Purple of Cassius

Purple of Cassius is a purple pigment formed by the reaction of <a href="/facts/Gold_salt/VJcqLTxH">gold salts</a> with <a href="/facts/Tin(II)_chloride/4TdjMLEk">tin(II) chloride</a>. It has been used to impart <a href="/facts/Glass/hezRsAGf">glass</a> with a red coloration (see <a href="/facts/Cranberry_glass/n7qmIHlT">cranberry glass</a>), as well as to determine the presence of <a href="/facts/Gold/1znknojp">gold</a> as a <a href="/facts/Chemical_test/6J3u77Ic">chemical test</a>.
Generally, the preparation of this material involves gold being dissolved in <a href="/facts/Aqua_regia/AHr1mobH">aqua regia</a>, then reacted with a solution of tin(II) chloride. The tin(II) chloride reduces the <a href="/facts/Chloroauric_acid/JV38w41G">chloroauric acid</a> from the dissolution of gold in aqua regia to a colloid of elemental gold supported on <a href="/facts/Tin_dioxide/tLGukNFB">tin dioxide</a> 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 <a href="/facts/Hamburg/kmAAsYtT">Hamburg</a>, in 1665. <a href="/facts/Berzelius/HakSv3sP">Berzelius</a> later made a detailed study of the purple of Cassius. The colour also attracted attention from <a href="/facts/Michael_Faraday/NDcc3lyt">Michael Faraday</a>.
<a href="/facts/Richard_Adolf_Zsigmondy/UBHPehV1">Richard Adolf Zsigmondy</a>, who earned the 1926 <a href="/facts/Nobel_Prize_for_chemistry/8Nb8Ajjy">Nobel Prize for chemistry</a>, says that "Several of the red gold divisions prepared with <a href="/facts/Formaldehyde/Ck8RLfqk">formaldehyde</a> as well as those reduced with <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a> appeared perfectly clear in ordinary daylight (like good <a href="/facts/Red_wine/Qs8V3v0x">red wine</a>). They did not settle out their gold, and I was therefore able to call them rightly <a href="/facts/Chemical_solutions/MGchhMQV">chemical solutions</a>. In <a href="/facts/Thomas_Graham_(chemist)/RQhhKdCs">Thomas Graham</a>’s <a href="/facts/Dialysis_(chemistry)/7I8t5LCA">dialysis</a>, however, they behaved like colloidal suspensions: the gold particles did not pass through the parchment membrane. This showed my gold divisions their proper place, namely, that they belonged to the colloidal suspensions."

Partially translated from the <a href="/facts/German_Wikipedia/q7qgYs13">German Wikipedia</a> article, Goldpurpur.

<ul><li>L. B. Hunt (1976). <a href="http://www.goldbulletin.org/assets/file/goldbulletin/downloads/Hunt_4_9.pdf">"The True Story of Purple of Cassius"</a> (PDF). Gold Bulletin. 9 (4): 134–139. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1007%2Fbf03215423">10.1007/bf03215423</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:138485909">138485909</a>.</li></ul>

Purple of Cassius open-in-new

Purple of Cassius