Neutrality. The same chemist determines the neutrality of glycerine thus:  50 c.c. of glycerine mixed with 100 c.c. of water and a few drops of alcoholic phenolphthalein[A] are titrated with hydrochloric acid or sodium hydroxide; not more than 0.3 c.c. normal hydrochloric acid or normal soda solution should be required to render the sample neutral; raw glycerines contain from .5 to 1.0 per cent. of sodium carbonate.

[Footnote A:  Sulman and Berry prefer litmus as indicator.]

Determination of Free Fatty Acids. A weighed quantity of the glycerine is shaken up with some neutral ether in a separating funnel, the glycerine allowed to settle, drawn off, and the ether washed with three separate lots of water.  The water must have been recently boiled, and be quite free from CO_{2}.  All the free fatty acid is now in the ether, and no other soluble acid.  A drop of phenolphthalein is now added, a little water, and the acidity determined by titration with deci-normal baryta solution, and the baryta solution taken calculated as oleic acid.

Combined Fatty Acid. About 30 grms. of the glycerine are placed in a flask, and to it is added about half a grm. of caustic soda in solution.  The mixture is heated for ten minutes at 150 deg.  C. After cooling some pure ether is added to it, and enough dilute H_{2}SO_{4} to render it distinctly acid.  It is well shaken.  All the fatty acids go into the ether.  The aqueous solution is then removed, and the ether well washed to remove all H_{2}SO_{4}.  After the addition of phenolphthalein the acid is titrated, and the amount used calculated into oleic acid.  From this total amount of fatty acids the free fatty acid is deducted, and the quantity of combined fatty acids thus obtained.

Impurities. The following impurities may be found in bad samples of glycerine:—­Lead, arsenic, lime, chlorine, sulphuric acid, thio-sulphates, sulphides, cyanogen compounds, organic acids (especially oleic acid and fatty acids[A]), rosin products, and other organic bodies.  It is also said to be adulterated with sugar and glucose dextrine.  Traces of sulphuric acid and arsenic may be allowed, also very small traces indeed of lime and chlorine.

[Footnote A:  These substances often cause trouble in nitrating, white flocculent matter being formed during the process of washing.]

The organic acids, formic and butyric acids may be detected by heating a sample of the glycerine in a test tube with alcohol and sulphuric acid, when, if present, compound ethers, such as ethylic formate and butyrate, the former smelling like peaches and the latter of pine-apple, will be formed.

Oleic Acid, if present in large quantity, will come down upon diluting the sample with water, but smaller quantities may be detected by passing a current of nitrogen peroxide, N_{2}O_{4} (obtained by heating lead nitrate), through the diluted sample, when a white flocculent precipitate of elaidic acid, which is less soluble than oleic acid, will be thrown down.  By agitating glycerol with chloroform, fatty acids, rosin oil, and some other impurities are dissolved, while certain others form a turbid layer between the chloroform and the supernatant liquid.  On separating the chloroform and evaporating it to dryness, a residue is obtained which may be further examined.