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

48 CHANGES OF CONDITION.

tion has been obtained, the ether can be driven off completely and

air introduced to oxidize the excess phosphorus, without injury
to the gold colloid (hydrosol). This preparation of colloidal gold

solution is very sensitive to external conditions; it is not possible

to produce equally good solutions in all laboratories.

2. Colloidal Gold by Reduction with Carbon Monoxide. Prepare
   carbon monoxide by heating oxalic acid crystals with concentrated

sulfuric acid, and passing the gas through sodium hydroxide solu-

tion to remove carbon dioxide. Pass a slow stream of the carbon

monoxide into 120 cc. of 0.03% chlorauric acid. There is pro-
duced a violet color which changes to violet-red and later to a dark

red. Do not carry the reduction too far, as the solution will then

assume a permanent violet color.

3. Colloidal Gold by Reduction with the Hydrogen Flame.
   1
   It is

easy to prepare a colloidal gold solution by merely directing a
pointed hydrogen flame, 1 to 2 cm. long, against a 0.03% solution

of chlorauric acid contained in a wide, porcelain dish. After

about five minutes the solution begins to be colored; soon a rich

red color is produced. Occasional stirring hastens the formation
of the colloidal gold.

Use a gold solution prepared by any one of these methods in the

following experiments.

I. Synthesis of Purple of Cassius. From a mixture of colloidal

stannic oxide and colloidal gold, the gold-stannic oxide adsorption

compound is precipitated on the addition of electrolytes. In order

to prepare the colloidal stannic acid solution, allow 5 cc. of tin

tetrachloride (No. 50) to become hydrolyzed by the addition of

150 to 200 cc. of water, and pour this solution into 500 cc. of

water to which a few drops of ammonia have been added. Dialyze

the clear mixture for five days (cf. No. 21), changing the outside

water two or three times daily, until it shows no test for chlorides.

If during this process a hydrogel separates in the dialyzing tube,

it may be peptonized (see p. 40) in a beaker by the addition of

about three drops of ammonia, whereupon, after a time, the jelly

will go over into a perfectly clear hydrosol. A mixture of this

hydrosol with an equal volume of gold solution remains unchanged,

but on addition of a salt (ammonium chloride) a beautiful deep-

(^1) J. Donau, Monatsh. 34, 949 (1913). Colloidal solutions of silver, platinum
and molybdenum blue can be prepared by this method.