Читать книгу Legal Chemistry - Alfred Naquet - Страница 7

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This method was formerly executed as follows: Nitrate of potassa was fused in a crucible, and the substances to be destroyed added in small portions to the fused mass. The organic matter soon acquired a pure white color; owing, however, to the imperfect admixture of the organic matter with the salt used for its decomposition, it was necessary to take a large excess of the latter.

The following process, suggested by M. Orfila, remedies this inconvenience: The organs are placed in an evaporating dish, together with one tenth of their weight of caustic potassa, and a quantity of water varying with the weight of the substances taken. An amount of nitrate of potassa equal to twice the weight of the organic matter is next added, and the mixture evaporated to dryness. The residue is then thrown by fragments into a Hessian crucible heated to redness, the portions first taken being allowed to become perfectly white before more is added.

Whichever process has been employed, the fused mass is decanted into a porcelain crucible, which has previously been heated in order to avoid danger of breakage. The portion remaining in the vessel is taken up by boiling with a small quantity of distilled water, and the solution so obtained likewise added to the crucible. The mass is then heated with sulphuric acid until all nitrous fumes are expelled, as these could give rise to an explosion, when, in the search for arsenic, the substance is introduced into Marsh's apparatus. As soon as the nitric acid is completely expelled, the liquid is allowed to cool; the greater portion of the sulphate of potassa formed now separating out in crystals. The fluid is next filtered and the crystalline salt remaining on the filter, washed, at first with a little distilled water, then with absolute alcohol, which is subsequently removed from the filtrate by boiling. This method is scarcely applicable otherwise than in the detection of arsenic, as in other instances the presence of a large amount of sulphate of potassa would be liable to affect the nicety of the reactions afterwards used. Its application, even in the search for arsenic, is not to be strongly recommended; on the contrary, the separation of the potassa salt by filtration is indispensable, as otherwise a double salt of zinc and potassium, which might be formed, being deposited upon the zinc used in Marsh's apparatus, would prevent the disengagement of hydrogen, and every chemist is too well aware of the difficulty of thoroughly washing a precipitate, not to fear the possible loss of arsenic by this operation.