METAL-AMMONIA COMPOUNDS 165
ammonia salts has led to corresponding results: ammonia is usually found in
the cation.^1
Whether other components than ammonia take part in the formation of
the complex ions is ascertained, first by conductivity measurements, from
which the number of ions contained in the solution is determined, and second
by chemical reactions from which the composition of the ions can be deduced.
If the molecular conductivity, a, of 1/512 normal or 1/500 normal solutions
at 25° is compared as follows:^2
Co(NH 3 ) 6 X 3 Co(NH 3 ) 6 X 3 Co(NH 3 ) 4 X 3 Co(NH,),X,
a 402 245 117 7
and at the same time the conductivity of the following types of simple salts
under the same conditions:
Na 3 PO 4 MgCl 2 NaCl
a 370 249 125
is taken into consideration, it follows that the first of the above complex
salts must be a quaternary electrolyte, the second a ternary one, the third a
binary one, while the fourth is not an electrolyte at all; this is expressed by
the following formulas:
[Co(NH 3 ) 6 ]X 3 [Co(NH 3 ) 6 X]X 2 [Co(NH 3 ) 4 X 2 ]X [Co(NH 3 ) 3 X 3 ]
A confirmation of this view is furnished by the chemical reactions of the
compounds. If, for example, X is a halogen, then in a solution of the com-
pound [Co(NH 3 ) 6 Br]Br 2 only two-thirds of the bromine is precipitated by
the addition of silver nitrate. In a similar way it has been found, in numerous
cases, that the deductions from conductivity measurements are in accord
with the chemical behavior of substances, and thus it is shown that in addi-
tion to ammonia, also certain atoms and radicals which usually act as anions
(e.g. the halogens, NO 2 , and CO 3 ) may participate in the formation of complex
cations.
In one series of metal-ammonia salts the compounds as they exist in the
solid state contain water which cannot be removed without destroying the
compounds. This water remains as a constituent of the salts even when
their negative components are replaced by others, and it must therefore
belong to the cation (cf. No. 134)
[Co(NH 3 ) 6 H 2 O]Cl 3 + 3AgNO 3 = [Co(NH 3 ) 6 H 2 O](NO 3 ) 3 + 3 AgCl.
The water apparently plays the same part in these compounds as ammonia
in the first-mentioned series, as is evident from the fact that the conductivity
is not materially changed when molecules of ammonia in one of the above
compounds are replaced successively by water molecules:
[Co(NH 3 ) 6 ]Br 3 [Co(NH 3 ) 6 H 2 O]Br 3 [Co(NH 3 ) 4 (H 2 O) 2 ]Br 3
a 402 390 380
(^1) For exceptions to this general rule see No. 115 and p. 169 (top).
(^2) These values are taken from measurements on [Co(NH
3 ) 6 ]Br 3 ,
[Co(NH 3 ) 6 Br]Br 2 , [Co(NH 3 ) 4 Cl 2 ]Br, and [Co(NH 3 ) 3 (NO 2 ) 3 ] Werner and Mio-
lati; Z. phys. Chem. 13, 35 (1893); 14, 506 (1894); 31, 225 (1896).