ALUM

ALUM is a salt so useful in commerce and the arts, that the knowledge of its component parts, and of the best method of preparing it, must be of importance. In the article CHEMISTRY (Encycl.), the opinions which were then held respecting its composition, and the practice which was generally followed in its preparation, have been detailed at full length; but some of these opinions have since been controverted, and if they be erroneous, it must be expedient to vary in some degree the mode of preparation. In particular, the opinion that it is merely an excess of acid which prevents the formation of alum by evaporation of the ley, has been shown to be false by Citizen Vauquelin, who contends, of course, that the addition of putrid urine to the ley is a very bad practice. This eminent chemist had long suspected, that the crystallization of alum is not prevented by an excess of acid, and that pot-ash is not of use simply to saturate this acid, but to perform an office of more importance. To bring his suspicions to the test of experiment, he dissolved very pure ALUMINE in sulphuric acid of equal purity, and evaporated the solution to dryness, for the purpose of expelling the superabundant acid. He then redissolved the dry and pulverulent residue in water, and reduced the solution to different degrees of specific gravity, with a view to seize the point most favourable to crystallization; but with every possible precaution he could obtain nothing but a magma (see MAGMA), formed of saline plates, without consistence or solidity. This solution, however, though it constantly refused to afford crystallized alum alone, afforded it immediately by the addition of a few drops of the solution of pot-ash; and as he had employed these two substances in the requisite proportion, the rest of the solution, to the very end, afforded pure alum, without any mixture of sulphate of pot-ash. Into another portion of the same solution of pure alumine he dropped the same quantity of carbonate of soda, as he had added of that of pot-ash to the former; but no crystallization was formed, even by the help of evaporation, nor did lime and barytes produce any better effect. But if the common opinion that pot-ash, in the formation of alum, is of use only to abstract the excess of acid, be true, soda, lime, barytes, and all the substances which by a more powerful force would take this acid from alum, ought to give the same result. Another argument presented itself, which seemed decisive: If the alkalis, pot-ash, and ammoniac, do nothing more than unite to the superabundant acid of the alum, the sulphates of pot-ash and of ammoniac ought not to occasion any change in pure alum in its acidulated state; whereas if these alkalis enter as a constituent part into the alum, and are necessary to its existence, they ought to produce the same effects as pure pot-ash or ammoniac. He therefore added to a third portion of the solution of sulphate of alumine before-mentioned some drops of the solution of sulphate of pot-ash; immediately upon which octahedral crystals of alum were formed. The sulphate of ammoniac presented the same effect. This result gave still greater confirmation to his first notions, though it did not yet afford a demonstration perfectly without objection; for it might have happened that the two salts he made use of might determine the crystallization of the alum, simply by absorbing the superfluous acid, of which they are very greedy; but to determine this possible fact, he mixed in the uncrystallizable solution of alumine some sulphate of pot-ash with excess of acid, and obtained a crystallization no less abundant than with the neutral sulphate of pot-ash. This last experiment leaves therefore no doubt with regard to the influence and mode of action of pot-ash and ammoniac in the fabrication of alum; and this action is still more strongly confirmed by the examination of the alums which have been formed by the processes above related; for in this manner it is proved that they contain considerable quantities of the sulphates of pot-ash and ammoniac. These experiments led M. Vauquelin to an examination of the different alums of commerce, of which he found not one that did not afford sulphate of pot-ash, or of ammoniac, or of both. His methods of analysis are very accurate; but to detail them at length would swell this article to little purpose. To such of our readers as are not chemists they would hardly be intelligible; and the experienced chemist will devise methods of analysis for himself. It may be proper, however, to observe, that M. Vauquelin proved, to his own satisfaction, that the sulphate of pot-ash, or of ammoniac, is necessary to render alum capable of being precipitated by its earth, or to cause it to pass, as it were, to the earthy state (A). He proved likewise, that such aluminous waters as do not contain pot-ash, may remain, as long as may be desired, on their materials, without being saturated with too great a quantity of earth, or suffering alum to precipitate. From the whole of his experiments our author drew the following conclusions, which he considers as of importance to the arts, to chemistry, and to natural history. 1. It is not, at least in the greatest number of circumstances, the excess of acid which impedes the crystallization of alum, but it is the want of pot-ash or ammoniac: For it is difficult to imagine that the sulphuric acid could remain disengaged after so long remaining upon alumine in a state of extreme division, and always superabundant. It is true that the aluminous waters reddish the vegetable tinctures; but this property is not owing to a disengaged acid. This portion of acid is a constituent part of these waters; and it appears to have more affinity with the neutral sulphate of alumine than with a new quantity of this earth at the temperature of the atmosphere. 2. The sulphate of pot-ash may be used, as well as pure pot-ash, to cause the crystallization of alum. It even (A) It may be proper to notice, that Scheele seems to have known this long before, and that he mentions it expressly in his paper on Pyrophorus. even has the advantage over the latter salt, because if the aluminous waters do not really contain a disengaged acid, the pot-ash, in its combination, will precipitate a portion of alumine, and diminish the product of the boiling; whereas the sulphate of pot-ash does not produce the same effect; but if the lixiviums contain disengaged acid, which must very seldom be the case, it is not converted into alum by the sulphate of pot-ash, and is lost with regard to the product. Our author therefore is of opinion, that when the waters really contain an excess of acid, or a very oxidized sulphate of iron, the use of pot-ash is preferable to that of the sulphate of pot-ash. But when economy is an object, that in many places it would be profitable to use the sulphate of pot-ash; because it is a salt indirectly produced in many manufactories, where of course it may be obtained for nothing. In particular, the residues of the distillation of aquafortis by the sulphuric acid would be excellent for this operation, and much preferable to putrid urine, because this fluid always contains phosphoric salts, which decompose a portion of the sulphate of alumine, and considerably diminish the product. 3. Alumine cannot be used in the treatment of mother waters, as Bergman proposes. This earth is incapable of favouring the crystallization of alum, besides which, it decomposes a portion of alum by the assistance of ebullition; in which circumstance it seizes the acid necessary to its solution, and precipitates it in the form of that powder which is called alum saturated with its earth. 4. Many alum ores must naturally contain pot-ash, because perfect alum is often obtained from the first crystallization of new alum waters without the addition of this alkali. It is true that an objection may be made with regard to the wood used in calcining these ores, which may be supposed to have furnished the alkali; but it is not probable that the small quantity of wood employed, in comparison to the quantity of ore and the alum it affords, could supply enough of pot-ash for the crystallization. 5. All the earths and stones which have given, or shall hereafter afford, by analysis with the sulphuric acid, perfect alum without addition of pot-ash, must contain this alkali naturally. For it is well proved, that alum cannot exist without pot-ash or ammoniac; and as there is little probability that this salt should be found combined in earths or stones, unless perhaps in very rare cases, we may almost constantly be assured, when alum is obtained from any of these substances, that its formation was effected by pot-ash. The quantity of alum will immediately show in what proportion this alkali existed in the substances analysed. 6. The alum of commerce ought not to be considered as a simple salt, but as a combination in the state of a triple and sometimes quadruple salt of sulphate of alumine, sulphate of pot-ash, or of ammoniac. Among these last we may distinguish two species; the one without excess of acid, insoluble in water and insipid, being what is improperly called alum saturated with its own earth; and the other, which contains an excess of acid soluble in water, very sapid and astringent, is the common alum. There is likewise a pure sulphate of alumine, very astringent, very difficult of crystallization, in the form of brilliant pearl-coloured plates without consistence, and which cannot be rendered insoluble by the addition of a new quantity of its base. This last salt may with the greatest propriety be called the sulphate of alumine. 7. It follows from the comparative analysis, and the knowledge acquired respecting the different states of the combination of alumine with the sulphuric acid united at the same time with other bases, that we must distinguish seven states in this combination, and that it is necessary to express them according to the rules of the methodical nomenclature. Here follow the series, the nature, and the names of these seven sulphates of alumine. 1. Sulphate of alumine, or the artificial combination of sulphuric acid and alumine. This salt is astringent; it crystallizes in laminæ or flexible leaves, soluble in water. It has never been described nor named by chemists. 2. Acid sulphate of alumine is the foregoing salt, with excess of acid, from which it differs by reddening blue vegetable colours. It is easily made by dissolving that salt in the sulphuric acid, but it is not easy to convert this into the neutral sulphate of alumine but by boiling it a long time with its earth. This salt, like the first, has not been described. 3. Saturated sulphate of alumine and of pot-ash is the alum of the chemists saturated with its earth. It is pulverulent, insipid, insoluble, not crystallizable, and is easily converted into true alum by the addition of sulphuric acid. 4. The acid sulphate of alumine and of pot-ash greatly resembles common alum, and is easily prepared chemically; but M. Vauquelin found no alum but that of La Tolfa which is exactly of the same nature with it. 5. The acid sulphate of alumine and of ammoniac has all the properties of alum, and may be used for the same purposes; but though it is easily made in the laboratories, our author never found it pure in commerce. 6. The acid sulphate of alumine, pot-ash, and ammoniac. It is remarkable enough, says M. Vauquelin, that this should be the nature of the alum most frequently made in the arts, and that to express its combination so many words should be necessary. This, however, may be avoided, by reserving the name of alum to this substance, which will be sufficient to distinguish it perfectly. 7. The acidulous sulphate of alumine and of pot-ash, our author says, he is less acquainted with than with the preceding series. The name by which he characterizes it was suggested to him, and he thinks it proper, because by adding to the solution a small quantity of pot-ash more than is necessary to obtain octahedral crystals, it manifestly passes to the cubic form. From these deductions, the physician, the chemist, and the manufacturer, with whom the uses of alum are greatly multiplied, will hereafter possess a knowledge of the substance they employ, and may appreciate its effects on the animal economy, and other bodies to which it is so frequently applied. See Annales de Chimie, xxii. 238, and Nicholson's Journal, Vol. I. p. 318, &c.