1902 Encyclopedia > Glycerin

Glycerin




GLYCERIN, GLYCERINE, or GLYCEROL (in pharmacy GLYCERINUM), a sugar-like substance obtainable from most natural fatty bodies by the action of alkalies and similar reagents, whereby the fats are decomposed, water being taken up, and glycerin being formed together with the alkaline salt of some particular acid (varying with the nature of the fat). Owing to their possession of this common property, these natural fatty bodies and various artificial derivatives of glycerin, which behave in the same way when treated with alkalies, are known as glycerides. This decomposition into an organic acid and a substance of more or less neutral character is a typical kind of reaction with numerous classes of organic bodies, and is termed saponi-fication, from the circumstance that the ordinary process of soap-making consists simply in the formation from natural fatty bodies and alkalies of the alkaline salts of the fatty acids thence derivable, soap being a mixture of these alkaline salts in various proportions, according to the particular pur-poses for which it is required, and its price. In the ordinary process of soap-making the complementary product, glycerin, remains dissolved in the aqueous liquors from which the soap is separated, and is usually thrown away; in many other instances, however, in which a substance capable of undergoing a reaction of saponification is thus treated, the product complementary to the alkaline salt is the more important of the two.

The fact that soap is obtainable by boiling together oily or fatty substances and alkalies (such as potashes and natron or mineral alkali) was known at an early period of history, being mentioned by Pliny, Galen, Aetius, and Paulus iEgineta. On the other hand, substances referred to in the Old Testament and translated " soap " {e.g., Jeremiah ii. 22, " For though thou wash thee with nitre [properly, natron], and take thee much soap;" Malachi iii. 2, " For he is like a refiner's fire, and like fuller's soap ") refer to the alkali itself (viz., borith = vegetable alkali, potash; or neter = mineral alkali, soda), and not to the substances prepared from oily bodies and these alkaline matters. Similarly Homer makes no mention of soap whilst describing the laundry operations of Nausicaa. The discovery of glycerin, however, is of modern origin, the body having been first described in 1783 by Scheele under the name of oelsiiss (principe doux des huiles—sweet principle of oils), and more fully investigated subsequently byChevreul,Berthelot, and many other chemists, from whose researches it results that glycerin is a trihydric alcohol indicated by the formula C3H5(OH)3, the natural fats and oils, and the glycerides generally, being substances of the nature of compound ethers formed from glycerin by the replacement of the hydrogen of the OH groups by the radicals of certain acids, called for that reason " fatty acids." 1 The relationship of these glycerides to glycerin will be more conveniently shown by an example, such as the series of bodies formed from glycerin by replacement of hydrogen by " stearyl" (C]gH350), the radical of stearic acid (C18H35O.OH):—

Glycerin. Monostearin. Distearin. Tristearin.
CH2 . OH CH2 . 0(C18H350) CH2 . 0(C18H350) CHj . 0(C18H3s0)
CH .OH CH . OH en . O(CUH35O) CH . O(C18H35O.\
CH2 . OH C'H2 . OH CH2 . OH CH2 . O(C18H350)

These four substances evidently form a progressive series, each number of which differs from the preceding one in the same way, viz., that one more stearyl group replaces hydrogen in the original OH groups.

The process of saponification may be viewed as the gradual progressive transformation of tristearin, or some analogously constituted substance, into distearin, monostearin, and glycerin, or as the similar transformation of a substance analogous to distearin or to monostearin into glycerin. If the reaction is brought about in presence of an alkali, the acid set free becomes transformed into the corresponding alkaline salt; but if the decomposition is effected without the presence of an alkali (i.e., by means of water alone, or by an acid fluid), the acid set free and the glycerin are ob-tained together in a form which usually admits of their ready separation. It is noticeable that with few exceptions the fatty and oily matters occurring in nature are sub-stances analogous to tristearin, i.e., they are trebly replaced glycerins. Amongst these glycerides may be mentioned the following:—

Tristearin—C3H6(O.C18H350)3. The chief constituent of hard animal fats, such as beef and mutton tallow, &c.; also con-tained in many vegetable fats in smaller quantity. Triolein—CjH^O.CjgHjjOlj. Largely present in olive oil and other saponifiable vegetable oils and soft fats; also present in animal fats, especially hog's lard. Tripalmitin—C3H5(O.C16H310)3. The chief constituent of palm oil; also contained in greater or less quantities in human fat, olive oil, and other animal and vegetable fata. Triricinolein—C3H6(0. ClaH3302)3. The main constituent of castor oil.

Other analogous glycerides are apparently contained in greater or smaller quantity in certain other oils. Thus in cows' butter, tributyrin, C3H6(O.C4H70)3, and the analogous glycerides of other readily volatile acids closely resembling butyric acid, are present in small quantity; the production of these acids on saponification and distillation with dilute sulphuric acid is utilized as a test of a purity of butter as sold. Triacetin, C3H6(O.C„H30)3, is appar-ently contained in cod-liver oil. Some other glycerides isolated from natural sources are analogous in composi-tion to tristearin, but with this difference, that the three radicals which replace hydrogen in glycerin are not all identical; thus kephalin, myelin, and lecithin are glycerides in which two hydrogens are replaced by fatty acid radicals, and the third by a complex phosphoric acid derivative. In no case, however, is the existence of a natural glyceride analogous to distearin or monostearin substantiated with certainty, bodies of these classes being either formed synthetically by reversing the reactions of saponification, or being produced by the partial saponification of substances analogous to tristearin.

Glycerin is also a product of certain kinds of fermentation, especially of the alcoholic fermentation of sugar; thus it is a constituent of many wines and other fermented liquors, being formed together with small quantities of various other substances by reactions subsidiary only to the main change taking place, and hence varying in their nature and extent with circumstances. According to Pasteur, about ^th of the sugar transformed under ordinary conditions in the fermentation of grape juice and similar saccharine liquids into alcohol and other products becomes converted into glycerin. In certain natural fatty substances, e.g., palm oil, it exists in the free state, so that it can be separated by washing with boiling water, which dissolves the glycerin but not the fatty glycerides; but how far its occurrence in this form is due to the breaking up of the glyceride by a spontaneous saponification is open to some question.





Properties.—In a state of purity glycerin is a viscid, colourless liquid of sp. gr. L264, possessing a somewhat mawkish sweet taste ; when exposed to a high degree of cold for a long time it sometimes solidifies to a crystalline mass, which then melts at about 7° C. The crystals when once melted often do not resolidify again readily, even when in contact with the solid substance, although sometimes contact with a crystal of the solid at a temperature of about 0° suffices to produce solidification of the whole. This solidification of glycerin is, however, a very exceptional phenomenon, only occurring with extremely pure sub-stance under certain conditions not thoroughly under stood, and then only after long continued exposure to a low temperature, as during a cold winter. When con-taining a minute quantity of water glycerin never solidi-fies, and to this circumstance several of its useful applica-tions are due. A weak aqueous solution, when chilled sufficiently, allows crystals of ice to form, the glycerin accumulating in the unfrozen portion as alcohol does when a mixture of spirit and water is partially frozen. When heated alone it partially volatilizes, but the greater part decomposes; by reducing the pressure to about TT2th of an atmosphere, it can, however, be readily distilled unchanged, boiling under a pressure of 50 millimetres of mercury at about 20° C. In an atmosphere of steam, also, it distils without decomposition under ordinary barometric pressure. In water and alcohol it dissolves readily in all proportions: in ether it is insoluble. Under certain conditions, such as prolonged contact with poor cheese and chalk at about 35° to 40° G, it can be made to ferment partially, becoming changed into alcohol; but under any circumstances, only a small fraction, at most a tenth, becomes thus transformed, the rest remaining unaltered. It possesses remarkable solvent powers on many substances, whence it is employed for numerous purposes in pharmacy and the arts. Its viscid character, and its non-liability to dry and harden by exposure to air, also fit it for various other uses, such as lubrication, &c, whilst its peculiar physical charac-ters, enabling it to blend with either aqueous or oily matters under certain circumstances, render it a useful ingredient in a large number of products of varied kinds. Applied to the living skin (and similarly to untanned leather) it pro-duces a remarkable softening effect, whence it is largely employed as a cosmetic, either by itself or in admixture with other substances. Taken internally it is alleged to be valuable as a substitute for cod-liver oil for phthisical patients, not possessing the disagreeable fishy flavour of that valuable food, and having a fattening tendency. When it is given in moderately small repeated doses to the lower animals, it does not appear to possess any marked! injurious action peculiar to itself; when, however, large doses of glycerin are subcutaneously injected into dogs, amounting to from 8 to 10 grammes per kilogramme of animal operated on (0'8 to l'O per cent, of the weight of the dog, corresponding to from 1 B> to 1 \ lb of glycerin for the weight of an average man), death ensues within twenty-four hours, accompanied by symptoms analogous to those of acute alcoholism (Dujardin, Beaumetz, and Audigrj). Like sugar it possesses antiseptic qualities, so that meat, albumin, &c, immersed in it do not for long periods of time undergo putrefactive changes.

Manufacture.—The simplest modes of preparing glycerin in a state of pnrity are based on the saponification of fats, either by alkalies or analogous basic substances, or by superheated steam, and on the circumstance that, although glycerin cannot be distilled by itself under the ordinary pressure without decomposition, it can be readily volatilized in a current of superheated steam ; in this way the glycerin formed is separated from the non-volatile sub-stances present. It was by means of saponification of olive oil or lard with litharge (lead oxide), wheieby a lead soap insoluble in water, or nearly so, is formed, together with glycerin, that the existence of glycerin was first demonstrated by Scheele, who obtained it as a bye-product in the formation of the " lead-plaster" of pharmacy made from lard and lead oxide. For a long time this was the only known method of preparing glycerin, the aqueous solution obtained being treated with sulphuretted hydrogen to remove any soluble lead compounds, filtered, and evaporated until almost all the water was driven off, leaving the glycerin behind as a syrupy fluid. By evaporating down the spent leys of the soapmaker (after the soap is separated therefrom by "salting out," and any excess of alkali neutralized with sulphuric acid), and treating the residue with alcohol, glycerin can also be obtained, the alcoholic solution of it thus formed being simply evaporated to drive off the alcohol; but this process is far too costly for ordinary purposes. An im-provement on this method was patented in 1858 by H. Beynolds, the concentrated leys being passed into a vessel where they are met by a stream of superheated steam at about 200° C. ; the glycerin then passes over with the aqueous vapour, whilst the inorganic salts present are left behind. In the manufacture of stearic acid for candle-making (see CANDLE) one of the older processes was the saponification of tallow with lime, forming an insoluble lime soap and an aqueous .solution of impure glycerin, from which the pure substance can be readily obtained by distillation with super-heated, steam. Less pure products were formerly obtained by treating the crude solution with sulphuric acid to separate lime, boiling to remove small quantities of volatile acids, evaporation, and filtration through animal charcoal to decolorize; or by evaporat-ing, dissolving out by alcohol, and purifying by treating with lead oxide, filtering to separate an insoluble lead compound formed, removing lead from this filtrate by sulphuretted hydrogen, filtering again, and evaporating to a syrup. In practice all these older methods have, however, been superseded by the process patented in 1854 by "Wilson & Payne. This consists in heating the fatty matter to be saponified in an appropriate still to a temperature of 290° to 315° C. (550° to 600° Fahr.), and passing in heated steam in such a way that it rises up through the fatty matter in numerous streams; saponification is thus rapidly effected, and the liberated glycerin and fatty acids are volatilized and carried along with the steam to the condensing arrangement. If the temperature do not exceed 310° C. there is no fear of the glycerin being decomposed, whilst under suitable conditions even higher temperatures than this can be employed without causing its decomposition ; but there is always a great liability to destruction of glycerin when the tem-perature of 310° is exceeded. This arises from the tendency of the glycerin to char on heating, and to split up into water and acrolein (acrylic aldehyde), thus:—

C3H803=2H20+C3H40.

"When a series of chambers is used as the refrigerator, the compartments nearest the still are found to condense little but fatty acids, the water and glycerin chiefly accumulating in the more dis-tant chambers, the last of which is usually open to the air at the end; so that there is no excess of pressure in the still and con-densers ; the fatty acids readily separate from the aqueous solutions of glycerin, which only requires concentration by evaporation to be fit for the market. Since the date of Wilson's patent various special forms of apparatus for effecting the transformation have been patented by Wright & Fouehe, Gilbee, and others.

This method of saponification is, strictly, only an improvement on former processes invented for the purpose of decomposing the glycerides and obtaining the fatty acids without the use of alkalies, lime, &c.; in these older methods theextraction of glycerin was not an important feature, and they were frequently worked in such a way as to decompose the whole or greater part of the $.ycennparipassu with its liberation. Thus, as far back as 1825, Chevreul and Gay-Lussac sketched ont the idea of the process of saponification by superheated steam, and the method was actually carried out with certain modifications by various chemists and manufacturers. For example, in 1842, E. Price & Co. patented, in the name of Jones & Wilson, a process of the kind, which was largely worked for some years ; in this, the fatty matter freed from extraneous impurities is mixed with 6 to 8 per cent, of strong sulphuric acid, and then heated in copper boilers to about 180° C, by superheated steam blown through the mass for about two hours ; after which hotter steam at about 300° to 350° is blown through, when fatty acids distil, but little or no glycerin passes over unaltered, almost the whole being charred or decomposed, forming acrolein, &c. Here the saponification and destruction of glycerin are largely effected by the sulphuric acid, as well as by the steam itself. Price's process was suggested to the inventors by Tilghman's method, brought out early in 1854, which consisted in making an emulsion of melted fatty matter and water (or solution of alkali) by agita-tion, and then pumping it through a long coil of iron tubing kept at a temperature near that of melted iead under a pressure of about 2000 lb to the square inch. In this way complete saponification is effected, fatty acids and an aqueous solution of glycerin being obtained when water is used, and soap with more or less water and glycerin when alkaline liquor is employed. It is noticeable that in this process it is not necessary that the alkali should be caustic, as it must be for the ordinary process of soap-boiling ; sodium and potassium carbonates answer j ust as well as their respective hydrates (caustic soda, caustic potash).





Derivatives.—Among the numerous derivatives obtainable from glycerin by appropriate chemical reactions, may be more particularly mentioned nitro-glycerin, which is, strictly speaking, im-properly named, inasmuch as it does not belong to the class of true nitro-substitution derivatives, but is simply constituted like tri-stearin, the radical of nitric acid displacing the hydrogen of the OH groups. By treating glycerin with nitric acid (preferably by dropping pure glycerin into a mixture of nitric and sulphuric acids) the following reaction ensues, the glycerin becoming what would be systematically termed trinilrin or glycerotrinitrin :—

fCH,. OH <CH2.O.NO?
\ CH .OH + 3N02OH = 3H20 + -JCH.OKO,
(CH2 . OH <.CHz.0.N0^

By treating the resulting "nitro-glycerin'' with caustic potash, saponification ensues, potassium nitrate being formed and glycerin reproduced precisely as when tristearin is similarly saponified. Two other important products obtainable from glycerin are isopropyl iodide and allyl iodide, each of which serves as the starting-point of a large series of chemical products, many of them of utility in the arts. These substances are manufactured by heating glycerin with hydriodic acid, and are formed in virtue of the reactions :—

Glycerin. Isopropyl iodide.
C3H5(OHi3 -i- SHI = 2I2 + 3H20 + C3H,I
Allyl iodide.
C3Hj(0H)3 +- 3HI I2 + 3H20 + C3HSI

Again, glycerin is employed in the manufactuie of formic acid, which is prepared most conveniently by heating together glycerin and oxalic acid. The splitting up of oxalic acid into carbon dioxide and formic acid, which takes place only to a minute extent when oxalic acid is heated alone (owing to the further decomposition of the formic acid), then ensues with but little formation of bye-products, and especially with but little loss of formic acid through further decomposition. This atises from the occurrence of a cycle of changes highly interesting from a chemical point of view, and consisting essentially in the continual formation of a body analo-gous to monostearin, and its continual breaking up into formic acid, which distils over, and glycerin, which acts over again on a fresh portion of oxalic acid.

Technical Uses.—Besides its use as a starting-point in the production of "nitro-glycerin" and other chemical products, glycerin is largely employed for a number of purposes in the arts, its appli-cation thereto being due to its peculiar physical properties. Thus its non-liability to freeze (when not absolutely anhydrous, which it practically never is when freely exposed to the air) and its non volatility at ordinary temperatures, combined with its power of always keeping fluid and not drying up and hardening, render it valuable as a lubricating agent for clockwork, watches, &c., as a substitute for water in wet gas-meters, and as an ingredient in cataplasms, plasters, modelling clay, pasty colouring matters, dye-ing materials, moist colours for artists, and numerous other analo-gous substances which are required to be kept in a permanently soft condition. From its softening property when applied to the skin, it constitutes a chief ingredient in many toilet preparations, creams, and the like. Many of these indeed, sold under fancy names, are nothing but glycerin diluted with water or weak alcohol, or mixed with some oleaginous emulsion or paste, and variously scented. Its solvent power for numerous substances renders it valuable in pharmacy as an ingredient in numerous preparations. In some of these the glycerin acts not merely as a solvent but also as a preservative against decomposition, owing to its antiseptic qualities, which also led to its being em-ployed to preserve untauned leather (especially during transit when exported, the hides being, moreover, kept soft and supple); to make solutions of gelatin, albumen, gum, paste, cements, &c., which will keep without decomposition ; to preserve meat and other edibles ; to mount anatomical preparations ; to preserve vaccine lymph un-changed ; and for many similar purposes. Its solvent power is also utilized in the production of various colouring fluids, where the colouring matter would not dissolve in water alone ; thus aniline violet, the tinctorial constituents of madder, and various allied colouring matters dissolve in glycerin, forming liquids which remain coloured even when diluted with water, the colouring matters being either retained in suspension or dissolved by the glycerin present in the diluted fluid. It has been proposed to use glycerin as a medium for the extraction of the odoriferous principle of flowers, &c, and a3 a substitute for sugar in the manufacture of some sorts of tobacco, the aioma of which is liable to be deteriorated should fermentation of the saccharine matter set in. Certain kinds of copying inks are greatly improved by the substitution of glycerin, in part or entirely, for the sugar or honey usually added. In fine, the number of useful adaptations of glycerin as an ingredient in order to confer certain special properties is almost unlimited, and its use in these directions is increasing yearly.

Impurities.—For some of these purposes it is essential that the glycerin should be of considerable purity. The chief impurities liable to be present vary with the mode of preparation. Substances made by saponification of oils, &c, with oxide of lead or lime, are apt to retain more or less of the metallic compounds, whilst glycerin extracted from soap-leys may also contain mineral matters. Such impure substances are readily purified by distillation with steam or under greatly diminished pressure. Glycerin prepared by saponi-fying clarified tallow, &c, by superheated steam, rarely contains fatty acids ; if not deprived of practically all the water with which it is mixed in the distillate first obtained, it is less viscid and has a lower density, so that the specific gravity forms a good test as to whether it contains much water or not. Occasionally glycerin is met with intentionally adulterated with sugai-syrup, gum, mmeial matters, &c., but such falsifications are comparatively tare. They may be detected by the substance being not wholly soluble in alcohol, by its leaving a residue on ignition in air, by its precipi-tating a solution of basic lead acetate (after being dissolved in water), or by other special tests, according to the nature of the impurity sought for. Thus, whilst pure glycerin does not reduce alkaline copper solutions so as to precipitate cuprous oxide when boiled therewith, the precipitation is readily produced by certain kinds of sugar, either without any previous treatment (e.g., glucose), or after boiling for a short time with water acidulated with a mineral acid such as sulphuric acid (e.g., cano sugar). (C. R. A. W.)


Footnotes

1 The name is often further extended so as to embrace other acids, analogous to the true acids of fats and oils in composition, properties, and chemical characters, but not yet found to exist in natural oils and fats.




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