234 HEAVY METALS OF GROUPS I AND IIdilute hydrochloric acid. A precipitate will probably form, in
which case add 10 cc. of concentrated nitric acid to the flask and
boil until a precipitate is no longer obtained when tested as above.
Pour the solution into a casserole and evaporate in the hood over a
very small free flame until the liquid has assumed a sirupy consist-
ency and crystals just commence to form on the surface. Then
transfer the whole mass to a warmed sample bottle, which has
previously been weighed; let it cool and stopper the bottle.QUESTIONS- To prepare a solution of this salt for use as a laboratory
reagent, explain why it is necessary to add nitric acid. (Compare
with Question 1 under Mercurous Nitrate, page 232.) - To a solution of mercuric nitrate add a little hydrochloric
acid. Now add a little stannous chloride solution. What is the
precipitate, and what change in the valence of mercury must have
occurred before it could form?
PREPARATION 34
MERCURIC SULPHOCYANATE, Hg(SCN) 2
In most of its properties the sulphocyanate radical resembles
the halogens, with which it is often classed, in the same manner
that the ammonium radical, NH 4 , is classed with the alkali metals.
Mercuric sulphocyanate is insoluble in water, and may be pro-
duced by bringing together equivalent quantities of solutions of
mercuric nitrate and potassium sulphocyanate, but if a suffi-
cient excess of either of these reagents is used, the precipitate
dissolves in it. From a consideration of the principle of solu-
bility product one would predict that an excess of either ion
would cause a decrease in the solubility of the salt Hg(SCN) 2 ,
but this effect is outweighed by the tendency of Hg(SCN) 2 to
combine with either Hg++ or SCN~ ions to form fairly stable
complex ions (see page 119). A very neat expedient may be
adopted in this preparation to show when the proper amount
of reagent has been added, as follows: ferric sulphocyanate,
Fe(SCN) 3 , is a soluble substance which has an intense red color.
If to a given solution of mercuric nitrate a few drops of a ferric
salt solution are added, and then to this is gradually added a
solution of potassium sulphocyanate, the SCN~ ions will unite