Date of Award

12-2014

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

Advisor

Alice E. Bruce

Second Committee Member

Mitchell R. M. Bruce

Third Committee Member

Carl P. Tripp

Abstract

Two approaches are described for the detection of mercury(II), based on the selective reaction of Hg(II) with thiosemicarbazides to form oxadiazoles. The first approach uses solid state infrared spectroscopic techniques and builds on previous research using high surface area solid state materials chemically modified with thiosemicarbazides to detect Hg(II) in aqueous solution. The second method uses a ferrocenyl compound derivatized with thiosemicarbazide and electrochemical techniques to detect reaction with Hg(II) in organic solution.

Three different high surface area materials were prepared and tested for reaction with Hg(II). These included silica derivatized with 3-(triethoxysilyl)propyl succinic anhydride, polytetrafluoroethylene (PTFE) coated with a block copolymer, and an aqueous dispersion of PTFE coated with sodium polyacrylate (NaPA). The surfaces were chemically modified with 4, 4-dimethyl-3-thiosemicarbazide (DTSC). Each DTSC group reacts with one Hg(II) ion to form an oxadiazole ring. Therefore the amount of Hg (II) in solution can be determined by the increase in absorbance of the oxadiazole ring in the IR spectrum.

The PTFE beads coated with NaPA and derivatized with DTSC gave the most promising results in terms of preparation and efficiency of reaction with Hg(II). The surface self-assembly of NaPA on PTFE is complete within about 0.5 h at pH 4-5 and subsequent derivatization of the surface bound polymer with thiosemicarbazide (DTSC) is complete in approximately 1 h. The reaction of the surface modified PTFE beads with Hg(OAc)2 is very fast (within 5 minutes) and the lowest amount of Hg(II) that was detected by using this method is 8 μg of Hg(II) with 10 mg of surface modified PTFE beads in 6 mL solution. This corresponds to a concentration of 1.3 ppm. In theory, it would be possible to detect a lower concentration of Hg(II) by increasing the volume. However, a disadvantage of the method is that the overlapping amide band (for the thiosemicarbazide) at 1701 cm-1 and the oxadiazole band at 1644 cm-1 make it difficult to use curve fitting to calculate the amount of oxadiazole. Thus it was not possible to determine the limit of detection for Hg(II) using this method.

The research described in Chapter 3 focuses on the characterization of two new ferrocenyl compounds and a proof-of-concept study for electrochemical techniques employing these compounds for detection of mercury(II). The synthesis and characterization of two new compounds, 1-ferrocenoyl-4-dimethyl-3-thiosemicarbazide (Fc-DTSC) and N,N-dimethyl(5-ferrocenyl-1,3,4-oxadiazoI-2-yl)amine (Fc-oxadiazole) are described. The Fc-DTSC compound has two redox processes at E1/2 = 0.692 V and 0.872 V vs. Ag/AgCl (0.1M TBAH/DMF). The first redox process is assigned to the Fe(II)/(III) couple in ferrocene and the second is assigned to the sulfur in thiosemicarbazide. The Fc-oxadiazole compound has one redox process at E1/2 = 0.700 V, assigned to the Fe(II)/(III) couple. Reaction of Fc-DTSC with Hg(II) results in the disappearance of the peak at 0.892 V and a slight, positive shift in the Fe(II)/(III) couple. By using differential pulse voltammetry, which is a more sensitive technique than cyclic voltammetry, the limit of detection for Hg(II) was determined to be ~1ppm.

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