Potassium Ferricyanide: Red Prussiate from Yellow, with Chlorine

Red prussiate of potash, the second salt of the cyanotype. Gmelin's method, as Thorpe (1912) gives it: bubble chlorine into a cold 10 % solution of yellow prussiate until a drop no longer turns blue with a ferric salt, then crystallise; a little potash clears the green that forms. Yield 85 to 90 % of theory. Chlorine is a poison gas: outdoors, never alone. The picture: potassium ferricyanide crystals (Wikimedia Commons).