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

BARIUM DITHIONATE. 131

ammonia. Dissolve 28 g. of potassium sulphate (15% less than

the calculated amount) in the combined filtrates, and bring the

double salt to crystallization in the usual way. Recrystallize the

combined crops of crystals. Yield, about 150 g.

90. Sodium Thiosulphate, Na 2 S 2 O 3 • 5 H 2 O.

Add 16 g. of finely powdered roll sulphur (not flowers of sulphur)

to a solution of 126 g. crystallized sodium sulphite, Na 2 SO 3 -7 H 2 O,

in 250 c.c. of water, and boil the mixture in a flask until, at the

end of about two hours, the sulphur has dissolved. Filter, evapo-

rate to the point of crystallization, and concentrate the mother-
liquor to obtain further crops of crystals. The yield is nearly

quantitative.

The above reaction, Na 2 SO 3 + S <=» Na 2 S 2 O 3 , is reversible.

Heat 2-3 g. of sodium thiosulphate with about 5 g. of copper

powder in a test-tube so that the thiosulphate melts in its water

of crystallization and the latter partly boils away; the copper

becomes black (copper sulphide), and the aqueous extract when

treated with calcium chloride gives a precipitate of calcium

sulphite; calcium thiosulphate is easily soluble.

91. Barium Dithionate, BaS 2 O 0 • 2 H 2 O.

Prepare sulphur dioxide by allowing sulphuric acid to drop into

160 c.c. of commercial bisulphite liquor (see note on p. 71), and

pass the gas into a suspension of 50 g. of finely powdered pyrolu-

site in 250 c.c. of water until the latter is saturated and nearly

all of the black manganese dioxide has dissolved. During this
reaction, .which lasts about two hours, keep the mixture cooled

with water.

Dilute to 1.5 liters, bring the solution to boiling, and keep it at

this temperature while adding a solution of 200 g. of crystallized

barium hydroxide. Filter, and test the filtrate with ammonium
sulphide for manganese. If the manganese is shown to be all pre-

cipitated, remove the excess of barium hydroxide from the filt-

rate by passing carbon dioxide into the boiling solution. Filter,

evaporate to one-half, filter again, and allow the salt to crystallize.
Drain the crystals and evaporate the mother-liquor to obtain

another crop. Yield, about 120 g.