164 METAL-AMMONIA COMPOUNDS.
pressure is known as an isotherm. With pressure constant the plot of con-
centration against temperature is an isobar; each compound has a definite
limit (similar to a boiling point) beyond which it changes abruptly into the
next lower compound of the series. When pressure and temperature are both
variable, a curve appears for each compound quite of the nature of a vapor
pressure curve which is known as a tension curve. Along this curve the par-
ticular compound can exist in equilibrium with the vapor and the next com-
pound of the series. In the area between two curves only the one compound
can exist in equilibrium with the vapor.
By studying isobars and isotherms it is possible to determine the number and
composition of all compounds of gaseous ammonia and a given salt and to
characterize the individual compounds by the tension curves which separate
the fields of their existence. The decomposition temperature, T on the
absolute scale, is, for different compounds at the same pressure, proportional
to the mutual affinities of the salt and ammonia. These studies show, there-
fore, not only what particular ammoniates can exist but they give us a rational
means of measuring the affinities of salts toward ammonia. Fig. 22a is
the isotherm curve for the system NiCl 2 /NH 3. The curve shows that NiCla
forms a monammine which has a pressure of 74 mm. at 300°, a diammine with
a pressure of 540 mm. and a hexammine with a pressure of more than an
atmosphere. If the pressure is kept constant at 710 mm., Fig. 21b, shows that
the hexammine is stable up to 176°, the diammine up to 311° and the monam-
mine to 375°. Fig. 21c shows, by tension curves, the separate fields in which
the three ammoniates can exist.
The second method of preparing complex metal-ammonia compounds, i.e.
by the combination of metal salt and ammonia in aqueous solution, has been
up to the present time the one most used. It should be noted, however, that
the principal form in which the complex salt exists in the solution is not
necessarily that form in which it crystallizes out. The determination of the
stoiehiometric composition of dissolved metal-ammonia salts in the presence
of an excess of ammonia is possible by a combination of different physico-
chemical methods (solubility measurements, and determinations of ion con-
centrations by measuring the electromotive force). Thus it has been found
that the composition of the silver-ammonia ion which preponderates in
aqueous solution is [Ag(NH 3 ) 2 ]+, whereas the salt separating from such a solu-
tion has the formula 2 AgCl • 3 NH 3.
The question, with which of the components of the salt is the ammonia
combined, can be answered by the physico-chemical behavior of the com-
pound (conductivity and transference number) and also in a purely chemical
way. A solution of a cupric salt to which an excess of ammonia is added
possesses a deep-blue color instead of the usual light-blue of copper ions, and
the addition of sodium hydroxide solution no longer causes a precipitation of
cupric hydroxide. From this, and from the fact that electrolysis of the solu-
tion causes copper to migrate as before towards the negative pole, the con-
clusion is drawn that the copper ions have disappeared and been replaced by
new cations containing copper, and that these can have been formed only by
the adding of ammonia to the copper. The investigation of other metal-