Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

24 THE ELEMENTS.

in which the gold is chemically united with some other element, are treated
either with chlorine water, whereby chlorauric acid H[AuCl 4 ] is formed,
or with potassium cyanide solution, which, together with atmospheric oxygen,
converts the gold into potassium aurocyanide, K[Au(CN)2].
For the following experiments, employ a sand that is very poor in gold;
in case such is not at hand, mix sand with a little high-grade gold ore, or
moisten the sand with a gold solution and dry it by ignition.

Krohncke's Method of Preparing the Gold Solution. Mix 10

to 100 g. of ore intimately with one-fourth its weight of sodium
chloride (to form NaAgCl 2 with the silver present) and a little

potassium chlorate; moisten the mixture well with concentrated

hydrochloric acid in a small flask, and allow it to stand for 12

to 24 hours at the room temperature with occasional shaking.
Then heat it on the water bath until the greater part of the free

chlorine has been expelled. Finally, dilute with water, whereby

any silver chloride is precipitated, and filter. Evaporate the

filtrate to a small volume on the water bath, and carry out with
this solution the two reactions given below.

Doring's Method of Preparing the Gold Solution. Place the

sample of ore in a glass-stoppered flask, add 1 cc. of bromine and

about the same amount of ether, and shake frequently. After

two hours — vapors of bromine must still be visible — add

50 cc. of water and allow the mixture to stand another two hours

in a warm place. Filter the solution and evaporate it to one-
fourth of its former volume.
Tests. To detect the presence of gold in either of the above
solutions, treat one part with freshly-prepared ferrous sulphate
solution, whereupon the liquid first assumes a reddish-violet
coloration, and then becomes turbid through separation of gold.
With the solution prepared by the Krohncke method a consider-
able quantity of the ferrous sulphate should be used. Treat a
second portion of the solution with a few drops of bromine water
and a little stannous chloride solution, whereby the solution
becomes colored at first blue, then brownish violet, and later red;
in this test an adsorption compound of gold and stannic hydroxide
is formed (Purple of Cassius); cf. No. 25.
According to the above directions, the presence of about 0.05
mg. of gold in the ore can be detected.