54015 CN

V.120

THE ELECTRO-CHEMISTRY OF URANIUM AND THE SINGLE POTENTIALS OF SOME OXIDES OF URANIUM.*

By CHESTER A. PIERLÉ.

I. INTRODUCTION.

THE element uranium although classed with chromium, molybdenum, and tungsten in the periodic system is peculiar in that it resembles members of other groups of elements as well. Beyond the electrolytic analysis (Smith, Am. Chem. Journ., 1879, i., 329), the conductivity of the aqueous solutions of its salts (Dittrich, Zeit. Phys. Chem., 1899, xxix., 449), the effect of light on the potential ot uranous uranyl solutions using platinum electrodes (Baur, Zeit. Phys. Chem., 1908, Ixiii., 683, and Titlestad, Zeit. Phys. Chem., 1910, lxxi., 257) a similar investigation by Luther and Michie (Zeit. Elektrochem., 1908, xiv., 367), very little is found in the literature concerning the electro chemistry of uranium. McCoy and Bunzel have used potential measurements for determining the rate of oxidation of uranous solutions (Fourn. Am. Chem. Soc., 1909. xxxi., 367).

In the paper here presented are contained accounts of experiments on the electrolysis of various uranium com. pounds. These experiments were made in order to ascertain the electrolytic behaviour of different uranium compounds in aqueous solutions, non-aqueous solutions, and in fusions. The hope at first was that a method of preparing uranium electrolytically might be found. While metallic uranium was not thus prepared in the pure state, quite a number of compounds of uranium were formed electrolytically. The conditions of the formation of these compounds have been detailed, and their single potentials have been ascertained whenever a sufficient amount of pure material was formed in the experiment.

The difficulties involved in the electrolysis of urauium compounds are many. The metal does not deposit as such from aqueous solutions of either uranyl or uranous salts, but it is deposited as a poorly adhering oxide of variable composition, most probably in the form of the uranyl radicle, UO2, or its hydrates. Prepared in other ways, such as the reduction with metalic potassium (Peligot, Ann. Chim. Phys., 1842, [3], v., 1) or metallic sodium (Zimmermann, Liebig's Ann., 1882, ccxvi., 14), by electrolysis of fused double sodium uranous chloride (Moissan, Comptes Rendus, 1896, cxxii., 1088), reduction in

* A portion of the thesis submitted to the Graduate School of the University of Wisconsin in partial fulfilment of the requirement for the degree of Doctor of Philosophy. From the Journal of Physical Chemistry, xxxiii., No. 8.

the electric furnace with carbon (Moissan, Comptes Rendus, 1893, cxvi., 369), and the reduction with aluminium (Aloy, Bull. Soc. Chim., 191, [3], xxv., 153), the metal has never been obtained absolutely pure. It is a metal that burns at a low temperature (170°), reacts with cold water, and combines with nitrogen, carbon, and oxygen so readily that its preparation in absolutely pure form is well nigh impossible.

The uranous salt solutions are easily oxidised, and for this reason an attempt to electrolyse a uranous solution always results in the forma ion of uranyl salts around the anode and a deposition of oxide on the cathode. It may also be possible that the reaction of the metal with water is the explanation of the deposit of the oxide at the cathode. Whatever the explanation may be it has thus far been experimentally impossible to plate pure metallic uranium, especially from aqueous solutions.

Muthmann and Fraunberger measured the single potential of metallic uranium against a normal solution of potassium chloride, and reported a value of 0.19 volt (4kad. Wiss. München, 1904, xxxiv., 201). No analyses indicating the purity of the metal used are given. My own measurements with a sample containing 91:49 per cent metallic uranium in normal potassium chloride solution gave a single potential of -0 099 volt, which is substantially the same as that developed by the same sample of metal immersed in uranyl nitrate solutions containing 143 grms. per litre. It is possible that Muthmann and Fraunberger attach a different meaning to the sign before the figure which they give. On the other hand, the uranium they had was undoubtedly of a different state of purity to my own which was contaminated with ferric oxide. Neither result can, of course, be considered as final since the metal used was not pure.

II. EXPERIMENtal.

(NOTE. In all the electrolytic experiments the potential impressed is given in volts and the current density is stated in amprères per square decimetre).

1. Pridine as a Solvent for Uranyl Salts. Tests upon the solubility of uranyl nitrate, chloride, acetate, sulphate, and oxalate were made by treating small portions of the different salts with pyridine. Uranyl nitrate was found to be the most easily dissolved, though uranyl chloride and acetate are also appreciably soluble. They are more soluble in cold pyridine than in hot. When heated in a water-bath at the boiling temperature a flocculent yellow precipitate was thrown down in each Uranyl sulphate and uranyl oxalate were only slightly soluble in pyridine. The yellow precipitates.

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