Potentiometric and Coulmetric Studies and the Mechanisms of the Precipitation of Metal Sulfides from Acid Solutions by Thioacetamide

Citation

Bowersox, David Frederic (1958) Potentiometric and Coulmetric Studies and the Mechanisms of the Precipitation of Metal Sulfides from Acid Solutions by Thioacetamide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/C5X1-V153. https://resolver.caltech.edu/CaltechETD:etd-10072004-095819

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The standard potential of the half cell [...] has been measured in cells with negligible liquid junction potentials. The value calculated at 25[degrees]C. in solutions 0.2 to 4.0 volume formal in perchloric acid is -0.711 volt; the value for a hypothetical half cell 1 molal in iodine is -0.625 volt. The constant for the reaction [...] was calculated from solubility measurements to be 0.87. Conditions under which electrolytically generated chlorine can be employed for the coulometric titration of iodide to iodine monocyanide have been determined. Titrations in the presence of bromide were made. One to three mg. of iodide were determined with an accuracy of 0.1%. The conditions were determined under which tetrapositive lead can be generated quantitatively. The determination of lead(IV) with electrolytically generated iron(II) was proved feasible. In confirmatory titrations with ferrous ion an accuracy of 0.1% was attained in the determination of quantitles of lead(IV) in the range from 0.25 to 1.50 microgram. This showed the feasibility of alternately generating lead(IV) and iron(II) in a coulometric cell, that is, the use of the lead-iron system as dual intermediates. Attempts to titrate glycols with excess lead(IV) were not successful because the reaction of glycols with lead(IV) was slow and incomplete when the reactants were present in low concentrations. The precipitation of cadmium as sulfide from acid solutions by thioacetamide has been studied. In solutions having pH values of 2 or less the rate of precipitation is controlled by the rate of hydrolysis of the thioacetamide to give hydrogen sulfide, and is first order with respect to the hydrogen ion and to the thioacetamide concentrations. In solutions having pH values from 6.3 to 3.3 a direct reaction occurs in which the rate of cadmium sulfide precipitation is first order with respect to the hydrogen ion concentration. The velocity constant for the expression [...] is [...] in 0.15 M sodium formate solution. The energy of activation for this reaction was calculated to be 20.8 kcal. per mole. The rate of precipitation of cadmium as sulfide from acid solutions by thioacetamide in this pH range is slower in the presence of chloride. At a pH of 4.0 this effect has been quantitatively studied at 25[degrees] and 90[degrees]C. with chloride concentrations from [...]. The data obtained conforms to the rate expression [...] where [...] and [...] are [...] and [...] [...] mole[...] min[...], respectively, at 90[degrees]C. The validity of these proposed constants was checked by the calculation of an equilibrium constant for the reaction [...] at each chloride concentration by the use of these constants. The value of this constant is [...] mole, per liter at 90[degrees]C. The precipitation of nickel sulfide by thioacetamide has been studied in solutions in which the hydrogen ion concentration was maintained at values from [...]. Rate measurements have shown that the precipitation reaction conforms to the expression for the direct reaction [...] where the velocity constant is [...] at 90[degrees]C. Nickel sulfide was precipitated by the direct reaction with thioacetamide at hydrogen ion concentrations at which precipitation by hydrogen sulfide is not possible. The energy of activation for the reaction was calculated to be 20.8 kcal. per mole. Precipitation by the hydrolysis of thioacetamide was not observed. The effectiveness of the separation of lead(II) and cadmium(II) from nickel(II) by the use of thioacetamide in solutions at 90[degrees]C. has also been studied. It was found that nickel sulfide was formed only by the direct reaction shown above. The precipitation of nickel sulfide was independent of the concentration of lead(II) or of cadmium(II), or of the amount of hydrogen sulfide in the reaction solution. It was found that the quantity of nickel sulfide which precipitated with the lead or cadmium sulfide was insignificant under certain conditions, and that an effective separation can be made. Rate measurements were made to determine the mechanisms of the precipitation of zinc sulfide from acid solutions by thioacetamide. Two mechanisms, similar to those found for lead and cadmium, were observed. In solutions having pH values of 1 or 2, the rate of precipitation of zinc sulfide is first order with respect to both the thioacetamide and hydrogen ion concentrations. The second order velocity constant for this reaction was found to be [...] at 90[degrees}C., the same value as that reported for cadmium and lead. In solutions having pH values from 6.3 to 2.5 the precipitation is first order with respect to both the thioacetamide and the zinc(II) concentrations and inversely half order with respect to the hydrogen ion concentration. The velocity constant for the expression [...] was found to be [...] in solutions 0.30 M in sodium formate, and the energy of activation for the reaction was calculated to be 20.1 kcal. per mole. This type of precipitation is similar to that observed for lead, cadmium, and nickel in the same pH range. The value of the energy of activation is almost equal to those calculated for the precipitation of cadmium and nickel. Separations of lead(II) from zinc(II) by the precipitation of lead sulfide from solutions 0.3 M in hydrogen ion and 0.6 M in chloride were made with thioacetamide or hydrogen sulfide at 90[degrees]C. The two methods were comparable in effectiveness.

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