THE VAPOR PRESSURE OF COBALT BETWEEN 1693K AND 1753K AND CHEMICAL ACTIVITIES OF COBALT AND PLATINUM IN C0-PT ALLOYS AT 1693K

The Knudsen effusion technique was used to determine seven values of the vapor pressure of pure solid cobalt between 1693 K and 1753 K and the vapor pressure of cobalt over five cobalt-platinum alloys at 1693 K, ranging in composition up to fifty-one atomic per cent platinum. Within the temperature range, the vapor pressure of cobalt is given by logP (atm.0 - -22,950/T + 7.57. Third law calculations gave a value for the heat of sublimation of cobalt at 298.15 K of 105,000 plus or minus 800 cal./mole. The activities of cobalt in the experimental alloys were calculated from the quotient of vapor pressures. Cobalt activity coefficients at higher platinum concentrations were extrapolated from a plot of N sub Co/N sub Pt versus ln gamma sub Co, and platinum activity coefficients were determined by graphical integration of the Gibbs-Duhem equation using the same plot. The activities of both cobalt and platinum in the alloys showed large negative deviations from Raoult's law. The integral free energies of mixing were determined to be negative, with the most negative value of -8300 cal./mole occuring at about forty-five atomic percent platinum. By then assuming the ideal entropy of mixing to be a limiting maximum value, it was found that the integral enthalpies of mixing must be negative. (Author)