The Decomposition Potentials and Conductivities of Magnesium Oxide and of the Alkaline Earths in Fused Fluorides

Abstract

The decomposition potentials of saturated solutions of (1) magnesium oxide in a fused bath of magnesium, barium and sodium fluorides ; of (2) calcium oxide in calcium and sodium fluorides; of (3) strontium oxide in strontium and sodium fluorides ; and of (4) barium oxide in barium and sodium fluorides are measured with platinum and with graphite anodes at temperatures between 900° and 1,100° C. The deplorization by graphite is given by the difference between these decomposition voltages. Depolarization is calculated by the third heat theorem and also by the Nernst equation for an oxygen concentration cell. The calculations agree to 0.01 volt ; the measurements do not extend beyond this place of figures. In the cases of magnesium oxide and of calcium oxide, the experimental decomposition voltages agree to within a few hundredths of a volt with the calculated values. This agreement shows the oxides are decomposed primarily and not by secondary reactions. Decomposition potentials decrease linearly with rise in temperature in all cases. Thermodynamic calculations could not be carried out for barium oxide and for strontium oxide for lack of specific heat data at high temperatures. The electrical conductivities of the above solutions are determined to 0.5 per cent for varying oxide concentrations and for different temperatures. The conductivity varies linearly with the temperature except for the more concentrated solutions of strontium and barium oxides. In general the effect of adding oxide is to lower the conductivity of the solution, though with calcium and strontium oxides small additions cause a rise in conductivity. A new constant temperature regulator capable of maintaining temperatures to within one degree was constructed; it is described in the thesis on file in the library at the Massachusetts Institute of Technology. A new technique was developed for measuring the electrical conductivity of such corrosive fused salts, which can not be held at high temperatures in conductivity cells made of electrically insulating material. These resistances are measured by an adaptation of the Carey‐Foster method to a 1,000‐cycle alternating current measurement of electrical conductivities in the neighborhood of 0.1 ohm or less. This method eliminates errors due to lead resistances and spurious electromotive forces.