A method is described for determining the standard free energies of formation of the tantalum silicides from emf measurements on cells with solid thoria‐yttria electrolytes. Measurements were conducted over the temperature range 900°–1100°C in a purified argon atmosphere. Corrections for electronic conduction were established with Ta, Ta2O5 electrodes. Difficulties were encountered because the hard, refractory nature of the compounds prevented fabrication of dense electrodes and because silica films interfered with the measurements. At 1300°K, the measured standard free energies of formation in kcal per gram atom of silicon are: TaSi2 , −8.1 false(+1.8/−3.7false) ; Ta5Si3 , −24.0±3 ; Ta2normalSi , −27.2±3.1 ; Ta4.5normalSi , −37.3±7.2 .

By assuming the Coulomb forces between the ions of an electrolytic solution and by combing Poisson’s equation and Boltzmann’s principle, Debye and Hükel have developed a theory of strong electrolytes, satisfactory, however, only up to moderate concentrations. The approximate character of their method has been extensively investigated by a number of authors. Proceeding from the laws of statistics, Fowler showed that only for dilute solutions is Poisson’s equation valid in the Debye-Hükel treatment, where phenomena on a molecular scale are involved, the omitted fluctuation terms becoming appreciable at stronger concentrations. Although Halpern§ claims that at no concentration of physical interest is such a neglect permissible, we may assume from more recent work|| that Fowler’s original conclusions hold. Another objection is the use of the Colulomb law of force between two ions of charge ε i and ε j F ij = ε i ε j /D r ij 2 , (1.1) Which is not justifiable for small values of r ij . In water, for example, the main part of D is contributed by the orientation of the water dipoles, and in the vicinity of an ion, a saturation effect will exist changing the value of D considerably. In addition, the polarization, van der Waals and exchange forces between the ions come into play at their close approach.