A Mercury Selective Electrode
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A Mercury Selective Electrode Jon Scaggs*, Dale D. Russell*, S. P. Duttagupta+, and Michael W. Hill* *Department of Chemistry, +Department of Electrical Engineering Boise State University, Boise ID 83725 ABSTRACT A novel mercury ion selective electrode has been constructed to have strongly coordinating macrocyclic ligand binding site covalently attached to polythiophene film using 11,4,10-trioxa-7,13-diazacyclopentadecane-1-thiophenylmethane (TDCD-thiophenylmethane) as a specific chelator. In amperometric mode, the current is proportional to mercury (II) concentrations down to 2 ppb. Reversible cyclic voltammetric waves show that both mercury (I) and mercury (II) are stabilized by the ligand. Monomethyl mercury and vapor phase elemental mercury are also electrochemically determined. Dissolved oxygen and heavy metal cations show no significant interference even at ten times the mercury concentration. The polymer surface shows robust performance for two years or longer. Fabrication strategies are being developed to optimize the probe performance. The variables under investigation include substrate surface roughness, the ratio of derivitized thiophene (TDCD-thiophenylmethane) to underivatized thiophene in the surface polymer, ratio of thiophene with two chelating rings to thiophene with only one, number and thickness of electrodeposited polymer layers, composition of polymer layers, and number of -CH2- units separating the chelating ring from the thiophene monomer units. The probe exhibits linear response to mercury concentration, and detects mercury in several forms. BACKGROUND Environmental Applications of the Mercury Sensing Probe The impetus for this work is the acute need to monitor heavy metals in real time, through a broad range of concentrations, in environmental applications. There is increasing demand for field portable instrumentation capable of in situ measurement of contaminants for a number of applications including biomedical analysis, industrial process control, environmental assessment, contaminant and remediation monitoring, and “smart” waste site planning [1]. Analytical devices are sought, that are capable of operation in harsh environments, or of incorporation into sensor arrays for depth profiling and time-resolved contaminant fate and transport studies [1]. For field survey work, there is great interest in rugged, portable, hand held devices capable of detecting contaminates through a wide range of levels, and even of speciating them [2]. The western United States has regions of high mercury contamination from both erosion of natural deposits, and from human activities such as gold mining. The mercury amalgam process for recovery of gold has left heavily contaminated mine sites that continue to discharge mercury into the surrounding air, soil, and water [3]. Mercury in the soil or water is transported in a diffusive pattern away from a point source, and it enters into the biosphere via plants, microorganisms, and direct ingestion by larger organisms [4]. Devices capable of measuring mercury level
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