Chromium Trioxide: Zettnow's Dichromate and Sulphuric Acid Method (Thorpe, 1912)

Chromic anhydride (chromium trioxide), the crimson crystals used in chromium plating, by Zettnow's method: potassium dichromate is broken up with strong sulphuric acid, the potassium sulphate left to crystallise out, and the chromic anhydride crystallised from the hot acid liquor in three crops. Nitric acid of at least 1.46 sp. gr. washes the sulphuric acid off the crystals without dissolving them. Zettnow got 85 to 90 per cent of the theoretical yield. Source: T. E. Thorpe, Dictionary of Applied Chemistry vol. II (1912), p. 54. Picture: the book's text.