140 COMPLEX HALOGEN ACIDS AND SALTS.
brilliant red prisms separate, which on the next day should be
washed with water and then with alcohol and dried quickly over
lime in a vacuum desiccator. On adding copper sulphate to the
main filtrate (to which the washings should not have been added)
a further yield of the crystals is obtained. The entire product is
about 25 g.
A sample of the salt dissolves completely in a small amount of
2-normal sodium hydroxide, which points to the existence of the
copper as a part of the complex; after some time copper sulphide
begins to separate slowly. Ammonia is readily detected in this
solution by its odor or by Nessler's reagent. Concentrated potas-
sium hydroxide added to the fresh solution immediately pre-
cipitates red potassium copper tetrasulphide. Concentrated
hydrochloric and nitric acids act but slowly on the dry salt, con-
centrated sulphuric acid not at all.COMPLEX HALOGEN ACIDS AND THEIR SALTS. — COM-
PLEX CYANOGEN COMPOUNDS.
Among the complex compounds formed by the union of two simple sub-
stances, the particular class in which both of the simple substances are halo-
gen or cyanogen compounds requires a special treatment, — first, for the
reason that the number of such compounds is very great, and many of them
are of considerable practical importance; and second, because their classifica-
tion and interpretation on the basis of the old theories of valence have for a
long time caused chemists much perplexity.
For the diagnosis of such complex compounds, purely chemical tests are
first made as to the reactivity of their constituents. If the typical reactions
of the simple salts do not occur with these double compounds, but instead
new specific reactions are found, then a complex structure of the substance
is indicated. Such cases are abundant in analytical chemistry, as, for example,
with the iron-cyanogen compounds.
Further, the existence in aqueous solution of complex metal-containing
ions can be proved electrochemically, and with this proof began the clearing
up of the constitution of these substances. The metal migrates, as a constit-
uent of the anion, toward the positive pole and there increases in concentra-
tion. Thus an anomalous change in concentration takes place with respect
to the metal when it is compared with that occurring in the electrolysis of
simple metal salts. From this Hittorf, in the years 1853-1859, deduced
the existence of complex ions in solution. Previously Porett (1814) had
noticed, incidentally, the analogy existing between potassium ferrocyanide
and the salts of the oxy-acids.
Finally, the degree of the complexity of dissolved double compounds can
be estimated by means of any one of the methods of determining ion concen-