A Few Notes on Aloes.
Related entry: The Purgative Action of Aloes
By WILLIAM A. TILDEN, B. SC. LOND., Y. C. S.,
Demonstrator of Practical Chemistry to the Pharmaceutical Society.
In the list of subjects for investigation issued to the members of the Conference is the following question, No. 176:—"Compound Decoction of Aloes loses bitterness after some time; to what is this due?"
Before attempting to answer this question, a few points in the chemistry of aloes require notice.
In the last edition of Pereira's 'Materia Medica' four proximate principles are enumerated as forming the most important constituents of aloes.
- Aloetin, aloesin, amorphous aloin, bitter principle of aloes.
- Crystallized or hydrated aloin.
- Aloesic acid; supposed by some to be gallic acid.
Experiments made by myself, in addition to those already published by Mr. Groves and other chemists, induced me to adopt an opinion respecting the constitution of aloes somewhat modified from the foregoing.
I. Aloetin.—The first of these bodies certainly forms a constituent very important as to quantity of all the varieties of aloes. There can be no doubt that it is the product of the alteration of crystallizable aloin, partly by the action of heat, partly by the oxidizing action of the air. I regard it as a mixture of anhydrous aloin, which is capable in the presence of water of recovering its crystalline condition, and the brown oxidized substance referred to further on.
II. Crystallizable aloin is the body to which especially all the varieties of aloes owe their bitterness. Its isolation is usually thought to be a matter of some difficulty, but the following simple process will furnish any desired quantity,—pounds if necessary.
Select a specimen of Barbadoes aloes, the most powerfully odorous that can be procured, bright-looking, and not the most waxy; break it up and dissolve it in a quantity of boiling distilled water, to which a few drops of sulphuric, sulphurous, or hydrochloric acid have been added. The proportions employed may be those of the Pharmacopoeia for Extractum Aloes, viz. one pound to a gallon. Let the liquid stand a night to deposit resin, then pour it off and evaporate quickly till, if 1 lb. of aloes have been used, about 2 lbs. of liquid remain.
This left for twenty-four hours will deposit an abundant crop of yellow crystalline matter. The fluid portion poured off and duly concentrated yields a first-rate extract. The yellow crystals must be well drained and pressed, and will yield pure aloin by recrystallization once or twice from water mixed with a small proportion of rectified spirits. If the selection of the aloes be looked to, the product will amount to about 20 per cent. of the material employed.
Aloin has been said to be with great facility decomposed or altered by the simple application of heat to its aqueous or alcoholic solution. I have found, however, that it will bear without appreciable change comparative rough treatment in this way, provided the solution is quite neutral or slightly acidified. A little pure aloin dissolved in distilled water may be evaporated to dryness and heated till it fuses, and then redissolved in water, and this operation repeated several times, but the aloin undergoes but slight change of color, and will still crystallize on letting the solution stand for an hour or two, or almost immediately on stirring. The transparent yellow varnish left by evaporating solutions of it consists merely of anhydrous aloin; treatment with water restores to it its crystalline state. It is of course already known that if kept in a moist state on a water-bath for some time, the pure substance becomes gradually brown, and assumes the appearance of Socotrine aloes; but this is a comparatively slow process, and even after some time a considerable quantity of the aloin is still capable of crystallizing.
A further illustration of its stability is exhibited in the following experiment and accompanying specimen. About ten years ago, a paper by Kosmann appeared in the Journal de Pharmacie, the object of which was to show that aloes was a mixture of glucosidic bodies. The experiments by which grape sugar was obtained, and its presence indicated by the asserted production of alcohol and carbonic acid, were performed by Kosmann solely upon Cape aloes. I have made a number of experiments which convince me that he is quite incorrect in his statements, but as I hope to reproduce the subject at a future meeting, I will cite only one experiment made with pure aloin. Some aloin was dissolved in about an equal weight of oil of vitriol (it forms a clear orange syrup); the solution was gently heated for a few minutes, and then poured into water and kept boiling for about two hours.
Saturated by excess of pure carbonate of barium, filtered and evaporated on a water-bath, a minute quantity of barium retained in solution precipitated by dilute sulphuric acid and the liquid further concentrated, unaltered aloin was deposited in yellow crystals. A part of the solution which had been thus treated was submitted to the fermentation test. Three tubes full of mercury were inverted in a small mercurial trough. Into the first was introduced some washed yeast and distilled water. Into the second some washed yeast and a weak solution of sugar. Into the third some yeast and the boiled solution of aloin. The first and third gave no bubbles of gas larger than a pin's head; the second tube was completely filled with CO2 in half an hour.
To ascertain if the aloin prevented fermentation, two similar tubes were set up. The first contained yeast, distilled water and sugar; the second had in addition a portion of the solution which had been boiled and tested as above. Both gave gas in about half an hour nearly equally. A portion of the same sample of yeast was used in all these cases. There is consequently no sugar produced by boiling aloin with acids, and the aloin undergoes practically no change.
The copper test is inapplicable, inasmuch as pure aloin which has undergone no manipulation reduces alkaline copper solution rapidly, and freely. (I have found that many other bodies besides the glucoses do this; amongst others tannin and orcin)
Aloin gives no apparent change with tartar emetic nor with ferrous salts, but with ferric salts it strikes an olive coloration, which is destroyed by reducing agents.
III. The substance termed resin, which abounds in all kinds of aloes, is not very happily so-called, for it is soluble in considerable quantity in hot water. It is said to yield chrysammic acid by treatment with nitric acid, and is therefore related in some way to the soluble part of aloes; but this is a point upon which nothing is known at present.
IV. There can be no doubt that the "aloesic acid," supposed to be present in aloes, has no existence. The reaction with iron salts, ascribed to it, is due to the crystallizable aloin, and the acidity to test-paper presented by an infusion of aloes is a property of the half-oxidized substance contained in the uncrystallizable "aloetin."
V. In addition to those bodies, there is in all aloes a small but notable proportion of vegetable albumen. It is left when either kind is exhausted with rectified spirit. Its presence probably promotes the change to which solutions of aloes are always subject.
Pure aloin, then, in pure solutions, is liable only to very tardy alteration. Exposed to the air, it gradually absorbs oxygen, and the solution deepens in color; but the change which is thus slow under such circumstances, is very rapid indeed if a small quantity of any alkali is introduced. The solution then becomes in a few hours of a deep brown color; and after the lapse of three or four days, if the air be admitted, the aloin entirely disappears, and is transformed into a substance, or mixture of substances, which no longer possesses any bitterness, but is perfectly insipid. An experiment was made by dissolving pure aloin in water with an equal weight of carbonate of potassium; the solution, left in an imperfectly closed flask for about a week, entirely lost its bitter taste. Nitrate of barium was added to remove the carbonate, and the filtered liquid mixed with acetate of lead. The result was a dirty greenish precipitate, which was removed and basic acetate of lead added. This gave a bright orange precipitate, which was collected and analysed. Its composition, compared with that of aloin, is shown by the subjoined numbers:
From which it appears that whilst in aloin the carbon stands to the oxygen nearly as 1 to ½. in the oxidized substance it is, roughly speaking, in the proportion of 1 to 2.
Some extract of Socotrine aloes was boiled with carbonate of potash and water, in the proportion directed for the preparation of compound decoction of aloes, the remaining ingredients being omitted. Keeping this solution in the way described, it also became tasteless, and gave the same reactions.
Mr. William Young, pharmaceutical chemist, proposed the question which stands in the Conference list, and I am indebted to him for the specimens upon the table, and also for his permission to quote from a letter with which he has favored me.
He says, "For more than ten years I have observed that decoct. aloes co. loses its bitterness on keeping, but I cannot say that it loses its aperient property. I have frequently taken a fluid ounce of various degrees of bitterness, and have always found it produce the desired effect. But this is a matter which does not affect the pharmaceutist so much as the fact that the public cannot be persuaded that a medicine which is not uniform in taste is rightly prepared. I venture to assert that if a customer were to purchase successively at one establishment four ounces of decoct. aloes co. weekly, and each sample being a week older than the one immediately preceding, no two samples would be alike. Of course if, as I understand is the custom in some large establishments, a large quantity is prepared and kept some weeks before use, a greater uniformity would be arrived at; but that puts the small tradesman at a great disadvantage, who perhaps prepares a pint at the time, and sends it out fresh and intensely bitter. I know an instance of a chemist who nearly lost a valuable customer in the following way. He had been in the habit of dispensing a ℥xij mixture, containing ʒvj vini aloes. When he first prepared it he had a pint of the vin. aloes in stock, which probably had been made five or six years, and had not the slightest taste of aloes in it, but it pleased the patient. At length the stock was exhausted, and the mixture prepared with a fresh supply of vin. aloes recently prepared. The patient could hardly be convinced that a mistake had not been made; and it was found that ʒss of the recently prepared vin. aloes imparted more bitterness to the ℥xij mixture than the whole ʒvj of the old. I have tasted samples of dec. aloes comp. concent. 1 to 3, almost devoid of bitterness; and a maker of that article informs me that it is a most unsatisfactory preparation."
The active constituent of aloes is still unknown. That the purgative property is not due to aloin was first shown by Robiquet, and is proved, I think, by the fact of its complete disuse after a very short trial. Mr. Young says that he has not noticed any variation of power in the specimens of different degrees of bitterness which he has tried; but, on the other hand, I have myself taken large doses of the oxidized alkaline solution of aloin, or of extract of aloes, without perceiving the slightest effect.
There is in Druitt's 'Surgeon's Vade Mecum' a prescription which, I am informed by the author, is the most active form in which any kinds of aloes can be administered. Barbadoes aloes is made into a mass with strong sulphuric acid, and in that state rolled out into pills. Dispensing difficulties may have stood in the way of the more extensive employment of this form, but if it bears out the character attributed to it, it would seem that a half oxidized condition of the aloes is the most advantageous.
The questions which still remain to be solved with reference to aloes are numerous. Amongst others, two very important points seem to me to require examination. These are the nature and properties of the resinoid matter, and the cause of the differences between the several varieties of this important drug known to commerce.—Pharm. Journ., London, Nov. 5th, 1870.