The estimation of actual glycerol in this is necessarily a matter of considerable importance, and a very large number of processes, which are constantly being added to, have been suggested for the purpose. Hitherto, however, only two methods have been generally adopted, viz. the acetin and the bichromate processes. Unfortunately the results obtained by these do not invariably agree, the latter, which includes all oxidisable matter as glycerol, giving sometimes considerably higher results, and it has been suggested that a determination should be made by both methods, and the average of the two results considered the true value. This involves a considerable amount of time and trouble, and it will generally be found sufficient in a works laboratory to determine the glycerol by one method only in the ordinary course, reserving the other process for use as a check in case of dispute or doubt.
Acetin Method.—This consists in converting the glycerol into its ester with acetic acid, the acetic triglyceride, or triacetin being formed. This is then saponified with a known volume of standard alkali, the excess of which is titrated with acid, and the percentage of glycerol calculated from the amount of alkali absorbed.
From 1 to 1.5 grammes of the glycerine is weighed into a conical flask of about 150 c.c. capacity, 7 or 8 c.c. of acetic anhydride added, together with about 3 grammes of anhydrous sodium acetate, and the whole boiled on a sand-bath under a reflux condenser for one to one and a half hours, after which it is allowed to cool, 50 c.c. water added, and the ester dissolved by shaking, and gently warming, the reflux condenser still being attached as the acetin is very volatile. The solution is then filtered from a white flocculent precipitate, which contains most of the impurities, into a larger conical flask, of some 500-600 c.c. capacity, and after cooling, rendered just neutral to phenol-phthalein by means of N/2 caustic soda solution, the exact point being reached when the solution acquires a reddish-yellow tint; 25 c.c. of a strong caustic soda solution is then added, and the liquid boiled for about fifteen minutes, the excess of alkali being titrated after cooling, with N/1 or N/2 hydrochloric acid. A blank experiment is carried out simultaneously, with another 25 c.c. of the soda solution, and the difference in the amounts of acid required by the two, furnishes a measure of the alkali required to saponify the acetin formed, and hence the amount of glycerol in the crude glycerine may be calculated.
Example.—1.4367 grammes crude glycerine, after treatment with acetic anhydride, and neutralising, was saponified with 25 c.c. of a 10 per cent. caustic soda solution.
| The blank experiment | required | 111.05 c.c. | N/1 | hydrochloric acid. |
| Flask containing acetin | " | 75.3 c.c. | " | " |
| —— | ||||
| 35.75 c.c. | " | " |
Hence, the acetin formed from the glycerol present in 1.4367 grammes of the crude glycerine required 35.75 c.c. N/1 caustic alkali for its saponification, so that the percentage of glycerol may be calculated from the following formula:—
| 35.75 × 0.03067 × 100 | ||||
| Per cent. glycerol | = | ——————— | = | 76.3. |
| 1.4367 |
Bichromate Method.—This process was originally devised by Hehner (Journ. Soc. Chem. Ind., 1889, 4-9), but the modification suggested by Richardson and Jaffe (ibid., 1898, 330) is preferred by the authors, and has been practised by them for several years with perfectly satisfactory results.
Twenty-five grammes of the crude glycerine are weighed out in a beaker, washed into a 250 c.c. stoppered flask, and made up to the graduation mark with water. Twenty-five c.c. of this solution are then measured from a burette into a small beaker, a slight excess of basic lead acetate solution added to precipitate organic matter, the precipitate allowed to settle, and the supernatant liquid poured through a filter paper into another 250 c.c. flask. The precipitate is washed by decantation until the flask is nearly full, then transferred to the filter, and allowed to drain, a few drops of dilute sulphuric acid being added to precipitate the slight excess of basic lead acetate solution, and the contents of the flask made up with water to 250 c.c. This solution is filtered, 20 c.c. measured from a burette into a conical flask of about 150 c.c. capacity, 25 c.c. of a standard potassium bichromate solution containing 74.86 grammes bichromate per litre added, together with 50 c.c. of 50 per cent. sulphuric acid, and the whole placed in a boiling water-bath for one hour, after which it is allowed to cool, diluted with water to 250 c.c., and this solution run in to 20 c.c. of a 3 per cent. ferrous ammonium sulphate solution until the latter is completely oxidised, as shown by no blue coloration being produced when one drop is brought into contact with one drop of a freshly prepared solution of potassium ferricyanide on a spot-plate. The ferrous ammonium sulphate solution is previously standardised by titration with a potassium bichromate solution of one-tenth the above strength, made by diluting 10 c.c. of the strong solution to 100 c.c. with water.
The reaction taking place in the oxidation may be represented by the equation:—