Determination of inorganic sulfur species in alkaline solutions using high performance liquid chromatography with polarographic detection

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Uddin, Zamir
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The Department of Chemistry seeks to provide students with a foundation in the fundamentals and application of chemical theories and processes of the lab. Thus prepared they me pursue careers as teachers, industry supervisors, or research chemists in a variety of domains (governmental, academic, etc).

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The existing methods for the simultaneous determination of inorganic sulfur compounds are cumbersome and tedious. It is desirable to have an analytical method for the rapid determination of as many sulfur compounds as possible in aqueous solutions of varied pH and in the presence of dissolved oxygen. A method is presented based on the limiting polarographic currents of inorganic sulfide, sulfite, thiosulfate and polysulfide. High performance liquid chromatography (HPLC) is used to affect the separation of S('2-), SO(,3)('2-) and S(,2)O(,3)('2-) which are detected amperometrically at a dropping mercury electrode (DME) in a flow-through electrolysis cell. The pretreatment of sample solutions, except dilution, is unnecessary. The method is fast and sensitive, being able to detect as little as 1 (mu)g of S('2-), SO(,3)('2-), S(,2)O(,3)('2-) and S(,x)('2-) with a signal-to-noise ratio of greater than 10, using simple instrumentation. The detection of S('2-), SO(,3)('2-) and S(,2)O(,3)('2-) is achieved by an anodic process based on the oxidation of the mercury electrode with the formation of slightly dissociated Hg(II) compounds at the surface of the DME. Polysulfide produces an anodic and a cathodic electrode current;The maximum number of sulfur atoms, x, present in the polysulfide anion (S(,x)('2-)) is determined polarographically to be 5. The number is verified by spectrophotometric and gravimetric methods. The relative value of the diffusion coefficients of various S(,x)('2-) species is determined and is found to decrease drastically with the increase in x;Flow injection detection of S('2-) and S(,x)('2-) is described and it is demonstrated that these compounds, when present separately, can be determined in a flowing stream with excellent sensitivity, reproducibility and high sample throughput (e.g., ten determinations in ca. 15 minutes);Anionic sulfur species like S('2-), SO(,3)('2-) and S(,2)O(,3)('2-) must be separated prior to electrochemical detection because their polarographic half-wave potentials are too close to permit satisfactory resolution in a mixture. The method is tested to determine S('2-), SO(,3)('2-) and S(,2)O(,3)('2-) in the process stream samples derived from the caustic desulfurization of coal. Although qualitative information about S(,x)('2-) is obtained, the polarographic method alone cannot be used for the quantitative determination of S(,x)('2-) because the value of x for a given unknown solution of Sx('2-) cannot be determined unambiguously by polarography.

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Tue Jan 01 00:00:00 UTC 1985