General Impurities and Adulterations may be detected and determined by shaking 1 grm. of the sample of acid in a graduated tube with 25 c.c. of ether, the pure acid dissolves, while any oxalic acid, nitrates, picrates, boric acid, alum, sugar, &c., will be left insoluble, and after removal of the ethereal liquid, may be readily identified and determined.  For the detection and determination of water and of oxalic acid, 50 c.c. of warm benzene may be advantageously substituted 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 temperature of about 160 deg.  C. in a platinum basin, and finish in an air bath.  Weigh until constant weight is obtained.  Afterwards incinerate over a bunsen burner, and weigh the ash.