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Reasons are outlined for thinking that accepted values for the activity of Al 2 O 3 in solution in cryolite are in error, and a new analysis is made of literature cryoscopic data. A general treatment is given for the effect of incongruent melting caused by the nonstoichiometry of solid cryolite. The effect is substantial, and accurate calculations require a knowledge of the effect of the solute on the activity of NaF in the solution, since that controls the composition of the solid separating. The results in the case of alumina as solute show that the slope of the log (activity) vs log (concentration) plot is about 3 in dilute solution and 1.5 in concentrated solution, corresponding to species containing 1 and 2 atoms of oxygen, respectively. Any calculations employing the earlier relation ϰ (wt pct a Al O 2 3 Al 2 O 3 ) 2.77 , which was based on electrochemical measurements of activity, need to be redone.

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In the electrodeposition of cobalt in chloride electrolytes the evolution of hydrogen is a parasitic reaction. On a rotating platinum disc electrode the current efficiency was calculated as the charge used for anodic dissolution of cobalt at a potential where no other reactions were taking place, divided by the total cathodic charge used for cobalt deposition. The results show that the current efficiency could be measured accurately in this way. In part I the current efficiency and deposition potential are studied as a function of current density and pH. The results show an increase in current efficiency with increasing current density, pH and temperature. The results also indicate a change in the reaction mechanism for electrodeposition when the pH is changed.

The temperature dependence of density, viscosity, and surface tension was determined in the system 55 mol % NaF + 45 mol % AlF3. The interfacial tension between aluminum and the above melt plus the melt 55 mol % KF + 45 mol % AlF3 was measured at 750 °C. These molten salts are potential low-temperature electrolytes for aluminum electrolysis. It was found that the temperature dependence of density and surface tension could be described by linear equations within the measured temperature ranges. The temperature dependence of viscosity can be described by a quadratic equation.