The electrode potential of mineral crystals
Tiews, Robert John
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In this thesis a thorough investigation of three problems connected with the electro-chemistry of lead sulfide has been discussed. It was definitely proven that a surface potential on a galena crystal immersed in a solution containing lead ion in respect to the center of the crystal does not exist, or is of such small magnitude as to be unmeasureable. The problem of determining the free energy of lead and sulfur has been outlined and the methods used in attacking this problem with galena electrodes have been presented together with the results of this work. It was found that galena electrodes are not reproducible and did not give steady values in either sulfide ion or lead ion solutions. The reasons for this phenomenon were two-fold. First, the potential of galena is not characteristic of galena, but is characteristic of the potential shown by a noble metal. This was illustrated by the fact that galena electrodes and platinum electrodes exhibited substantially the same potential in the same solution of lead ion. Second, impurities ln the galena crystals made each crystal exhibit its own characteristic value. The application of galena to the problem of producing a null electrode together with the importance of such an electrode has been discussed. The same difficulties which were encountered with free energy determinations on galena were also encountered in this application of galena . Finally, galena was discarded because of its erratic behavior. The reason for developing pure lead sulfide electrodes was because of the impossibility of obtaining pure, naturally occurring PbS. Four types of synthetic PbS electrodes were prepared and measurements with three of these types carried out. The lead sulfide electrodes formed by fusing PbS showed the best promise when the mechanical difficulty of obtaining them without impurity was completely surmounted. Initially, several general conclusions instructive to future work in this field as well as suggestions of applications of the pure lead sulfide electrodes have been outlined. The most important conclusion is that naturally occurring PbS cannot be used for quantitative measurements. The suggested use which should prove most interesting for further investigation is the application of the pure electrodes to solubility measurements and measurements of the rate of attainment of saturation of PbS in water.
Thesis (M.A.)--Boston University
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