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

POTASSIUM COBALTOTHIOCYANATE. 147

drain off the liquid, and wash the crystals with the ether. Bring

the ether-hydroferrocyanic-acid compound into an Erlenmeyer

flask, which is provided with an inlet and an outlet tube, and heat

it in an atmosphere of dry hydrogen at 80° to 90° on the water

bath. The ether is removed in this way in about an hour. Yield,

about 12 g. The hydroferrocyanic acid is nearly colorless at first,

but it quickly develops a light-blue color by contact with the air.

110. Cobaltous Salt of Hydromercurlthlocyanic Acid, Co[Hg(SCN)J.
     Prepare two solutions, one with 30 g. of mercuric chloride and
     44.5 g. of potassium thiocyanate in 500 c.c. of water, and another
     with 20 g. of cobalt nitrate in 50 c.c. of water. Mix the two
     clear solutions; a shower of small, deep-blue crystals of the diffi-
     cultly soluble double salt begins to fall at once. After standing
     12 hours, drain the crystals, wash them with water, then with
     alcohol, and dry the product in the steam closet. The yield is
     nearly quantitative.
     A sensitive test for mercury is based upon the formation of this

salt; the test is so delicate that less than 0.01 mg. of mercury can
be detected. Evaporate one small drop of dilute mercuric chloride

solution on a microscope slide and moisten the residue, by means

of a platinum wire, with a tiny droplet of cobaltous acetate dis-

solved in a concentrated aqueous solution of ammonium thio-
cyanate. Under the microscope, tiny, dark-blue needles will soon

be seen; these often unite to form concentric aggregates. Rub-

bing with a platinum wire serves to start the crystallization.

Powdered cobaltous mercurithiocyanate in contact with con-

centrated sodium carbonate solution decomposes after a few hours:
on filtering a residue of dull red cobaltous carbonate remains on

the filter and the filtrate contains sodium mercurithiocyanate.

111. Potassium Cobaltothiocyanate, K 2 [Co(SCN) 4 ].
     Prepare a hot, saturated solution of 14.5 g. of crystallized cobalt

nitrate and 24 g. of potassium thiocyanate; let it cool, and after

several hours remove the crystals of potassium nitrate which have

separated and rinse them with 40 g. of amyl acetate, added in small

portions, until they are nearly colorless. Add these washings to

the .main part of the liquid in a separatory funnel and shake thor-

oughly. Separate the two layers that form; this will require very