Synthetic Inorganic Chemistry

MERCURIC SULPHOCYANATE 235

with the Hg++ ions as long as any of the latter are present, the
solution remaining colorless and the precipitate, Hg(SCN) 2 , form-
ing towards the end; but as soon as the Hg++ ions are exhausted,
then the SCN~ ions unite with Fe+++ ions, producing the red
compound, which indicates that the reaction is complete.
Mercuric sulphocyanate has a peculiar property: when ignited
it burns with the production of a very voluminous coherent ash,
which, from the form it assumes, is called " Pharaoh's Serpent."
It should not be burned indoors because of the production of
poisonous vapors.

Materials: mercuric nitrate from preceding preparation,

potassium sulphocyanate, KSCN, 25 grams,

ferric chloride for indicator.

Apparatus: 2-liter common bottle.

suction filter and trap bottle.

8-inch porcelain dish.

Procedure: Dissolve the mercuric nitrate in 1 liter of water,

adding enough nitric acid to prevent the formation of any basic

salt. To this add 10 drops of a ferric chloride solution; then add

gradually, with constant stirring, a solution of the potassium

sulphocyanate in 500 cc. of water until a red color appears and

persists after stirring. Collect the precipitate on a suction filter,

and dry it on paper towels.

The dried salt may be made into the so-called Pharaoh's serpent

eggs by mixing it with 1.5 grams of dextrine and water to obtain

a paste, placing the latter in conical molds about 1 cm. wide

and 1 cm. deep, and letting it dry out and harden.

QUESTIONS

1. What is the degree of ionization of the soluble halides of
   mercury, i.e., HgCl 2 , Hg(CN) 2? Do these salts form, in this
   respect, any exception to the general rule regarding the ionization
   of salts?


2. Describe at least three instances which have previously fallen
   under your observation in which a reagent in limited amount will
   give a precipitate, but, added in excess, will cause the precipitate
   to redissolve.