FeO is determined through titration, using a cold acid digestion of ammonium metavanadate, and hydrofluoric acid in an open system. Ferrous ammonium sulphate is added after digestion and potassium dichromate is the titrating agent.
This cold digestion will dissolve silicates and some sulphides. Pyrite may not be totally dissolved. The extent of dissolution is affected by the ferric iron content higher Fe(3+) concentration increases the solubility of pyrite. Concentrations of S(II) has a tendency to reduce Fe(3+) to Fe(2+). This is minimized in an open system. If the S concentration is greater than 10% an alternative hot digestion method is used for FeO.
When titrating, the endpoint is determined by colour. Certain solutions high in MnO2 or certain matrices make the endpoint determination difficult or impossible.
Modified method from Wilson (1955. Bull Geol. Sur. Gt. Britain , volume 9, pp. 56-68)