The determination of mercury at trace levels by flow-injection analysis with electrochemical detection

摘要

Electrochemical investigations are described which were performed with respect to the development of a flow-injection, electroanalytical method for the quantitative determination of Hg(II) in acidic media. The method utilizes the deposition of Hg(II) at underpotential on a Au electrode, followed by Differential Pulse Anodic Stripping Voltammetry (DPASV). The linear dynamic range of calibration for the proposed method is shown to be over three orders of magnitude in concentration of Hg(II). Using the optimum conditions found for DPASV, the detection limit in 1.0 M H(,2)SO(,4) is approximately 5 x 10(\u27-10) M Hg(II) using a deposition time of 5.0 min;The underpotential deposition (UPD) of Hg(II) on Au is demonstrated to occur within the region of potential between 0.9 and 0.4 V, vs. SCE, at a rate which is limited by the rate of mass transport of Hg(II) to the surface of the electrode. The mass-transport limitation is shown to be valid only when the surface coverage of the deposit is very small, i.e., much less than the equivalent of a monolayer. Furthermore, it is shown that the reduction of the very first quantities of Hg(II) at underpotential occurs with n(,app) = 1.60 equivalents mole(\u27-1) at all potentials within the UPD region;The optimization of the experimental variables of DPASV, applied within the scheme of Flow-Injection Analysis, is described. Described are the effects of flow rate during the stripping scan; modulation amplitude; rate of potential scan; cycle period of pulses; flow rate during deposition when the injected, sample volume is constant; and the time of deposition;The method is shown to be highly selective for Hg(II) with the exception of Ag(I) which interferes with the quantitative determination of Hg(II) when both ions are present at comparable concentrations;The application of three methods of sample dissolution in attempts to analyze two NBS Standard Reference Materials is described. All three dissolution procedures lead to low results by the proposed method. Incomplete destruction of all organic matter, or/and loss of Hg due to volatilization are proposed to account for the incomplete recovery of Hg as Hg(II) in the final digest.