Purple of Cassius
Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride. It has been used to impart glass with a red coloration (see cranberry glass), as well as to determine the presence of gold as a chemical test.
Generally, the preparation of this material involves gold being dissolved in aqua regia, then reacted with a solution of tin(II) chloride. The tin(II) chloride reduces the chloroauric acid from the dissolution of gold in aqua regia to a colloid of elemental gold supported on tin dioxide to give a purple precipitate or coloration.
When used as a test, the intensity of the color correlates with the concentration of gold present. This test was first observed and refined by a German physician and alchemist, Andreas Cassius (1600–1676) of Hamburg, in 1666.
- L. B. Hunt (1976). "The True Story of Purple of Cassius" (PDF). Gold Bulletin. 9 (4): 134–139. doi:10.1007/bf03215423.
- "Richard Adolf Zsigmondy: Properties of Colloids". Nobel Lectures, Chemistry 1922-1941. Amsterdam: Elsevier Publishing Company. 1966.