This paper describes a simple, direct reading and inexpensive system for determining end-points of oxidation-reduction reactions. The method depends upon the sudden change at the end-point in the potential difference between a platinum and a tungsten electrode which are immersed in the solution.

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The two electrodes in this arrangement are identical and are kept at the same temperature. Solutions I and II, the junctions of which are at different temperatures, may contain the same electrolyte at different concentrations, or different electrolytes.Other arrangements consisting of two similar electrodes, at different temperatures, and a single solution in which the temperature gradient lies, are also occasionally given this name.

Since lattice energy calculations have indicated that the most stable sulfides of the periodic system could be expected in the neighborhood of the elements of the rare earth and actinide series, a thorough study of the sulfides of typical representatives of these elements was made. This paper describes the results of the study of the sulfides of thorium and uranium.The earliest careful study of thorium and uranium sulfides is that of Picon4r6sB who studied methods of preparation of ThS2 and US2 and the properties and reactions of these compounds. Strotzer and Zumbusch7 and Strotzer, Schneider and Biltz8 extended the study to polysulfides above the MS2 compounds and to oxidation states below the four plus state. However, due to very extensive oxygen contamination during preparation, the compositions and composition ranges given for the various phases below MS2 were greatly in error.In the present work both uranium and thorium were found to have compounds of the 2f oxidation state, US and ThS, which have the NaCl crystal structure. No compounds below the 2+ oxidation state were found. The solid solution ranges for these compounds appear to be relatively small. The next phase found above the MS phase was a M& phase corresponding to the 3f oxidation state. Both U2S3 and Th&s were found to have the same orthorhombic structure with apparently relatively small solid solution ranges. Th83 is an unusual thorium compound as i t is the only definitely established compound observed to date which contains thorium in the 3+ oxidation state. ThSz prepared by conversion of the oxide by H2S in a graphite container had the orthorhombic PbCt structure. US2 was prepared in a similar manner. Upon heating under reduced pressures, both lost sulfur through a solid solution range. US2 was less stable than ThSz and lost sulfur a t lower temperatures. A compound, Th&, of hexagonal structure, has been prepared by heating liquid ThS2 above 1950' under reduced pressures. It has a homogeneity range of approximately ThS1.71--1.,6. Because of the ready loss of sulfur, the study of the uranium sulfide range down (1) This work was performed under Manhattan District Contract to a composition corresponding to Th&2 did not give definite results. Experimental I. Preparation of the Sulfides of Uranium and Thorium.-Four different methods of preparation will be described below. The first three methods were also used for the preparation of the cerium sulfides and the apparatus used and details of the procedures may be obtained from the paper by Eastman, Brewer, Bromley, Gilles and Lofgreno on the cerium sulfides.Method 1.-This method is suited only for ThS2 and US2 since it involves the treatment of the oxides with excess H2S in a carbon system. Thus one obtains the highest sulfide stable at the temperature of the preparation. The temperature of the reaction should be varied between 1200-1300°; the rate of reaction a t these temperatures is fairly rapid. During the course of the reaction, the oxysulfides ThOS and UOS are first formed at lower temperature ...

With the object of testing experimentally the third law of thermodynamics as applied to crystalline solutions, the chief thermodynamic properties of such a solution of silver bromide and silver chloride have been studied. Measurements described and recorded include: the free energy of formation, obtained from measurements of the e.m.f. of appropriate silver-silver halide electrodes against hydrogen; the heat of formation, from measurements of heat absorbed in dissolving the various phases in a thiosulfate solution; the specific heats between 15°K and 298°K of the pure constituents and the solid solution. The entropy of silver bromide and of silver chloride is calculated from the specific heat curves. The entropy change in the formation of the solid solution at 298°K is calculated from the experimental data and is found to agree with the result calculated statistically. The latter is identical in form and magnitude with the entropy as calculated thermodynamically for an ideal solution, though the heat content and free energy show that the solution is far from perfect. From the specific heat curves it appears that the entropy of mixing persists essentially unchanged in magnitude to the lowest temperature of the measurements, and presumably to the absolute zero. The existence of finite entropies, or entropy differences, at the absolute zero is regarded as established thermodynamically, and the requisite interpretation of statistical ideas to secure concordance with this result is discussed. Finally, although the existence of a valid but limited third law is accepted, and a concise general statement of it suggested, it is pointed out that the ``principle of the unattainability of the absolute zero'' must be regarded as extra-thermodynamic in character.