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

ELECTROLYTIC PREPARATION. 125

Heat again, at first moderately then more strongly, and stir

vigorously all the while until the mass has become dry. Finally
heat for 5 minutes at a dull red heat. Unless stirred as directed,

the melt will solidify to a hard cake which can be removed from

the crucible only with difficulty. When it is cold, break up the

mass and boil it with 1.5 liters of water while conducting a vigorous
stream of carbon dioxide into the liquid. When the manganate

is completely decomposed and a drop of the solution gives a clear,

violet-red spot on filter paper, with no trace of green, allow the

precipitate to settle, and decant the liquid as carefully as possible
from the sludge of manganese dioxide. Filter with suction

through a felt of asbestos on a Biichner funnel. Concentrate the

filtrate to one-half, filter again through asbestos, and evaporate

until crystallization begins. Collect the crystals on a porcelain

filter-plate and wash with a little cold water. Obtain a second

crop of crystals from the mother-liquor and examine with a micro-

scope to see if it is free from crystals of potassium chloride.

Finally recrystallize. The yield should be 50 to 60 g. of the pure

salt. The yield can be improved somewhat by using chlorine
instead of carbon dioxide.

84. Electrolytic Preparation of Potassium Permanganate.
    If a solution of an alkali hydroxide, or better of an alkali carbonate, is
    clectrolyzed with an anode of manganese, permanganate is formed by anodic
    oxidation. For the reaction to succeed it must be carried out at a rather
    high temperature, since otherwise only manganate is formed. For the cathode,
    nickel or iron wires may be used, but they must be surrounded with a porous
    clay diaphragm to avoid reduction of the permanganate.

Place a porous cup, 10 cm. high and 4 cm. wide, in a liter beaker,
and inside the cup place an iron wire to serve as the cathode.

For the anode use a good sized lump of manganese (obtained by

the Goldschmidt process; cf. No. 2); fasten it by means of a

fine platinum wire to a stout iron wire and allow only the man-
ganese to dip into the liquid. Use as the electrolyte 200 c.c. of

a solution of potassium carbonate saturated at 0° (solubility:

105 g. of K 2 CO 3 in 100 g. of water) which should stand at the same

level inside and outside the porous cell. In order to effect a rapid
electrolysis it will probably be necessary, on account of the high

resistance of the diaphragm, to draw from a current supply of

more than 6 volts. Raise the temperature of the bath to 50-60°

and maintain it at this point by using an external flame whenever