POTASSIUM PERSULPHATE. 133
The older name for hyposulphurous acid is hydrosulphuroua acid. It
should not be confused with thiosulphuric acid, H 2 S 2 O 3 , the sodium salt of
which is commonly known as "hypo."
Take two samples of the same solution of sulphurous acid, allow
one of them to stand a short time in contact with a zinc rod, and
afterward test the reducing power of each solution towards diluteindigo.
- Potassium Persulphate, Electrolytically.
In concentrated solutions of acid sulphates, dissociation takes place for
the most part only partially:
KHSO 4 <=• K+ + HSO 4 --
On electrolyzing, and particularly with high-current densities, the acid sul-
phate ions on becoming discharged at the anode unite in pairs to form per-
Bulphuric acid, H 2 S 2 O 8 , the potassium salt of which is insoluble.
Inside a large beaker filled with ice water, place a glass cylinder,
or a small beaker 14 cm. high and 6 cm. in diameter. Suspend in
this, by means of a wire triangle, an 11 cm. long, 2.7 cm. wide
glass tube open at both ends (a test-tube with its bottom cut off).
Use for the cathode a loop Of platinum wire,
placed as near the surface of the solution as
possible and outside the inner tube; for the
anode melt a platinum wire into a glass tube
so that it projects 1.5 to 2.0 cm., and insert
this through the inner tube until it reaches
nearly to the bottom of the beaker (see Fig. 21).
Fill the inside beaker one-half full of a satu-
rated solution of acid potassium sulphate. Use
a current density at the anode of 100 amperes
per 100 sq. cm. of electrode surface. Measure
the protruding anode wire and estimate its
surface. The current will amount to less than
an ampere with a moderately stout platinum
wire. The temperature of the electrolyte should be lower than
15° and may quite easily be maintained at 6° to 8°. A few min-
utes after closing the circuit, crystals of the difficultly soluble
persulphate are seen to separate. Continue the electrolysis 40
minutes in one experiment and an hour in another. Collect the
salt on a hardened filter, wash it with alcohol and then with ether.
Weigh the salt, after drying in a desiccator, and analyze it as soon