Objec., precau., &c. Objections have been raised to this mode of testing, from the great frothing which often occurs with organic mixtures, and from antimony and imperfectly charred organic matter also forming crusts somewhat resembling, to the inexperienced eye, those produced by arsenic. But these objections are invalid, because there are easy means of purifying the liquid before testing it, and of discriminating between true arsenical spots or deposits and false ones. Another objection is, that both zinc and sulphuric acid sometimes contain arsenic; but to obviate this difficulty, we have only to use them when perfectly pure; and to test them by means of the apparatus before pouring the suspected liquid into it. Indeed, these objections apply with equal force to all those tests which depend on the production of nascent hydrogen. The precaution necessary to success, and to reliable results, is to set the apparatus with simple zinc, acid, and water, and after it has worked a short time to test the evolved gas for arsenic (as above); when, if no trace of that substance is detected, the suspected fluid, in which the organic matter (if necessary) has been destroyed by any one of the methods hereinafter pointed out, may be added, and the operation continued. Care should also be taken not to light the jet of gas before all the atmospheric air is expelled from the apparatus, as without this precaution an explosion may take place.
Modification of Marsh’s Test.—Davy. This process consists in the use of sodium amalgam instead of zinc and sulphuric acid, both of which are liable to be contaminated with arsenic. Sodium, on the other hand, has never been found to contain arsenic, and mercury only very rarely; but should it exist in that metal, it can be easily removed by digesting the mercury in dilute nitric acid, and afterwards well washing it with water.
One part by weight of sodium to 8 or 10 parts of mercury forms a very good amalgam. The mercury is placed in a test-tube, and the sodium gradually added in small portions; the metals readily combine, forming an alloy, liquid whilst hot, but hard and brittle when cold.
The author uses this amalgam by placing the suspected solution, or solid substance, along with a little water in a test-tube, then adding a small piece of amalgam about the size of a grain of wheat, and quickly covering it with a piece of white filtering paper or the lid of a porcelain crucible moistened with a dilute solution of silver nitrate slightly acidified with nitric acid. If arsenic is present, a dull black or deep brown stain on the paper or porcelain will be developed on the moistened part, owing to the silver being reduced to the metallic state by the arseniuretted hydrogen. The solution may be made by dissolving 20 gr. of nitrate of silver in an ounce of distilled water acidulated with 2 drops of strong nitric acid.
It is advisable to place between the moistened paper or lid and the tube a small disc of bibulous paper, to prevent any particles of the liquid producing minute black spots, and thus interfering with the results. 1⁄1000th part of a grain of arsenious acid in 1 c. c. of distilled
water gives a very decided effect in a few moments, but much smaller quantities may be detected, e.g., the 1⁄100000th or even 1⁄1000000th part of a grain in 1 c. c.
This method is applicable not only to arsenic as arsenious acid, but also to other compounds of arsenic, soluble or insoluble in water, e.g., orpiment and realgar, the alkaline arsenates, and even the metal itself if in powder. Organic matter interferes but very little with this method. Antimony, as in Marsh’s process, will produce, with the sodium amalgam, results similar to those of arsenic; this, when brought into contact with the nitrate of silver, forms a black antimonide of that metal.
Fleitmann, however, pointed out that antimoniuretted hydrogen is not evolved from strongly alkaline solution, and, as in this case, the action of the sodium amalgam is to render the mixture quickly alkaline, only a very small quantity of antimony present will be evolved, and by previously rendering the mixture strongly alkaline the evolution of that gas may be almost entirely prevented.
It may be occasionally necessary to determine whether the stains on the paper moistened by the silver solution are due to arsenic or antimony. It is then best to digest the paper-stain in sulphide of ammonium, the metal present being converted into a sulphide, and dissolving in the excess of the alkaline salt, leaving the silver sulphide undissolved; the alkaline solution when evaporated will, in the case of arsenic, leave a bright yellow residue, almost insoluble in hydrochloric acid; whereas in the case of antimony an orange-coloured residue will remain soluble in that acid. Dr Russell observes that hydrogen alone is capable of reducing silver solution to the metallic state, but acknowledges that this action is exceedingly slow. Pellet, on the other hand, maintains that pure hydrogen when passed through solutions of soda and nitrate of silver has no action at the ordinary temperature; but he states that the silver salt which has been fused possesses an alkaline reaction in solution, and hydrogen thus produces a slight precipitate, which can be prevented by adding a drop or two of nitric acid.
Davy, however, found in his experiments only the faintest possible effect of the reducing action of pure hydrogen in solutions of caustic soda and nitrate of silver.