~Resinous and Tarry matters~ are not unfrequently present. They are left insoluble on dissolving the sample in boiling water. The separation is more perfect if the hot solution be exactly neutralised by caustic soda.

~Sulphuric Acid, Hydrochloric Acid, and Oxalic Acid~, and their salts are detected by adding to the filtered aqueous solution of the sample solutions of the picrates of barium, silver, and calcium. These salts are readily made by boiling picric acid with the carbonates of the respective metals and filtering: other soluble salts of these methods may be subst.i.tuted for the picrates, but they are less satisfactory.

~Nitric Acid~ may be detected by the red fumes evolved on warming the sample with copper turnings.

~Inorganic Impurities and Picrates of Potash and Sodium~, &c., leave residues on cautious ignition.

~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 subst.i.tuted 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 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 pota.s.sium salt.

~Pota.s.sium 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 quant.i.ty of picric acid in the sample taken.

~a.n.a.lysis 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 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, _a.n.a.lyst_, xi., 12-34, and Allen's "Commercial Organic a.n.a.lysis," 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 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.

[Footnote A: Organic matter up to .6 per cent. is not always prejudicial to the nitrating quant.i.ties of a glycerine.]

~Silver Test.~ A portion of the sample of glycerine to be tested should be put in a small weighing bottle, and a quarter of its bulk of N/10 silver nitrate solution added to it, then shake it, and place in a dark cupboard for fifteen minutes. It must be p.r.o.nounced bad if it becomes black or dark brown within that time (acrolein, formic, and butyric acids).

The German official test for glycerine for pharmaceutical purposes is much more stringent, 1 c.c. of glycerine heated to boiling with 1 c.c. of ammonia solution and three drops of silver nitrate solution must give neither colour or precipitate within five minutes.

~Nitration.~ Fifty grms. of the glycerine are poured from a beaker into a mixture of concentrated nitric acid (specific gravity 1.53) and sulphuric acid (1.84), mixed in the proportions of 3 HNO_{3} to 5 H_{2}SO_{4} (about 400 c.c. of mixed acids). The mixed acids should be put into a rather large beaker, and held in the right hand in a basin of water, and the glycerine slowly poured into them from a smaller one held in the left. A constant rotatory motion should be given to the beaker in which the nitration is performed. When all the glycerine has been added, and the mixture has been shaken for a few minutes longer, it is poured into a separator, and allowed to stand for some time. It should, if the glycerine is a good one, have separated from the mixed acids in ten minutes, and the line of demarcation between the nitro-glycerine and the acid should be clear and sharp, neither should there be any white flocculent matter suspended in the liquid. The excess of acids is now drawn off, and the nitro-glycerine shaken once or twice with a warm solution of carbonate of soda, and afterwards with water alone. The nitro-glycerine is then drawn off into a weighed beaker, the surface dried with a piece of filter paper, and weighed; 100 parts of a good glycerine should yield about 230 of nitro-glycerine. A quicker method is to take only 10 c.c. of the glycerine, of which the specific gravity is already known, nitrate as before, and pour into a burette, read off the volume of nitro-glycerine in c.c. and multiply them by 1.6 (the specific gravity of nitro-glycerine), thus: 10 grms. gave 14.5 c.c. nitro-glycerine, and 14.5 x 1.6 = 23.2 grms., therefore 100 would give 232 grms. nitro-glycerine. The points to be noted in the nitration of a sample of glycerine are: the separation should be sharp, and within half an hour or less, and there should be no white flocculent matter formed, especially when the carbonate of soda solution is added.

~Total Acid Equivalent.~ Mr G.E. Barton (_Jour. Amer. Chem. Soc._, 1895) proposes to determine thus: 100 c.c. of glycerine are diluted to 300 c.c.

in a beaker, a few drops of a 1 per cent. solution of phenolphthalein and 10 c.c. of normal caustic soda solution are added; after boiling, the liquid is t.i.trated with normal hydrochloric acid (fatty acids are thus indicated and roughly determined).

~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 t.i.trated 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 quant.i.ty 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 t.i.tration 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 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 t.i.trated, and the amount used calculated into oleic acid. From this total amount of fatty acids the free fatty acid is deducted, and the quant.i.ty of combined fatty acids thus obtained.

~Impurities.~ The following impurities may be found in bad samples of glycerine:--Lead, a.r.s.enic, 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 a.r.s.enic 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 quant.i.ty, will come down upon diluting the sample with water, but smaller quant.i.ties may be detected by pa.s.sing 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.

~Sodium Chloride~ can be determined in 100 c.c. of the glycerine by adding a little water, neutralised with sodium carbonate, and then t.i.trated with a deci-normal solution of silver nitrate, using pota.s.sium 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 to 105 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.

~Alb.u.minous Matters.~ An approximate determination of the alb.u.minous 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 alb.u.minous 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 t.i.trated 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.

~The Lead Oxide Method.~--Two grms. of sample are mixed with about 40 grms. of pure litharge, and heated in an air bath to 130 C. until the weight becomes constant, care being taken that the litharge is free from such lead compounds and other substances as might injuriously affect the results, and that the heating of the mixture takes place in an air bath free from carbonic acid. The increase in weight in the litharge, minus the weight of substance not volatilisable from 2 grms. of glycerine at 160 C., multiplied by the factor 1.243, is taken as the weight of glycerine in the 2 grms. of sample. The glycerine must be fairly pure, and free from resinous substances and SO_{3}, to give good results by this process.

~a.n.a.lysis of the "Waste Acids" from the Manufacture of Nitro-Glycerine or Gun-Cotton.~ Determine the specific gravity by the specific gravity bottle or hydrometer, and the oxides of nitrogen by the permanganate method described under nitro-glycerine. Now determine the total acidity of the mixture by means of a tenth normal solution of sodium hydrate, and calculate it as nitric acid (HNO_{3}), then determine the nitric acid by means of Lunge nitrometer, and subtract percentage found from total acidity, and calculate the difference into sulphuric acid, thus:--

Total acidity equals 97.46 per cent.--11.07 per cent. HNO_{3} = 86.39 per cent., then (86.39 x 49)/63 = 67.20 per cent. H_{2}SO_{4}.

Then a.n.a.lysis of sample will be:--

_ Sulphuric acid = 67.20 per cent. | Nitric acid = 11.07 " |- Specific gravity = 1.7075.

Water = 12.73 " _|

This method is accurate enough for general use in the nitric acid factory.

The acid mixture may be taken by volume for determining nitric oxide in nitrometer. Two c.c. is a convenient quant.i.ty in the above case, then 2 x 1.7075 (specific gravity) = 3.414 grms. taken, gave 145 c.c. NO (barometer = 748 mm, and temperature = 15C.) equals 134.9 c.c. (corr.) and as 1 c.c.

NO = .0282 grm. HNO_{3} 135 x .0282 = .378 grm. = 11.07 per cent. nitric acid.

~Sodium Nitrate.~ Determine moisture and chlorine by the usual methods, and the total, NaNO_{3}, by means of nitrometer--0.45 grm. is a very convenient quant.i.ty to work on (gives about 123 c.c. gas); grind very fine, and dissolve in a very little hot water in the cup of the nitrometer; use about 15 c.c. concentrated H_{2}SO_{4}. One cubic cent. of NO equals .003805 grm. of NaNO_{3}. The insoluble matter, both organic and inorganic, should also be determined, also sulphate of soda and lime tested for.

~a.n.a.lysis of Mercury Fulminate (Divers and Kawakita's Method).~--A weighed quant.i.ty of mercury fulminate is added to excess, but measured quant.i.ty of fuming hydrochloric acid contained in a retort connected with a receiver holding water. After heating for some time, the contents of the retort and receiver are mixed and diluted, and the mercury is precipitated by hydrogen sulphide. By warming and exposure to the air in open vessels the hydrogen sulphide is for the most part dissipated. The solution is then t.i.trated with pota.s.sium hydroxide (KOH), as well as another quant.i.ty of hydrochloric acid, equal to that used with the fulminate. As the mercury chloride is reconverted into hydrochloric acid by the hydrogen sulphide, and as the hydroxylamine does not neutralise to litmus the hydrochloric acid combined with it, there is an equal amount of hydrochloric acid free or available in the two solutions. Any excess of acid in the one which has received the fulminate will therefore be due to the formic acid generated from the fulminate. Dr. Divers and M. Kawakita, working by this method, have obtained 31.31 per cent. formic acid, instead of 32.40 required by theory. (_Jour. Chem. Soc._, p. 17, 1884.)

Divers and Kawakita proceed thus: 2.351 grms. dissolved, as already described, in HCl, and afterwards diluted, gave mercury sulphide equal to 70.40 per cent. mercury. The same solution, after removal of mercury, t.i.trated by iodine for hydroxylamine, gave nitrogen equal to 9.85 per cent., and when evaporated with hydroxyl ammonium chloride equal to 9.55 per cent. A solution of 2.6665 grms. fulminate in HCl of known amount, after removal of mercury by hydrogen sulphide, gave by t.i.tration with pota.s.sium hydrate, formic acid equal to 8.17 per cent. of carbon.

Collecting and comparing with calculation from formula we get--

Calc. I. II. III.

Mercury 70.42 70.40 ... ...

Nitrogen 9.86 9.85 9.55 ...

Carbon 8.45 ... ... 8.17 Oxygen 11.27 ... ... ...

_______

100.00

~The a.n.a.lysis of Cap Composition.~--Messrs F.W. Jones and F.A. Willc.o.x (_Chem. News_, Dec. 11, 1896) have proposed the following process for the a.n.a.lysis of this substance:--Cap composition usually consists of the ingredients--pota.s.sium chlorate, antimony sulphide, and mercury fulminate, and to estimate these substances in the presence of each other by ordinary a.n.a.lytical methods is a difficult process. Since the separation of antimony sulphide and mercury fulminate in the presence of pota.s.sium chlorate necessitates the treatment of the mixture with hydrochloric acid, and this produces an evolution of hydrogen sulphide from the sulphide, and a consequent precipitation of sulphur; and pota.s.sium chlorate cannot be separated from the other ingredients by treatment with water, owing to the appreciable solubility of mercury fulminate in cold water.

In the course of some experiments on the solubility of mercury fulminate Messrs Jones and Willc.o.x observed that this body was readily soluble in acetone and other ethereal solvents when they were saturated with ammonia gas, and that chlorate of potash and sulphide of antimony were insoluble in pure acetone saturated with ammonia; these observations at once afforded a simple method of separating the three ingredients of cap composition. By employing this solution of acetone and ammonia an a.n.a.lysis can be made in a comparatively short time, and yields results of sufficient accuracy for all technical purposes. The following are the details of the process:--

A tared filter paper is placed in a funnel to the neck of which has been fitted a piece of rubber tubing provided with a clip. The paper is moistened with a solution of acetone and ammonia, the cap composition is weighed off directly on to the filter paper and is then covered with the solution of acetone and ammonia and allowed to stand thirty-four hours. It is then washed repeatedly with the same solution until the washings give no coloration with ammonium sulphide, and afterwards washed with acetone until washings give no residue on evaporation dried and weighed. The paper is again put in the funnel and washed with water until free from pota.s.sium chlorate, dried and weighed.