132 OXY-ACIDS AND SALTS OF SULPHUR.
A sample of the salt dissolves clear in water and remains
unchanged upon the addition of a little nitric acid. By boiling
with nitric acid, however, it is oxidized to sulphate, and oxides of
nitrogen are evolved.
- Sodium Tetrathionate, Na 2 S 4 O 6 • 2 H 2 O.
Triturate 50 g. of sodium thiosulphate, 26 g. of iodine, and 5 g.
of water in a mortar to a bright brownish-yellow paste. After a
short time rinse the mass with 50 c.c. of alcohol into an Erlenmeyer
flask. At the end of about three hours drain the precipitated
sodium tetrathionate and wash it with alcohol until the washings
are free from iodine.
Dissolve the crude product in 20 to 25 c.c. of lukewarm water,
and by adding alcohol in portions of 10 c.c.—in all 50 c.c.—
bring about a separation of crystals. After about ten hours,
during which time the mixture has stood out of contact with the
air in an Erlenmeyer flask, or in a vacuum desiccator, drain the
crystals, wash with alcohol, and dry in a desiccator over sulphuric
acid. The yield is about 20 g. of compact, colorless, crystalline
aggregates.
Reactions: A solution prepared in the cold gives no precipitate
with copper sulphate even on boiling; with mercuric nitrate a
yellow precipitate is obtained which becomes black on boiling;
with mercuric chloride a yellow flocculent precipitate separates
slowly. Compare the behavior of sodium thiosulphate and
sodium sulphite with these reagents.
Analysis: Determine the water of crystallization in a 0.7-1.0 g.
sample by heating it to constant weight in the steam closet; deter-
mine the sulphur by dissolving in water, oxidizing with bromine,
and precipitating with barium chloride. The amount of water
contained in the salt prepared in this way is stated variously in
the literature.- Hyposulphurous Acid, H 2 S 2 O 4.
Zinc dissolves in aqueous sulphurous acid without evolution of hydrogen,
and forms a yellow solution of great reducing power. By studying the
properties of this solution and later by preparing the solid sodium salt, the
formula of free hyposulphurous acid was established as H 2 S 2 O 4.