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

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 SeIV 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 (0.001e0.1 mol L1) under strongly acidic conditions (HCl, HClO4, H2SO4, HNO3, 0.5 e5 mol L1 Hþ). Analytical applicability of the CVG system was tested by the determination of SeIV in natural water samples certified reference materials, using 0.01 mol L1 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 L1 HCl or HClO4 reaction media and in 0.1 mol L1 cysteine þ0.1 mol L1 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 L1 HCl or HClO4), the hydrolysis of amine boranes takes place in two steps, generating a first amount of H2 (0.67e1.15 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 SeIV 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.