Polarographic behavior of the platinum group metals
Sette, Maria Elisa P
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It has been reported in the literature that metals of the VIII group showed a catalytic effect on the discharge of hydrogen, giving rise to polarographic waves at less negative potentials. The case of platinum has been studied in detail and it was found that the waves were roughly proportional to the amount of the metal in solution. It was also observed that their heights depend on the concentration of the acid solution. Heyn found that if platinum was complexed with fluoride better waves were obtained that seemed to be linearly dependent on the concentration of the metal. West, Dean, and Breda showed the polarographic behavior of several ions including iridium and rhodium, using sodium fluoride as supporting electrolyte. No mention is made of the action of the other metals of the group. The aim of this research was to make a fundamental study of the polarographic behavior of the VIII b group metals, when complexed as fluorides. In the event that waves of analytical significance were obtained for any of the metals a quantitative method would be established. Only platinum and osmium gave waves of possible significance and an analytical method was developed for them, using the height of the catalytic waves obtained. Rhodium, iridium, ruthenium, and palladium showed erratic polarographic behavior under the conditions studied. The explanation for this failure in catalyzing the hydrogen evolution is attributed to formation of unreactive complexes of these metals with fluorides. The waves obtained for platinum in the preliminary experiments needed some improvements, in order to have analytical application. Three major drawbacks were observed: 1. the waves were not reproducible; 2. the wave heights were not linearly proportional to the concentration of platinum; 3. waves were characterized by erratic pen oscillations, making the accurate determination of the plateau very difficult. The variables that could have influence on the waves, such as effect of bubbling time of nitrogen, influence of the height of mercury column and concentration of fluoride were studied. The results led to a proposed method (see p. 23) which is much more sensitive than would be expected for a simple polarographic reduction due to the catalytic action of platinum. The presence of other platinum group metals which are not catalytically active masks the catalytic action of platinum and therefore interferes in its determination. Osmium in acid solution is known to react with the mercury of the cathode and therefore gives irreproducible waves. In an acetate buffer, Kolthoff and Perry reported that a quantitative method of restricted application was obtained. A special technique was employed to measure the current: a quick measurement was made at the potential corresponding to the maximum of the wave, and by comparison with a curve obtained with standard solutions the concentration at osmium was found. It was confirmed in this research that a reaction takes place between mercury and osmium tetroxyde in hydrochloric acid solution. But if osmium is complexed with fluoride, fairly good waves are obtained and a quantitative method could be set up. The polarograms have to be recorded as soon as the electrode is dipped into solution. If subsequent polarograms are run, with the same solution the height of the wave is found to be smaller. A precipitate appears in the solution after sometime and black specks, presumably osmium, are found on the surface of the mercury. The reaction mentioned above can be detected on the polarogram by an abrupt change on the shape of the wave. In view of these facts, a method was proposed (see pg. 36) which was again very sensitive, as in the case of platinum.
Thesis (M.A.)--Boston University
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