Use of a sensor in the electro-chemical determination of silicon dissolved in a molten metal, said sensor comprising a layer of a solid electrolyte capable of conducting oxygen ions, a reference electrode in contact with one surface of said layer of the solid electrolyte for providing a constant oxygen potential at a particular temperature of measurement, and an auxiliary electrode comprising SiO.sub.2 disposed in the immediate vicinity of the other surface of said layer of the solid electrolyte.