Purple of Cassius
Encyclopedia
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.
Tin(II) chloride
Tin chloride is a white crystalline solid with the formula 2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent , and in electrolytic baths for tin-plating...
. It has been used to impart glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
with a red coloration (see cranberry glass
Cranberry glass
Cranberry glass is a red glass made by adding gold chloride to molten glass. Tin, in the form of stannous chloride, is sometimes added in tiny amounts as a reducing agent...
), as well as to determine the presence of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
as a chemical test
Chemical test
In chemistry, a chemical test is a qualitative or quantitative procedure designed to prove the existence of, or to quantify, a chemical compound or chemical group with the aid of a specific reagent...
.
Generally, the preparation of this material involves gold being dissolved in aqua regia
Aqua regia
Aqua regia or aqua regis is a highly corrosive mixture of acids, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and hydrochloric acid, usually in a volume ratio of 1:3, respectively...
, then reacted with a solution of tin(II) chloride. The tin(II) chloride reduces the chloroauric acid
Chloroauric acid
Chloroauric acid is a inorganic compound with the formula HAuCl4. This pale yellow compound is a common precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
from the dissolution of gold in aqua regia to a colloid of elemental gold supported on tin dioxide
Tin dioxide
Tin dioxide is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry...
to give a purple precipitate or coloration.
When used as a test, the intensity of the color correlates with the concentration of gold present.