ubstituted for ether. Sugar may be
separated from the other impurities by treating the residue insoluble in
ether or benzene with rectified spirit, in which sugar and boric acid
alone will dissolve. If boric acid be present, the alcoholic solution will
burn with a green flame. Mono- and di-nitrophenic acids lower the melting
point (122 deg. C). Their calcium salts are less soluble than the picrate, and
may be approximately separated from it by fractional crystallisation, or
by precipitating the hot saturated solution of the sample with excess of
lime water. Picric acid may be determined by extracting the acidulated
aqueous solution by agitation with ether or benzene, and subsequently
removing and evaporating off the solvent. It may also be precipitated as
the potassium salt.

~Potassium Picrate~ [KC_{6}H_{2}(NO_{2})_{3}O]. When a strong solution of
picric acid is neutralised by carbonate of potash, this salt is thrown
down in yellow crystalline needles, which require 260 parts of cold or 14
parts of hot water for their solution. In alcohol it is much less soluble.

~Ammonium Picrate~ is more soluble in water than the above, and sodium
picrate is readily soluble in water, but nearly insoluble in solution of
sodium carbonate.

~Picrates of the Alkaloids.~--Picric acid forms insoluble salts with many
of the alkaloids, and picric acid may be determined in the following
manner:--To the solution of picric acid, or a picrate, add a solution of
sulphate of cinchonine acidulated with H_{2}SO_{4}. The precipitated
picrate of cinchonine [C_{20}H_{24}N_{2}O(C_{6}H_{2}N_{3}O_{7})_{2}] is
washed with cold water, rinsed off the filter into a porcelain crucible or
dish, the water evaporated on the water bath, and the residual salt
weighed. Its weight, multiplied by .6123, gives the quantity of picric
acid in the sample taken.

~Analysis of Glycerine.~[A] Glycerine that is to be used for the
manufacture of nitro-glycerine should have a minimum specific gravity of
1.261 at 15 deg. C. This can be determined, either by the aid of a Sartorius
specific gravity balance, or by using an ordinary specific gravity bottle.
One of 10 or 25 c.c. capacity is very convenient.

[Footnote A: See also Sulman and Berry, _Analyst_, xi., 12-34, and Allen's
"Commercial Organic Analysis," vol. ii., part i.]

~Residue~[A] left upon evaporation should not be more than 0.25 per cent.
To determine this, take 25 grms. of the glycerine, and evaporate it at a