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

Unknown and controversial aspects related to the mechanisms of hydrolysis of borane complexes and to the mechanisms of chemical vapor generation for trace element determination in strongly acidic media (0.01-10 M HCl) have been investigated and clarified. The overall hydrolysis rates of borane complexes (BH4-, H3N-BH3) in the acidity range of 0.2-10 M HCl were several orders of magnitude lower than those predicted by kinetics laws and obtained in the pH range of 3.8-14. The decomposition of the borane complexes takes place stepwise and proceeds through the formation of hydroboron intermediates, LxBH4-xn (x = 1, 2, 3), where L could be one or more species among the donor groups H2O, NH3, OH-, and Cl- and n is the charge of the hydroboron species (n = 0, +1, -1, depending on L). Some intermediates present surprisingly long lifetimes at elevated acidities and play a key role in determining both the overall hydrolysis rates of borane complexes and the reactivity of Hg(H), As(111), Sb(111), Bi(111), Se(IV), Te(IV), and Sn(IV) in chemical vapor generation for trace element determination. Atomic absorption experiments demonstrated that almost all trihydroboron species (LBH3n), dihydroboron species (L2BH2 n), and monohydroboron species (L3BHn) 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 those hydroboron species, which are stable in strongly acidic conditions, while the same species are unreactive in the generation of stannane, The collected experimental evidence is in agreement with the general reactivity of the elements in chemical vapor generation techniques and, together with other literature data, definitely rule out the hypothesis of nascent hydrogen as a possible mechanism of chemical vapor generation by borane complex derivatization.