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

METAL-AMMONIA COMPOUNDS. 163

again filtering. The solution, after being acidified with nitric

acid, gives no precipitate with silver nitrate.

A dilute solution of copper sulphate, or one containing ferric

chloride, when shaken with Millon's base, is entirely freed from all

dissolved salt.

Oxydimercuriammonium hydroxide. Spread 10 g. of Millon's

base in a thin layer on a watch-glass and allow it to dry for five

days over lime in a desiccator, adding in the first place 0.5 to 1 c.c.

of concentrated ammonia to the lime so that an atmosphere of

ammonia is produced. The product is explosive. Clamp a test-

tube containing a small amount of the substance in an upright

position and heat it behind the lowered window of the hood.

METAL-AMMONIA COMPOUNDS.

The importance of the metal-ammonia compounds lies in their great

number,^1 in their stability, and in the significance of the theoretical questions

pertaining to them. Among the best known of these compounds are those

containing tervalent cobalt, chromium, rhodium and iridium. Metal-ammonia

compounds are formed by the addition of gaseous ammonia to the solid salts,

or by the union of the two components in solution.

The first method, inasmuch as it represents the combination of two sub-

stances of widely different volatility (very difficultly volatile metal salt and

0 100 200 300 O 400

Isobar '

FIG. 22b

200

100 200

Tension Curves l

FIG. 22C

gaseous ammonia), offers opportunity for the investigation of these substances
from the standpoint of heterogeneous equilibrium.
Of the three variables, pressure, p, temperature, t, and concentration, c,
which determine the condition of a system, one may hold any one constant
and follow the variation of the other two with respect to each other. With
temperature constant, the curve obtained by plotting concentration against

(^1) Werner estimates the number of compounds of the general formula
MXB(NH 3 )m as 1700.