Ferric Ammonium Citrate: Ferric Hydroxide Dissolved in Citric Acid and Ammonia

Iron and ammonium citrate is the light-sensitive salt of the blueprint (cyanotype). The British Pharmacopoeia formula (US Dispensatory, 1907): throw down ferric hydroxide from ferric sulphate with ammonia, wash it, dissolve it in warm citric acid, add ammonia, and dry the syrup in thin layers below 38 °C to garnet-red scales. Keep it from light. The picture: ferric ammonium citrate (Wikimedia Commons).