Unveiling the mechanisms behind the chemical vapor generation of plumbane for trace analysis of lead

Abstract

The mechanisms controlling the generation of PbH4 by reaction of inorganic Pb(II) with aqueous NaBH4 were investigated both in the presence and in the absence of the additive K3Fe(CN)6. For the first time PbH4 has been identified in analytical chemical vapor generation (CVG) by using gas chromatographic mass spectrometry (GC-MS), which allows the use of deuterium labelled experiments. In the absence of the additive, under reaction conditions typically employed for trace lead determination by CVG, Pb(II) is converted to solid species and no volatile lead species can be detected by either atomic or mass spectrometry for Pb(II) concentration up to 100 mg L−1. In alkaline conditions Pb(II) substrates are unreactive towards NaBH4. In the presence of K3Fe(CN)6, deuterium labelled experiments clearly indicated that the generated PbH4 is formed by the direct transfer of hydride from borane to lead atoms. Kinetic experiments were carried out to evaluate the rate of reduction of K3Fe(CN)6 by NaBH4, the rate of hydrolysis of NaBH4 both in the presence and in the absence of K3Fe(CN)6, and the rate of dihydrogen evolution following NaBH4 hydrolysis. The effect of delayed addition of Pb(II) to NaBH4–HCl– K3Fe(CN)6, and K3Fe(CN)6 to NaBH4–HCl–Pb(II) reaction mixtures on the efficiency of plumbane generation was investigated by continuous flow CVG coupled with atomic fluorescence spectrometry. The collected evidences, complemented with thermodynamic considerations and literature data, have made it possible to clarify long-standing controversial aspects related to the mechanism of plumbane generation and the role of K3Fe(CN)6 additive.