An electrochemical method was tested in order to find a suitable experimental technique for the determinations of the activities of P2O5 in molten fluxes used for external dephosphorization of hot metal. This technique consists of equilibrating phosphorus‐containing copper with molten slag and measuring the equilibrium oxygen partial pressure by means of solid state galvanic cells. The activities of P2O5 in MgO—P2O5 were determined with this technique.

The activities of FexO in CaO + CaF2 + SiO2 + FexO quaternary slags were measured by means of solid‐oxide galvanic cell. The FexO activities in the slags are influenced by CaF2 as well as SiO2. At constant FexO mole fractions, e.g., , at low SiO2 mole fractions i.e., , the substitution of CaF2 for CaO has an effect of raising the FexO activity. At higher SiO2 mole fractions, e.g., , however, such an effect becomes insignificant.

Measurements of the activities of FexO in complex slags during the final stages of external dephosphorization by using a disposable electrochemical oxygen probe. Positive deviations of the FexO activities from the Raoult law. Iso‐activity curves drawn on the “ternary” diagram, (CaO + MgO + MnO + CaF2) + FexO + (SiO2 + P2O5) at 1673 K. Calculations of the activities of P2O5 in complex slags with an assumption of thermodynamic equilibrium between slags and hot metal. Beneficial effect of CaF2 in the slags with respect to lowering of the P2O5 activity.

An electrochemical technique involving magnesia‐stabilized zirconia as the solid electrolyte and a mixture of Mo + MoO2 as the reference electrode for the measurements of the activities of FexO in the SrO + FexO and BaO + FexO systems at 1673 K. Measurement of oxygen potentials established at the slag electrode by disposable solid state cells. Expression of the thermodynamic properties of SrO + FexO and BaO + FexO by the binary subregular model.