Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

POTASSIUM MERCURIC IODIDE. 145

solution is placed between a heated slide and cover glass under the

microscope.

If it is desired to obtain the dry salt, it should be collected on a

filter, and, without washing, pressed between filter papers and

dried in a vacuum desiccator.

Potassium lead iodide possesses the remarkable property of

being extremely soluble in acetone. If the preparation is treated

while still moist with 10 to 15 c.c. of acetone, there is produced,

even in the cold, a yellow solution from which the salt can be

precipitated by addition of two or three volumes of ether. On

evaporating the acetone solution, no well-formed crystals are

obtained. If a few drops of this solution are allowed to evaporate

on filter paper the salt is obtained in a state of very fine subdivision,

in which condition it is extremely sensitive to the least traces of

moisture. Even the moisture of the air suffices to decompose

the salt in a short time; the yellow color of lead iodide which

thereby appears indicates in the sharpest manner the presence of

traces of water. If another paper prepared in the same manner

is left in a vacuum desiccator over sulphuric acid, no noticeable

yellow color appears; on opening the desiccator, however, it

develops immediately.

107. Potassium Mercuric Iodide, KJLHgI,]. % H 2 O.

Precipitate mercuric iodide from an aqueous solution of 13.5 g.

mercuric chloride by adding a solution of 16.6 g. potassium iodide.

Wash the precipitate and then redissolve it in a hot solution of

16 g. potassium iodide in 10 c.c. of water. Filter off the small

amount of undissolved mercuric iodide and allow the double salt to

crystallize in a vacuum desiccator over sulphuric acid, breaking up

occasionally the crust which forms over the surface. When a thick,

pasty mass of crystals is obtained, drain off the liquid with suction,

and without washing dry the light-yellow, prismatic crystals in a

desiccator. Obtain more of the product from the mother-liquor.

Test-Tube Experiments. 1. Treat a little mercuric oxide with a

few cubic centimeters of potassium iodide solution. A colorless,

strongly alkaline solution containing potassium mercuric iodide
and potassium hydroxide is produced. The formation of the com-

plex salt makes it possible for the mercuric oxide to dissolve with

the liberation of free potassium hydroxide (cf. No. 112).