190 HYPOSULPHITE AND TRITHIONATE OF POTASH. 
ART. XLV.— ON THE PREPARATION OP THE HYPOSUL- 
PHITE AND OF THE TRITHIONATE OF POTASH. 
BV M. PLES3Y. 
M. Langtlois, to whom the discovery of trithionic acid 
is due, obtains it in combination with potash in the follow- 
ing manner : — He prepares some bisulphite of potash by 
passing sulphurous acid in excess into a solution of pure 
carbonate of potash. The salt thus obtained is introduced 
into a vessel containing pure flowers of sulphur, and the 
whole digested on a sand-bath, the temperature of which 
should not cause it to boil. Sulphurous acid is disengaged, 
and some sulphate is produced ; the liquid assumes a yellow 
tint, which disappears at the end of two or three days, when 
the formation of the new compound is complete. The liquid, 
filtered whilst hot, becomes opake on cooling, and deposits 
some crystals, which are coated with a little sulphur. They 
are purified by dissolving them in a small quantity of warm 
water ; the filtered solution no longer becomes opake, and 
furnishes very beautiful prismatic crystals. 
When therefore sulphur is dissolved in the bisulphite, a 
liquid is obtained, which deposits a quantity of sulphur on 
boiling, and affords but a small quantity of the trithionate 
of potash and of an impure hyposulphite. The reaction 
does not take place in the manner first supposed ; and al- 
though sulphurous acid is disengaged under the circum- 
stances in which M. Langlois operated, we cannot have 
KO (S02) 2 + S=KOS 2 0*+ SO 2 - 
This evolution of sulphurous acid, observed by M. Lang- 
lois, does not always happen ; thus it does not take place at 
77° to 86°, and nevertheless the sulphur dissolves in the bi- 
sulphite. But if the bisulphate of potash is not adapted for 
the preparation of the hyposulphite, the neutral sulphite 
does perfectly well. 
