144 COMPLEX HALOGEN ACIDS AND SALTS.
through the two bottles placed in series. After about five hours
all the lead chloride should be dissolved. If any remains, allow
the solutions to stand overnight and then filter off any insoluble
residue on asbestos. Combine the two filtrates in a 600 c.c.
flask, cool the liquid with ice, and add an ice-cold solution of 8 g.
ammonium chloride in 80 c.c. of water; after a short time a heavy,
yellow, crystalline precipitate of ammonium plumbic chloride
begins to separate. After several hours filter rapidly on a hardened
filter, wash the precipitate with 50 c.c. of ice-cold alcohol and dry
it at 50°. Yield, about 20 g.
On treating a sample of the ammonium plumbic chloride with
water, it decomposes immediately, forming brownish-black hydrated
lead dioxide which remains to some extent in colloidal solution.
Lead Tetrachloride. Mix 20 g. of ammonium plumbic chloride
thoroughly, by means of a mechanically driven stirrer, with 60 c.c.
of ice-cold, concentrated sulphuric acid. In a short time heavy,
nearly colorless, oily drops of lead tetrachloride separate. By
repeated decantation and stirring up with fresh portions of sul-
phuric acid, it is possible to separate all of the ammonium sulphate
from the oil, and the latter then settles as a fairly clear layer. If
the oil is allowed to remain long in contact with the sulphuric
acid, lead sulphate is formed.
Lead tetrachloride decomposes with explosive violence when
strongly heated.- Potassium Lead Iodide, K[PbI ]. 2 H 2 O.
Potassium lead iodide is stable only when in contact with a cold, concen-
trated solution of potassium iodide. If the solution is diluted with water,
or even if it is merely heated, potassium iodide dissolves out of the solid
compound, and lead iodide remains behind. On concentrating and cooling
the solution, the double salt is again formed.
If a hot solution of 4 g. lead nitrate in 15 c.c. of water is mixed
with a hot solution of 15 g. of potassium iodide in 15 c.c. of water,
yellow lead iodide is at first precipitated. On cooling to roomtemperature, the crystals of lead iodide disappear and a very pale-
yellow, felted mass of crystal needles is produced. On heating,
the crystals of the double compound disappear with a re-formation
of lead iodide; on cooling, the double salt is again produced.
The change can be observed especially well if a drop of the hot