Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
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Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

    166/318 (page 150)
    process for estimating magnesia and the alkalies ; and when the operator possesses the means, the hydro-fluoric acid method is employed with the greatest advantage. Estimation of Organic Matter.—A convenient quantity, say 50 grains, of thoroughly washed and dried impalpable matter, is carefully weighed in a platinum crucible, and ignited at a very dull red, till all organic mat- ter is burnt off, as described page 136, and the loss of weight is deter- mined. Estimation of Lime existing as Carbonate, or in Combination with Or- ganic Matter.—After the destruction of organic matter, the contents of the crucible are transferred to a porcelain evaporating basin, and treated in the cold with muriatic acid diluted with three or four times its hulk of water; being frequently stirred with a glass rod, and allowed to stand ten or twelve hours. The dissolved portion is now separated by filtration, and the residue is washed free from acid, dried, ignited and weighed. The loss of weight shews how much the muriatic acid has taken up. The acid solution is concentrated by evaporation and neutralized with ammonia, and on adding a slight excess of the alkali, a precipitate is thrown down, consisting chiefly of peroxide of iron, alumina, and phosphoric acid. The precipitate is collected on a filter, protected from the air by a plate of glass, (page 145,) and washed. If now, the only thing to be estimated is lime, it is done by adding oxalate of ammonia to the filtered solution, and proceeding as already described, pages 135-6. Estimation of Carbonic Acid.—As the lime estimated has existed partly as carbonate, and partly in combination with organic acids, it becomes necessary to estimate the carbonic acid in a known weight, say 50 grains, of the impalpable matter, in which the organic matter has not been de- stroyed. This is done in the same way as has been pointed out page 148, in estimating the carbonic acid of the soluble saline matter. It may be remarked, that although muriatic acid is generally employed in estimating the carbonic acid of earthy carbonates, sulphuric acid deserves in this case a decided preference ; because, from the fine division of the matter operated on, there need be no fear of effecting complete decomposition; in all other respects, the advantages of sulphuric acid are manifest. Bearing in mind that 1 of carbonic acid corresponds to T28842 of lime —if it appears that there is enough of carbonic acid to account for the whole lime found, then, as a part of the lime—a very small part it will sometimes be—must have resulted from the decomposition of organic matter, a part of the carbonic acid must have existed in the impalpable matter in combination with some other base as magnesia or protoxide of iron. Should the quantity of these bases prove to be appreciable, the analysis both of the precipitate thrown down by ammonia, and of the solution filtered from the oxalate of lime, may be proceeded with by pro- cesses easily deducible from what has already been stated, or from what is contained in the next section. As this, however, rarely occurs, both the precipitate and the solution maybe retained, to be conjoined with the solution obtained in the following process. Treatment with Concentrated Muriatic Acid, and Estimation of Silica and Difficultly decomposable Silicates.—The residue left undissolved by the diluted muriatic acid is transferred to a porcelain capsule, and treated for an hour, with hot concentrated muriatic acid. The heat should be continued till the whole is evaporated to dryness ; the dry mass is then moistened with a little muriatic acid, and after being allowed to stand for half an hour, the soluble parts are taken up by water, and the insoluble residue is col-