Sodium Chloride can be determined in 100 c.c. of the glycerine by adding a little water, neutralised with sodium carbonate, and then titrated with a deci-normal solution of silver nitrate, using potassium chromate as indicator.

Organic Impurities of various kinds occur in crude glycerine, and are mostly objectionable.  Their sum may be determined with fair accuracy by Sulman and Berry’s method:  50 grms. of the sample are diluted with twice its measure of water, carefully neutralised with acetic acid, and warmed to expel carbonic acid; when cold, a solution of basic lead acetate is added in slight but distinct excess, and the mixture well agitated.  The formation of an abundant precipitate, which rapidly subsides, is an indication of considerable impurity in the sample.  To ascertain its amount, the precipitate is first washed by decantation, and then collected on a tared, or preferably a double counter-poised filter, where it is further washed, dried at 100 deg. to 105 deg.  C., and weighed.  The precipitate and filter paper are then ignited separately in porcelain, at a low red heat, the residues moistened with a few drops of nitric acid and reignited; the weight of the lead oxide deducted from that of the original precipitate gives the weight of the organic matter precipitated by the lead.  Raw glycerines contain from 0.5 to 1.0 per cent.

Albuminous Matters. An approximate determination of the albuminous matters may be made by precipitating with basic lead acetate as already described, and determining the nitrogen by the Kjeldahl method; the nitrogen multiplied by 6.25 gives the amount of albuminous matter in the precipitate.

The Determination of Glycerine. The acetin method of Benedikt and Canton depends upon the conversion of glycerine into triacetin, and the saponification of the latter, and reduces the estimation of glycerine to an acidmetric method.  About 1.5 grm. of crude glycerine is heated to boiling with 7 grms. of acetic anhydride, and 3 to 4 grms. of anhydrous sodium acetate, under an upright condenser for one and a half hours.  After cooling, 50 c.c. of water are added, and the mixture heated until all the triacetin has dissolved.  The liquid is then filtered into a large flask, the residue on the filter is well washed with water, the filtrate quite cooled, phenolphthalein is added and the fluid exactly neutralised with a dilute (2 to 3 per cent.) solution of alkali.  Twenty-five c.c. of a 10 per cent. caustic soda solution, which must be accurately standardised upon normal acid, are then pipetted into the liquid, which is heated to boiling for ten minutes to saponify the triacetin, and the excess of alkali is then titrated back with normal acid.  One c.c. of normal acid corresponds to .03067 grm. of glycerine.

Precautions.—­The heating must be done with a reflux condenser, the triacetin being somewhat volatile.  The sodium acetate used must be quite anhydrous, or the conversion of the glycerine to triacetyl is imperfect.  Triacetin in contact with water gradually decomposes.  After acetylation is complete, therefore, the operations must be conducted as rapidly as possible.  It is necessary to neutralise the free acetic acid very cautiously, and with rapid agitation, so that the alkali may not be locally in excess.