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

DIAMMINEDICHLOROCOBALT. 187

142. a-Diamminedicblorocobalt, a-[Co(NHj)aCla].
     The more stable of the two isomers mentioned in the introduction to Prepa-
     ration No. 141 can be prepared as equilibrium product by thermal decomposi-
     tion of the hexammine. At about 150° the hexammine has an ammonia
     tension of considerably more than an atmosphere whereas the ammonia
     tension of the diammine corresponds to only 10-20 mm.

Transfer about 2 g. of dry hexamminecobaltous chloride with

the aid of a short, dry funnel, to a dry, weighed test-tube, taking

care that none of the crystals adhere to the sides of the tube.

Weigh the tube and its contents to an accuracy of 4 significant

figures. Stopper the tube with a vacuum-tight, rubber stopper

carrying a right-angled piece of glass tubing with a glass stop-

cock. Connect this tubing with a short-armed manometer and

beyond the latter with the outlet-arm of a 100 c.c. distilling flask,

placing a screw-cock between the manometer and the distilling

flask. Fill the distilling flask about one-third full of concentrated

sulphuric acid for absorbing ammonia, insert a rubber stopper

carrying glass tubing that reaches almost to the surface of the

sulphuric acid and make connection through a rubber tube with

a screw-cock with the suction pump. Evacuate the entire ap-

paratus down to a pressure of 10-15 mm., close the stopcock at the

test-tube and dip the tube in a bath of melted paraffin or sulphuric

acid at 150°. Immediately after placing the tube in the hot bath,

open the glass stopcock very slowly and apply suction for one

minute to withdraw the ammonia. The latter is given off very

vigorously at the start and, if the stopcock is opened too quickly,

powdery solid material will be carried along with the gas. The

color of the solid changes from pink to blue, the latter being the
color of the unstable diammine. Close the stopcock and allow

the tube to remain in the hot bath at 150° for half an hour to give

the unstable diammine time to change to the stabler modification.

Then carefully open the stopcock again, apply a little more suction
and again allow the stoppered tube to remain quietly in the bath

for a half hour. Repeat this treatment two or three times until

a pure, pink-colored salt is obtained. Then, if the decomposition

has been complete, on closing the screw-clamp between the manom-
eter and the sulphuric acid, which was used to absorb escaping

ammonia, a pressure of about 40 mm. will be shown in the manom-