EXPERIMENTS 239The ions of cupric iodide are brought together but cupric iodide
is unstable and decomposes into insoluble cuprous iodide, Cul, and
free iodine. With excess of KI the soluble salt KCuI 2 , which is
ionized as K+ CuI 2 ~, is formed. Cupric chloride, CuCl 2 , and
cupric bromide, CuBr2, do not decompose in the same way into
cuprous salts. (Compare the relative reducing action of chloride,
bromide, and iodide, Experiments 12, 13, and 14, pages 166-168.)
(6) To 1 cc. of 0.2 2V Hg(NO 3 ) 2 add a few drops of 12V KI
and note a bright red precipitate. Add a little more KI and
note that the precipitate redissolves to give a colorless solu-
tion.(c) To 1 cc. of 0.12V AgNO 3 add a few drops of a saturated
NaCl solution. Note the white precipitate. Add 10 cc. of
saturated NaCl, shake, and note that the precipitate cannot
be seen to dissolve. To show that some does dissolve, filter,
and add a large amount of water to the clear nitrate, noting
an opalescent precipitate.Mercuric iodide, Hgl 2 , is very insoluble but with excess iodide
ions it readily forms the complex ion Hgl4~~. Similarly silver
chloride can, in the presence of a high concentration of chloride ions,
form to a limited extent the complex ion AgCl 2 ~. The latter is
very unstable, however, and dilution reduces the Cl~ ion concen-
tration sufficiently to allow the complex ion to dissociate, AgCU"
^± AgCl | + Cl~, with a reprecipitation of silver chloride.
The above experiments show the strong tendency of the heavy
metals of this chapter to enter into the formation of complex
negative ions. (See page 120.) The negative radical ions show
differing tendencies to enter into these complexes, and in the
decreasing order of the strength of this tendency are CN~, SCN~,
I", Cl~. Sulphate and nitrate ions show very little tendency to
join heavy-metal ions in the formation of complexes of this kind,
although it may be that the formation of the crystallized double
salts like the alums and the double sulphate of potassium and
copper is due to such a cause.- Sulphides, (a) To separate tubes containing diluted
heavy-metal salt solutions as in Experiment 3, (a) to (/), add
2 cc. of 62V (NH 4 ) 2 S in each case and note the character of the