Deduct the Na_{2}O found in (1). The difference is the percentage of Na_{2}O existing as carbonate.

4. _Alkali Combined with Organic Acids._--The sum of the percentages of Na_{2}O found at (1) and (3) deducted from the percentage found at (2) is a measure of the Na_{2}O or other alkali combined with organic acids.

5. _Determination of Acidity._--Take 10 grams of the sample, dilute with 50 cc. distilled water free from carbon dioxide, and t.i.trate with _N_/1 NaOH and phenolphthalein. Express in terms of Na_{2}O required to neutralize 100 grams.

6. _Determination of Total Residue at 160 C._--For this determination the crude glycerine should be slightly alkaline with Na_{2}CO_{3} not exceeding 0.2 per cent. Na_{2}O, in order to prevent loss of organic acids. To avoid the formation of polyglycerols this alkalinity must not be exceeded.

Ten grams of the sample are put into a 100 cc. flask, diluted with water and the calculated quant.i.ty of _N_/1 HCl or Na_{2}CO_{3} added to give the required degree of alkalinity. The flask is filled to 100 cc., the contents mixed, and 10 cc. measured into a weighed Petrie or similar dish 2.5 in. in diameter and 0.5 in. deep, which should have a flat bottom. In the case of crude glycerine abnormally high in organic residue a smaller amount should be taken, so that the weight of the organic residue does not materially exceed 30 to 40 milligrams.

The dish is placed on a water bath (the top of the 160 oven acts equally well) until most of the water has evaporated. From this point the evaporation is effected in the oven. Satisfactory results are obtained in an oven[20] measuring 12 ins. cube, having an iron plate 0.75 in. thick lying on the bottom to distribute the heat. Strips of asbestos millboard are placed on a shelf half way up the oven. On these strips the dish containing the glycerine is placed.

If the temperature of the oven has been adjusted to 160 C. with the door closed, a temperature of 130 to 140 can be readily maintained with the door partially open, and the glycerine, or most of it, should be evaporated off at this temperature. When only a slight vapor is seen to come off, the dish is removed and allowed to cool.

An addition of 0.5 to 1.0 cc. of water is made, and by a rotary motion the residue brought wholly or nearly into solution. The dish is then allowed to remain on a water bath or top of the oven until the excess water has evaporated and the residue is in such a condition that on returning to the oven at 160 C. it will not spurt. The time taken up to this point cannot be given definitely, nor is it important. Usually two or three hours are required. From this point, however, the schedule of time must be strictly adhered to. The dish is allowed to remain in the oven, the temperature of which is carefully maintained at 160 C. for one hour, when it is removed, cooled, the residue treated with water, and the water evaporated as before. The residue is then subjected to a second baking of one hour, after which the dish is allowed to cool in a desiccator over sulphuric acid and weighed. The treatment with water, etc., is repeated until a constant loss of 1 to 1.5 mg. per hour is obtained.

In the case of acid glycerine a correction must be made for the alkali added 1 cc. _N_/1 alkali represents an addition of 0.03 gram. In the case of alkaline crudes a correction should be made for the acid added.

Deduct the increase in weight due to the conversion of the NaOH and Na_{2}CO_{3} to NaCl. The corrected weight multiplied by 100 gives the percentage of _total residue at 160 C._

This residue is taken for the determination of the non-volatile acetylizable impurities (see acetin method).

7. _Organic residue._--Subtract the ash from the total residue at 160 C. Report as organic residue at 160 C. (it should be noted that alkaline salts of fatty acids are converted to carbonates on ignition and that the CO_{3} thus derived is not included in the organic residue).

ACETIN PROCESS FOR THE DETERMINATION OF GLYCEROL.

This process is the one agreed upon at a conference of delegates from the British, French, German and American committees, and has been confirmed by each of the above committees as giving results nearer to the truth than the bichromate method on crudes in general. It is the process to be used (if applicable) whenever only one method is employed.

On pure glycerines the results are identical with those obtained by the bichromate process. For the application of this method the crude glycerine should not contain over 60 per cent. water.

REAGENTS REQUIRED.

(_A_) _Best Acetic Anhydride._--This should be carefully selected. A good sample must not require more than 0.1 cc. normal NaOH for saponification of the impurities when a blank is run on 7.5 cc. Only a slight color should develop during digestion of the blank.

The anhydride may be tested for strength by the following method: Into a weighed stoppered vessel, containing 10 to 20 cc. of water, run about 2 cc. of the anhydride, replace the stopper and weigh. Let stand with occasional shaking, for several hours, to permit the hydrolysis of all the anhydride; then dilute to about 200 cc., add phenolphthalein and t.i.trate with _N_/1 NaOH. This gives the total acidity due to free acetic acid and acid formed from the anhydride. It is worthy of note that in the presence of much free anhydride a compound is formed with phenolphthalein, soluble in alkali and acetic acid, but insoluble in neutral solutions. If a turbidity is noticed toward the end of the neutralization it is an indication that the anhydride is incompletely hydrolyzed and inasmuch as the indicator is withdrawn from the solution, results may be incorrect.

Into a stoppered weighing bottle containing a known weight of recently distilled aniline (from 10 to 20 cc.) measure about 2 cc. of the sample, stopper, mix, cool and weigh. Wash the contents into about 200 cc. of cold water, and t.i.trate the acidity as before. This yields the acidity due to the original, preformed, acetic acid plus one-half the acid due to anhydride (the other half having formed acetanilide); subtract the second result from the first (both calculated to 100 grams) and double the result, obtaining the cc. _N_/1 NaOH per 100 grams of the sample. 1 cc. _N_/NaOH equals 0.0510 anhydride.

(_B_) _Pure Fused Sodium Acetate._--The purchased salt is again completely fused in a platinum, silica or nickel dish, avoiding charring, powdered quickly and kept in a stoppered bottle or desiccator.

It is most important that the sodium acetate be anhydrous.

(_C_) _A Solution of Caustic Soda for Neutralizing, of about N_/1 _Strength, Free from Carbonate._--This can be readily made by dissolving pure sodium hydroxide in its own weight of water (preferably water free from carbon dioxide) and allowing to settle until clear, or filtering through an asbestos or paper filter. The clear solution is diluted with water free from carbon dioxide to the strength required.

(_D_) _N_/1 _Caustic Soda Free from Carbonate._--Prepared as above and carefully standardized. Some caustic soda solutions show a marked diminution in strength after being boiled; such solutions should be rejected.

(_E_) _N_/1 _Acid._--Carefully standardized.

(_F_) _Phenolphthalein Solution._--0.5 per cent. phenolphthalein in alcohol and neutralized.

THE METHOD.

In a narrow-mouthed flask (preferably round-bottomed), capacity about 120 cc., which has been thoroughly cleaned and dried, weigh accurately and as rapidly as possible 1.25 to 1.5 grams of the glycerine. A Grethan or Lunge pipette will be found convenient. Add about 3 grams of the anhydrous sodium acetate, then 7.5 cc. of the acetic anhydride, and connect the flask with an upright Liebig condenser. For convenience the inner tube of this condenser should not be over 50 cm. long and 9 to 10 mm. inside diameter. The flask is connected to the condenser by either a ground gla.s.s joint (preferably) or a rubber stopper. If a rubber stopper is used it should have had a preliminary treatment with hot acetic anhydride vapor.

Heat the contents and keep just boiling for one hour, taking precautions to prevent the salts drying on the sides of the flask.

Allow the flask to cool somewhat, and through the condenser tube add 50 cc. of distilled water free from carbon dioxide at a temperature of about 80 C., taking care that the flask is not loosened from the condenser. The object of cooling is to avoid any sudden rush of vapors from the flask on adding water, and to avoid breaking the flask. Time is saved by adding the water before the contents of the flask solidify, but the contents may be allowed to solidify and the test proceeded with the next day without detriment, bearing in mind that the anhydride in excess is much more effectively hydrolyzed in hot than in cold water. The contents of the flask may be warmed to, but must not exceed, 80 C., until the solution is complete, except a few dark flocks representing organic impurities in the crude. By giving the flask a rotary motion, solution is more quickly effected.

Cool the flask and contents without loosening from the condenser. When quite cold wash down the inside of the condenser tube, detach the flask, wash off the stopper or ground gla.s.s connection into the flask, and filter the contents through an acid-washed filter into a Jena gla.s.s flask of about 1 litre capacity. Wash thoroughly with cold distilled water free from carbon dioxide. Add 2 cc. of phenolphthalein solution (_F_), then run in caustic soda solution (_C_) or (_D_) until a faint pinkish yellow color appears throughout the solution. This neutralization must be done most carefully; the alkali should be run down the sides of the flask, the contents of which are kept rapidly swirling with occasional agitation or change of motion until the solution is nearly neutralized, as indicated by the slower disappearance of the color developed locally by the alkali running into the mixture.

When this point is reached the sides of the flask are washed down with carbon dioxide-free water and the alkali subsequently added drop by drop, mixing after each drop until the desired tint is obtained.

Now run in from a burette 50 cc. or a calculated excess of _N_/1 NaOH (_D_) and note carefully the exact amount. Boil gently for 15 minutes, the flask being fitted with a gla.s.s tube acting as a partial condenser.

Cool as quickly as possible and t.i.trate the excess of NaOH with _N_/1 acid (_E_) until the pinkish yellow or chosen end-point color just remains.[21] A further addition of the indicator at this point will cause an increase of the pink color; this must be neglected, and the first end-point taken.

From the _N_/1 NaOH consumed calculate the percentage of glycerol (including acetylizable impurities) after making the correction for the blank test described below.

1 cc. _N_/1 NaOH = 0.03069 gram glycerol.

The coefficient of expansion for normal solutions is 0.00033 per cc.

for each degree centigrade. A correction should be made on this account if necessary.

_Blank Test._--As the acetic anhydride and sodium acetate may contain impurities which affect the result, it is necessary to make a blank test, using the same quant.i.ties of acetic anhydride, sodium acetate and water as in the a.n.a.lysis. It is not necessary to filter the solution of the melt in this case, but sufficient time must be allowed for the hydrolysis of the anhydride before proceeding with the neutralization.

After neutralization it is not necessary to add more than 10 cc. of the _N_/1 alkali (_D_), as this represents the excess usually present after the saponification of the average soap lye crude. In determining the acid equivalent of the _N_/1 NaOH, however, the entire amount taken in the a.n.a.lysis, 50 cc., should be t.i.trated after dilution with 300 cc.

water free from carbon dioxide and without boiling.

_Determination of the Glycerol Value of the Acetylizable Impurities._--The total residue at 160 C. is dissolved in 1 or 2 cc. of water, washed into the acetylizing flask and evaporated to dryness. Then add anhydrous sodium acetate and acetic anhydride in the usual amounts and proceed as described in the regular a.n.a.lysis. After correcting for the blank, calculate the result to glycerol.

WAYS OF CALCULATING ACTUAL GLYCEROL CONTENT.

(1) Determine the apparent percentage of glycerol in the sample by the acetin process as described. The result will include acetylizable impurities if any are present.

(2) Determine the total residue at 160 C.

(3) Determine the acetin value of the residue at (2) in terms of glycerol.

(4) Deduct the result found at (3) from the percentage obtained at (1) and report this corrected figure as glycerol. If volatile acetylizable impurities are present these are included in this figure.

Trimethyleneglycol is more volatile than glycerine and can therefore be concentrated by fractional distillation. An approximation to the quant.i.ty can be obtained from the spread between the acetin and bichromate results on such distillates. The spread multiplied by 1.736 will give the glycol.

BICHROMATE PROCESS FOR GLYCEROL DETERMINATION. REAGENTS REQUIRED.

(_A_) _Pure pota.s.sium bichromate_ powdered and dried in air free from dust or organic vapors, at 110 to 120 C. This is taken as the standard.

(_B_) _Dilute Bichromate Solution._--7.4564 grams of the above bichromate are dissolved in distilled water and the solution made up to one liter at 15.5 C.

(_C_) _Ferrous Ammonium Sulphate._--It is never safe to a.s.sume this salt to be constant in composition and it must be standardized against the bichromate as follows: dissolve 3.7282 grams of bichromate (_A_) in 50 cc. of water. Add 50 cc. of 50 per cent. sulphuric acid (by volume), and to the cold undiluted solution add from a weighing bottle a moderate excess of the ferrous ammonium sulphate, and t.i.trate back with the dilute bichromate (_B_). Calculate the value of the ferrous salt in terms of bichromate.

(_D_) _Silver Carbonate._--This is prepared as required for each test from 140 cc. of 0.5 per cent. silver sulphate solution by precipitation, with about 4.9 cc. _N_/1 sodium carbonate solution (a little less than the calculated quant.i.ty of _N_/1 sodium carbonate should be used as an excess to prevent rapid settling). Settle, decant and wash one by decantation.