Sodium nitrite is generally estimated in commerce by oxidation with permanganate in the usual manner, but this gives values which are slightly too high for the works chemist. Besides the nitrous acid the permanganate gives also any other oxidizable substances which may be present, so that, under certain conditions, slight errors may be caused. In spite of this, however, this method is adopted in various factories, such as the Notodden nitrite works. For the colour chemist, however, there is only one really dependable method, namely, the Sulphanilic Acid Method, which is fully as accurate with a little practice and is safer.