The Solubility of Ignited Ferric and Chromic Oxides

In the gravimetric determination of iron the metal is frequently precipitated as the hydroxide by ammonia. It is then collected by filtration, dried, ignited and weighed as the oxide. Such precipitates when wet, as hydroxides, dissolve instantaneously in acids. After the precipitate has been ignited it often becomes practically insoluble. Such ignition gives a range of temperature of 600° - l000°C, depending on the type of heating device used. Kolthoff and Sandell suggest the fusion of this ignited ammonia precipitate with alkali pyrosulfate in order to convert the oxides into sulfate which then can be dissolved. This procedure is troublesome because the alkali must then be removed before the iron is determined either volumetrically or gravimetrically. Chromium is not usually determined in this way because the strong heating forms some Cr2(CrO4)3 which gives high results unless the oxide is ignited in hydrogen