pisa, Italy.
³ C.N.R, Institute of Chemistry of Organometallic Compounds, S.S. of Pisa, Via. G Moruzzi, 1, 56124 Pisa, Italy. Electronic address: dulivo@pi.iccom.cnr.it.

PMID: 29153083
DOI: 10.1016/j.aca.2017.10.034

Abstract

The behavior of NaBH₄ (THB) and the amine boranes, NH₃BH₃ (AB), tertbutylNH₂BH₃ (TBAB), Me₂NHBH₃ (DMAB) was investigated in continuous flow chemical vapor generation of H₂Se from aqueous Se^IV coupled with atomic absorption spectrometry. Unexpected higher efficiency of H₂Se generation was obtained with amine boranes compared to THB (TBAB > AB > THB) using millimolar concentration of reductant (0.001-0.1 mol L^-1) under strongly acidic conditions (HCl, HClO₄, H₂SO₄, HNO₃, 0.5-5 mol L^-1 H⁺). Analytical applicability of the CVG system was tested by the determination of Se^IV in natural water samples certified reference materials, using 0.01 mol L^-1 TBAB in 0.5 M H₂SO₄. In order to explain this unexpected higher efficiency of amine boranes with respect of THB, the kinetic of hydrolysis of AB, TBAB and DMAB was investigated in acid media typically employed in chemical vapor generation for trace element determination. The kinetic was investigated by monitoring the rate the hydrogen gas evolved during hydrolysis, using a laboratory made thermostated reaction cell. Kinetics were measured for AB, TBAB and DMAB in 0.1, 0.5, 5 mol L^-1 HCl or HClO₄ reaction media and in 0.1 mol L^-1 cysteine +0.1 mol L^-1 HCl or HClO₄ buffer, for reaction times from 0 to 30 min. Under strongly acidic conditions, the rates of hydrogen evolution produced by amine boranes hydrolysis appear to be much slower than those predicted by a pseudo-first order reaction and using the rate constants reported in the literature. This suggests that, at elevated acidities (5 mol L^-1 HCl or HClO₄), the hydrolysis of amine boranes takes place in two steps, generating a first amount of H₂ (0.67-1.15 mol) much faster than the remaining about 2 mol. This evidence indicates a different mechanism of hydrolysis to the one accepted in the literature for amine boranes. The relatively high efficiencies of H₂Se observed with amine borane reduction of inorganic Se^IV at elevated acidities can be addressed to the action of borane intermediates, most probably amine borane cations, formed during amine borane hydrolysis in the same reaction conditions.

Keywords: Amine boranes; Hydride generation; Hydrogen evolution; Mechanism; Selenium determination; Tetrahydridoborate(-1).