Comment on “Understanding the effects of potassium ferricyanide on lead hydride formation in tetrahydroborate system and its application of lead in milk using hydride generation inductively coupled plasma optical emission spectrometry” by B. Deng, X. Xu, Y. Xiao, P. Zhu, Y. Wang

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

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.