Role of hydroboron intermediates in the mechanism of chemical vapor generation in strongly acidic media

Unknown and controversial aspects related to the mechanisms of borane complexes hydrolysis and to the mechanisms of chemical vapor generation for trace element determination in strongly acidic media (0.01-10 M HCl), have been investigated. The overall hydrolysis rates of borane complexes (BH4ƒ{, H3Nƒ{BH3) in the acidity range of 0.2 ¡V 10 M HCl resulted several order of magnitude lower than those predicted by kinetics laws obtained in the pH range of 3.8 ¡V 14. The stepwise decomposition of the borane complexes and the formation of hydroboron intermediates, some of which presented surprisingly long lifetimes at elevated acidities, play a key role in determining both the overall hydrolysis rate of borane complexes and the reactivity of Hg(II), As(III), Sb(III), Bi(III), Se(IV), Te(IV) and Sn(IV) in chemical vapor generation for trace element determination. Atomic absorption experiments demonstrated that almost all tri-hydroboron species (L-BH3), di-hydroboron species (L2BH2) and mono-hydroboron species ( L3BH) ( L could be one or more among the groups H2O NH3, OHƒ{, Clƒ{) could play an active role in the generation of elemental mercury and stibine. Some of these intermediates are inactive or play a marginal role in the generation of arsine, bismuthine and hydrogen selenide. Hydrogen telluride is preferentially formed by hydroboron species, which are stable in strong acidic conditions, while the same species are unreactive in the generation of stannane. The collected experimental evidences are in agreement with the behavior of borane complexes towards hydrolysis and with the general reactivity of the elements in chemical vapor generation techniques.