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

DITHIO-OXAMIDE. 93

remaining in the distilling flask by decantation with water, and

then heat it upon the water bath with about 100 grams of 2-normal

nitric acid, which oxidizes it to a mass resembling Prussian blue;

the substance so formed contains potassium and is known as

Williamson's Violet. Wash this pigment several times by decan-

tation with water, suck it free from liquid on a hardened filter

paper, and dry it in the hot closet. Yield, 40 grams.

Mercuric cyanide can be prepared from Prussian blue or William-

son's violet by boiling the pigment with 2.25 times its own weight

of mercuric oxide and forty times its weight of water. After the

blue color has disappeared, filter the solution and boil the turbid

filtrate with more of the Prussian blue until no more of it is de-

colorized and the solution reacts neutral. Add some animal

charcoal, boil the solution again, filter, and finally evaporate to

dryness. Recrystallize the residue from water.

Mercuric cyanide in aqueous solution is but slightly dissociated

electrolytically, and on this account it fails to give a precipitate

when treated with either silver nitrate or with sodium hydroxide.

For the same reason mercuric oxide will dissolve in solutions con-

taining potassium cyanide, forming mercuric cyanide and potas-

sium hydroxide; the experiment should be tried.

Cyanogen from Mercuric Cyanide. Heat about one gram of the

mercuric cyanide strongly in a dry test-tube; cyanogen gas is

evolved and burns with a yellowish-red flame, which is purple

at the edges.

Dithio-oxamide.

Cyanogen unites with one or with two molecules of hydrogen sulphide:

. C = N CSNH 2
I + H 2 S = I (cyanthioformamide).
C^N C=N
C = N CSNH 2
1 + 2 H 2 S = I (dithio-oxamide).
C = N CSNH 2

Dithio-oxamide is sometimes formed in qualitative analysis when hydrogen
sulphide is passed into the solution containing potassium cyanide in the test
for cadmium.

1. Saturate 100 g. of alcohol with hydrogen sulphide in a

closed flask. Then conduct into the liquid some cyanogen gas,

which is prepared by heating successively two portions of 10 g.