Chap. I. Now the smallest number of atoms which would correspond with the constituents in these proportions is, This would make the atomic weight of the acid 10·75, a number quite at variance with that resulting from the analysis of the pyrocitrates. It is obvious from this that some of these results are inaccurate, and that new researches are necessary before we can have any accurate knowledge either of the atomic weight or atomic constituents of this acid. History. SECTION IX.-OF MALIC ACID. This acid, which was discovered by Scheele in 1785, has received the name of malic acid, because it may be obtained in abundance from the juice of apples, in which it exists ready formed. Scheele has given us the following process for extracting it: Saturate the juice of apples with potash, and add to the solution acetate of lead till no more precipitation ensues. Wash the precipitate carefully with a sufficient quantity of water; then pour upon it diluted sulphuric acid till the mixture has a perfectly acid taste, without any of that sweetness which is perceptible as long as any lead remains dissolved in it; then separate the sulphate of lead, which has precipitated, by filtration, and there remains behind malic acid.* M. Braconnot has shown that when malic acid is obtained by this process of Scheele, it is contaminated with a mucilaginous matter, intermediate between sugar and gum, which masks all its properties.† Vauquelin ascertained that it may be extracted, with greater advantage, from the juice of the sempervivum tectorum, or common house-leek, where it exists abundantly, combined with lime. The process which he found to answer best is the following: To the juice of the house-leek add acetate of lead as long as any precipitate takes place. Wash the precipitate, and decompose it by means of diluted sulphuric acid in the manner directed by Scheele.‡ Gay-Lussac has shown that when malic acid is obtained in * Swedish Trans. and Crell's Annals for 1785. † Ann. de Chim. et de Phys. viii. 149. Ibid. xxxiv. 127. * this way, it still retains a quantity of lime, from which it may be freed by evaporating it to the consistence of a syrup;` and mixing it with alcohol. Malate of lime is precipitated, and the malic acid remains dissolved in the alcohol. But the acid even when thus purified, retains the mucilaginous matter with which it is contaminated when extracted from the juice of apples, as Braconnot has shown.+ Class I. Div. II. In the year 1815, Mr. Donovan announced the existence of Preparation. a new acid in the expressed juice of the berries of the pyrus aucuparia, to which he gave the name of sorbic acid. His method of procuring this acid was the following: The berries are to be collected when fully ripe. They are to be first bruised in a mortar, and then squeezed in a linen bag. They yield nearly half their weight of juice of the specific gravity 1-077. This juice is to be strained and mixed with a filtered solution of acetate of lead. Separate the precipitate on a filter, and wash it with cold water. A large quantity of boiling water is then to be poured upon the filter, and allowed to pass through the precipitate into glass jars. After some hours this liquid becomes opaque, and deposites crystals of great lustre and beauty. The matter remaining on the filter has now become hard and brittle. But it may be made to furnish more crystals by the following treatment: Boil it for half an hour with rather more diluted sulphuric acid than is sufficient to saturate the whole lead which it contains. Filter the liquid and treat it again with acetate of lead. The edulcorated precipitate, when treated as before with boiling water, yields an additional crop of crystals. This process may be repeated till almost the whole acid has been obtained united to lead in a crystallized state. Collect the crystals formed, and boil them for half an hour with 2-3 times their weight of sulphuric acid of the specific gravity 1.090, supplying water as fast as it evaporates, and taking care to keep the mixture constantly stirred with a glass rod. The clear liquor is to be poured off and put into a tall glass jar of small diameter. While still hot, a stream of sulphuretted hydrogen gas is to be passed through it. When the lead is all precipitated the fluid is to be filtered and boiled in an open basin till the sulphuretted hydrogen is disengaged. The liquid now consists of water, holding sorbic acid in solution.‡ * Ann. de Chim. et de Phys. vi, 351. Phil. Trans. 1815. + Ibid. vol. viii. p. 149. Chap. I. M. Braconnot has given us the following process, for extracting this acid from the berries of the pyrus aucuparia, which he recommends as easier than the process of Donovan : The berries are to be taken before they are ripe, pounded in a marble mortar, and the juice squeezed out by a strong pressure. The juice is to be heated in a capsule, to the boiling temperature, and carbonate of lime is to be thrown in as long as any effervescence continues. Evaporate the liquid to the consistence of a syrup, taking care to skim off the scum in proportion as it collects on the surface. A granular precipitate of sorbate of lime falls, which adheres strongly to the vessel, unless care be taken to stir the liquid from time to time. After an interval of some hours, decant off the syrupy liquid, and wash the precipitate with a little cold water. Then squeeze it in a cloth and dry it. The colour of this salt is fawn, indicating that it is not free from the colouring matter of the berries. Boil it for a quarter of an hour with a quantity of crystallized carbonate of soda equal to it in weight, and a sufficient quantity of water. A double decomposition takes place, and the liquid retains in solution sorbate of soda, mixed with a quantity of colouring matter. This colouring matter is removed by heating the liquid for some time with lime water, or milk of lime. The liquid when filtered becomes quite colourless. A current of carbonic acid gas passed through it precipitates the lime retained in solution. The liquid thus freed from colouring matter is to be precipitated by subacetate of lead, and the sorbate of lead may be decomposed, and the acid obtained in solution in water by digesting it with the requisite quantity of dilute sulphuric acid. I have repeated this process of Braconnot very exactly; but did not succeed in freeing the acid from the colouring matter. The berries which I employed were fully ripe, while Braconnot's probably were green. Perhaps this may account for the difference between our results. In the year 1818, Braconnot made a set of experiments on malic acid from the juice of the apple, and from the leaves of the sempervivum tectorum, or house-leek, and demonstrated that when freed from the mucilaginous matter with which it is usually mixed and disguised, it possesses exactly the properties of sorbic acid. Hence it follows that sorbic acid is not a peculiar acid, as Donovan supposed; but merely the malic acid of Scheele, in a state of greater purity than that chemist had * Ann. de Chim. et de Phys. vi. 241. + Ibid. vol. viii. p. 149. been able to obtain it. Of course we must drop one or other of these two terms, and as Scheele was undoubtedly the first discoverer of this acid, though he did not succeed in procuring it in a state of purity, it seems but fair to retain the name malic acid, which that illustrious chemist originally bestowed on it. Braconnot obtained pure malic acid from the juice of the house-leek, in the following manner: The expressed juice of the plant was evaporated to the consistence of a syrup, and alcohol being gradually mixed with it, threw down the precipitate usually considered as malate of lime. This precipitate was pressed strongly in a cloth to free it as completely as possible of the saccharine matter with which it was mixed. It was then dissolved in water, to which it communicated a brown colour. Sulphuric acid was added to the liquid in such quantity as only partially to decompose the salt, and the liquid was filtered in order to separate the sulphate of lime precipitated. The liquid, which had now a decidedly sour taste, being set aside for 24 hours, deposited a colourless salt, consisting of bimalate of lime. This salt being redissolved and crystallized slowly, formed six-sided flat prisms, terminated by bihedral summits, of the purest white colour. These crystals being dissolved in water were decomposed by sulphuric acid, and in order to get rid of all the sulphuric acid, the filtered liquid was digested over oxide of lead, and the whole being filtered, a current of sulphuretted hydrogen gas was passed through it to thrown down the dissolved lead. It was now evaporated to the consistence of a syrup, and dissolved in alcohol to get rid of some lime and oxide of lead, not separated by the previous processes. The alcoholic solution being evaporated to the consistence of a syrup did not crystallize in 24 hours; but being put into a stove it crystallized completely within the same interval of time. The crystals first formed were groups of small flattened globules of about a line in diameter, and by degrees the whole concreted into an opaque white mass tuberculated on the surface. Class I Div. IL Malic acid thus obtained is a white opaque substance, having Properties. an exceedingly sour taste, but destitute of smell, and exceedingly soluble both in water and alcohol. When dropt into solutions of nitrate of lead, nitrate of silver, or nitrate of mercury, it occasions no precipitate. In acetate of lead it throws down a white precipitate soluble in acetic acid, and even in boiling water. When acetate of lead is dropt into this acid, diluted with water, a copious white precipitate falls, Chap. I. which gradually redissolves in the liquid, and is slowly converted into very fine silky crystals, having a great deal of lustre. When these crystals are boiled in water they partially dissolve, but the greatest part of them melt into a resinous looking matter which may be drawn out into the threads, but which becomes brittle as soon as the temperature sinks. Neither lime water nor barytes water occasion any precipitate when dropt into this acid. The malates of potash and soda are incrystallizable, deliquescent, and insoluble in strong alcohol. But the bimalates of potash, soda, and ammonia, crystallize. When saturated with magnesia, or with oxide of zinc, it forms salts, which crystallize with the greatest facility. Scheele considered the acid produced by treating sugar with a quantity of fuming nitric acid equal to its own weight, and previously diluted with its own weight of water, to be malic acid. But from the experiments of Vogel, there is reason to believe that it constitutes a different and peculiar acid, which requires further examination.* Various analyses of malic acid have been made, but the results differ so much from each other that we can only reconcile them by supposing that two different acids have been hitherto Constituents, confounded under the name of malic acid. And the properties of this acid as they have been hitherto stated, are so vague, that this may very well have been the case. Frommherz, malic acid is composed of According to 100.000+ This is equivalent to 3:46 atoms hydrogen, 3.59 atoms carbon, and 6 atoms oxygen. Frommherz, therefore, considers it as a compound of 3 atoms hydrogen 0.4375 This would make the atomic weight of the acid 9-0625. He * Gilbert's Annalen, lxi. 233. Schweigger's Jahrbuch, xvii. 1. |