146 COMPLEX HALOGEN ACIDS AND SALTS.
- Heat a sample of the dry salt in a test-tube. In addition to
a little water, mercuric iodide distils off and condenses in the yellow
modification on the cooler part of the tube. After some time, or
more quickly on touching it with a glass rod, the sublimate changes
into the red form, and this can again be changed into the yellow
condition by heating (cf. Prep. 17). - Potassium Cobalticyanide, K.,[Co(CN) 6 ].
Triturate 30 g. of cobalt carbonate with a little water until it is
thoroughly wet; then suspend it in 100 c.c. of water and dissolve it
by adding a solution of 110 g. potassium cyanide in 400 c.c. of
water. Oxidize the potassium cobaltocyanide thus formed by
drawing a vigorous current of air through the liquid for an hour.
After filtering the dark-yellow solution, add to it 40 g. of glacial
acetic acid and evaporate it to crystallization under a well-ven-
tilated hood outside of the general laboratory. Drain the crystals,
and wash them with alcohol of about 66% by volume. Work up
the mother-liquor repeatedly as long as a sample of the crystals
obtained gives a deep-blue solution when heated with concentrated
sulphuric acid. The easily soluble potassium acetate remains in
the residual liquor. Recrystallize the combined fractions from a
solution which is slightly acidified with acetic acid. Yield, about
60 g. The product may be used in No. 109.
- Hydroferrocyanlc Acid, H 4 [Fe(CN)e].
Hydroferrocyanic acid can be obtained pure without difficulty, and dif-
fers in this respect from most of the related acids which are only stable in
the form of their salts. Hydroferrocyanic acid is easily soluble in water
and alcohol; but it can be readily precipitated as an addition product with
ether, and the ether can be removed from the compound by heating it in a
current of hydrogen at 80° to 90°.
Treat a solution containing 42 g. of potassium ferrocyanide
(0.1 mol.) in 350 c.c. of water with 120 c.c. of concentrated hydro-
chloric acid; if any precipitate of potassium chloride separates,
redissolve it by adding a little more water. Cool and add about
50 c.c. of ether. After standing for several hours, colorless, glisten-
ing, microscopic crystals separate which should be drawn off and
washed with dilute hydrochloric acid containing a little ether. To
remove any admixed potassium chloride, dissolve the product in
50 c.c. of alcohol, filter, precipitate again with 50 c.c. of ether,