SODIUM THIOSULPHATE 253going over into sulphate. If it is allowed to react with sulphur,
the latter enters into the compound in much the same way as
oxygen, and i/iiosulphate instead of sulphate is formed. The
sulphur so taken up certainly plays a different function from the
sulphur already contained in the compound, although it is perhaps
a question whether the thiosulphate is exactly the same compound
as sulphate, except that one oxygen atom is replaced by a sulphur.
Sodium sulphite is conveniently prepared by allowing sulphur
dioxide (sulphurous acid) to react with sodium carbonate. It is
practically impossible, however, to distinguish the exact point at
which the normal sulphite (Na 2 SO 3 ) is formed; therefore it is more
expedient to divide a given amount of sodium carbonate into two
equal parts, to fully saturate one part with sulphur dioxide, where-
by sodium bisulphite, NaHSO 3 , is formed, and to add the other
half of the sodium carbonate, thereby obtaining the normal
sulphite, Na 2 SO 3.
Materials: anhydrous sodium carbonate, Na2CO 3 , 106 grams
= 1 F.W.
sulphur dioxide; this gas is most conveniently
drawn from steel cylinders in which liquid
sulphur dioxide is held under pressure. It
can be prepared by the action of copper turn-
ings on 36 N H 2 SO 4.
sulphur (powdered roll sulphur), 48 grams.
Apparatus: two 500-cc. flasks with 2-hole rubber stoppers and
delivery tubes.
600-cc. beaker.
5-inch funnel.
8-inch porcelain dish.
4-inch crystallizing dish.
5-inch watch glass,
iron ring and ring stand.
Bunsen burner.
Procedure: Dissolve 53 grams of the sodium carbonate in 300 cc.
of hot water, and place about five-sixths of the solution in one
flask and the remainder in another flask. Connect these flasks
in series so that sulphur dioxide gas may be passed first into the
larger volume of solution, and what is there unabsorbed may pass
on through the second flask. Pass a vigorous stream of the gas