182 M. C. Lea on Picric Acid. 
Strength of aqueous so-/Ammonio sulphate of|Potash liver of sulphur} Cyanid of potassium. 
lution of picric acid. copper. (heat. ) 
sola Immediate precip-/Solution becomes Pale sherry red, 
itate. sherry wine red.| with heat becom- 
ing deep red. 
go0o0 No precipitate at|Deep yellow with/Deep yellow, tinge 
first,but by stand-| a tinge of sherry| of sherry color, 
ing a few minutes; color. deepened by heat- 
a distinct one. ing. 
sos Distinct precip. by 
standing. 
rotor No precipitate. /The yellow color was slightly deep- 
ened, the cyanid test is the more 
delicate of the two. 
Purification of Picric Acid. 
becomes to the last degree tedious and troublesome. As the 
picrate of lime is very soluble, it seemed probable that it might 
afford a convenient means of solution; it has indeed been already 
recommended for that purpose.t But I find it wholly inadmis- 
sible basic salt is formed which falls to the bottom with the 
excess of hydrate of lime, and great waste ensues. The insolu- 
bility of alkaline picrates in cold alkaline solutions which I have 
escribed in a previous number of this Journal, furnished me — 
with an excellent process. The crude acid is saturated with car-. 
bonate of soda, an excess of which is to be avoided as it tends to 
dissolve resinous matter. The hot solution is then easily filtered, 
and into the filtrate a few clear crystals of carbonate of soda are 
placed. On cooling, the picrate of soda crystallizes out almost 
as completely as the potash salt would have done, and all the 
wearisome delay in filtration is avoided. From the mother water 
more picric acid may be recovered by the addition of a little car- 
bonate of potash. In decomposing alkaline picrates to separate 
the acid sulphuric, (and not as usually recommended, chlorhydrie) 
acid should be used, because a moderate excess of sulphuric acid 
throws down a great portion of the acid which would otherwise 
* This Journal, Nov., 1858. ¢ See Gmelin, Eng. Ed., vol. xi, p. 214. 
