Proceedings of the Royal Society of London/Volume 2/On the Nature and Combinations of a newly discovered vegetable Acid; with Observations on the Malic Acid, and Suggestions on the State in which Acids may have previously existed in Vegetables
The acid here noticed by the author being obtained in greatest quantity from the fruit of the Sorbus aucuparia, is denominated by him sorbic acid, in order to distinguish it from other known vegetable acids. To prepare it, he presses the ripe fruit, previously bruised, in a linen bag, and thereby obtains nearly half its weight of juice. With this juice he mixes a solution of acetate of lead, and obtains a precipitate of sorbate of lead, which requires to be frequently washed with cold water. The purified powder is then boiled in a large quantity of water, which dissolves a part as a super-sorbate, leaving undissolved a sub-sorbate. The liquor being filtered and suffered to cool, deposits brilliant crystals of purified sorbate of lead.
To the crystals thus obtained he adds a quantity of dilute sulphuric acid, sufficient to separate nearly the whole of the lead; and having then separated the remainder by a current of sulphuretted hydrogen gas, he obtains the acid in a state of purity.
The acid to which this bears the nearest resemblance, is the malic; and indeed these two acids appear to the author to have been confounded even by Scheele, for apples contain a portion of sorbic as well as of malic acid; but that these acids are different, Mr. Donovan proves by decomposing malate of lead by sorbic acid. For if water be boiled on malate of lead, no crystals are to be obtained on cooling the liquor; but when the malate is boiled in sorbic acid, the malate is decomposed; and the liquor, when cooled, deposits the peculiarly brilliant crystals of sorbate of lead.
In order to be well assured of the difference between these acids, Mr. Donovan compared his acid with as many as seven different specimens of malic acid obtained from different sources, and was confirmed in the opinion that they are essentially different, by comparison of various neutral salts obtained from each; those containing sorbic acid being in general to be procured in permanent crystals, while those from malic acid yield merely deliquescent residua when reduced to dryness.
The author remarks, that the purest malic acid is that prepared from the Sempervivum tectorum, which, according to the observation of Vauquelin, appears to be free from every other acid; while the juice of apples, unless they be taken very young, appears constantly to contain a portion of sorbic acid.
This paper concludes with conjectures respecting the progressive changes of vegetable products, and possible conversion of bitter principle into malic acid, sorbic acid, and oxalic acid; but the author is fully sensible that little reliance can be placed on such speculations.