(Reinsch.) Solids (as the stomach, liver, &c.) are cut into small fragments and boiled in a glass vessel with water acidulated with about 1-4th of its volume of hydrochloric acid, until the tissues or fragments are entirely broken down into flakes or grains, when the whole, after filtration, is again heated to the boiling-point, and tested as described under Reinsch’s test (see ANTIMONY). Liquids do not require this preparation.
Reinsch’s test is inapplicable when, as sometimes happens, the arsenic sought after may be in the state of one of the sulphides—either
as orpiment or realgar—a not improbable contingency, when it is remembered that, although arsenious anhydride or white arsenic is the form most generally used for criminal or suicidal purposes, the yellow and the red varieties being largely employed in workshops where fireworks are manufactured, have not unfrequently been had recourse to. Again, when the examination of a corpse long buried and disinterred takes place, it must be borne in mind that the arsenious anhydride taken by the deceased has, by the decomposition of the body, become converted into sulphide. In these cases the hydrochloric acid necessary for the performance of Reinsch’s test fails to effect the solution of the sulphide.
Mr Blyth says: “It is found that the post-mortem change into orpiment is never quite complete, so that for the detection of arsenic in solid organic substances, such as the tissues of the body, the best general method is most decidedly to convert the arsenic, if present, into the volatile chloride; and according to Dr Taylor, there is always sufficient arsenic (if present at all) unchanged into sulphide to ensure success. The only necessary caution is that the substance be thoroughly dried, and that the reagents be pure. After drying it is placed in a retort with fuming hydrochloric acid, and slowly distilled by the heat of a sand-bath. The distillate contains chloride of arsenic (if arsenic was present), and may be submitted to further tests.”
Estim. This may be effected in various ways:—
1. Gravimetrically:—Arsenic is usually WEIGHED under the form of arsenate of lead, arsenate of sesquioxide of iron, tersulphide of arsenic, (metallic) arsenic, or (directly) as arsenious anhydride. The last three only, as the more simple and convenient, will be noticed here:—
As trisulphide:—The whole of the arsenic being precipitated by a stream of sulphuretted hydrogen, with the necessary precautions, in the manner already noticed, the precipitate, after being carefully collected, washed, and dried, is purified by redissolving it in pure ammonia water, and evaporating the resulting solution in a weighed watch glass or capsule by the heat of a water-bath. It is then dried at a temperature not above 212° Fahr., and finally weighed. Each grain of the tersulphide so found corresponds to ·80487 gr. of arsenious acid, or ·61 gr. of metallic arsenic.
As (metallic) Arsenic:—Obtained by one of the processes already given. Each gr. represents 1·32 gr. arsenious acid.
As Arsenious anhydride:—Obtained in a weighed capsule or tube, either by the crystallisation or sublimation test. The weight is the answer sought for arsenious anhydride. Each gr. of this is equiv. to ·75758 gr. of metallic arsenic.
Volumetrically. (Method of F. Mohr.) This depends on the fact that an aqueous solution of arsenious acid, or of an alkaline arsenite, when mixed with an excess of saturated solution of pure bicarbonate of soda and a little starch-paste, has its arsenious acid converted into arsenic acid by a solution of iodine. A standard solution of iodine is, therefore, an appropriate arsenim′eter for the above mixture. The solution of iodine is added until the blue starch-reaction just begins to appear, the arsenious solution having been previously exactly neutralised with pure carbonate of soda if acid, or with pure hydrochloric acid if alkaline. The results are accurate when no substance capable of oxidising or decomposing iodine is present in the liquid tested.