Part 45

_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.

Finally, the author mentions that where paper is used with the silver solution we must not forget that the silver alone will after some time blacken the paper, especially if exposed to light; but this gradual change is very unlike the quick effect produced by arseniuretted or antimoniuretted hydrogen. ('Chem. News,' xxxiii, 58-63.)

_Nascent Hydrogen Test._ The apparatus used may be similar to that figured in the _engr._ The plan followed in the laboratory of Giessen is to heat the long tube through which the gas passes to redness in several parts, to produce distinct metallic mirrors; and then to remove the tube from the hydrogen apparatus and transmit a very feeble stream of dry sulphuretted hydrogen through it, the metallic mirrors being at the same time heated by means of a common spirit lamp from the outer towards the inner border or extremity. If arsenic alone is present, yellow trisulphide of arsenic is formed within the tube; if antimony alone is present, an orange-red or black trisulphide of antimony is produced; and if the mirror consists of both metals, the two sulphides appear side by side, the sulphide of arsenic, as the more volatile, lying invariably before the sulphide of antimony. If dry hydrochloric acid gas be now transmitted through the tube, without application of heat, no alteration will take place if sulphide of arsenic alone is present, even though the gas be transmitted through the tube for a considerable time. If sulphide of antimony alone is present, this will entirely disappear; and if both sulphides are present, the sulphide of antimony will immediately volatilise, whilst the yellow sulphide of arsenic will remain. If a small quantity of ammonia be now introduced into the tube, the sulphide of arsenic is dissolved, and may thus be readily distinguished from sulphur, which perhaps may have separated.

_Reduction Test._ A small quantity of the suspected sample, in the state of powder, is mixed with twice its weight, or more, of some reducing agent or flux, and the mixture is placed at the bottom of a very small glass tube, and heated in the flame of a spirit lamp for some time, when the arsenic gradually sublimes, and condenses in the cooler portion of the tube, under the form of a metallic crust, mirror, or ring. A common test-tube, if of very small diameter, may be employed; but those known as the reduction tubes of Liebig, Rose, or Berzelius are undoubtedly the most convenient and efficient. (See _engr._)

Liebig's method is by using a mixture of equal parts of dry carbonate of sodium and cyanide of potassium. The suspected substance, perfectly dry and in powder, being first introduced into a Berzelius' tube, is then covered with 6 times the quantity of this mixture, and so that the whole will not more than half fill the bulb. A very gentle heat is next applied, to expel any adhering moisture from the powder and the tube, after which a strong heat is applied to the bulb, and continued for some time, to effect the entire reduction and sublimation of the arsenical compound.

The best fluxes to use are ferrocyanide of potassium dried at 212° F., calcined bitartrate of potassium, cyanide of potassium, and powdered charcoal.

The metallic ring is proved to be arsenical by the properties and tests previously noticed. Should it be imperfectly formed, or masked by decomposed organic matter, the portion of the tube which contains it may be cut off with a file, next coarsely powdered, then reintroduced into another arsenic tube, and the exposure to heat repeated.

The characteristics most simple and well-marked are--

The volatility of the deposit when heated, shown by its escaping from the hotter portion of the tube and condensing on the cooler part higher up or further on.

Its conversion into minute octahedral crystals of arsenious anhydride, when repeatedly chased up and down the tube by the cautious application of the flame of a spirit lamp first to one part, and then to another. The character of these crystals with respect to volatility, lustre, transparency, and form, is so exceedingly well marked that a practised eye may safely identify them, though their weight should not exceed the 1/100th or even the 1/250th part of a grain. A pocket lens is here serviceable. The form of the crystals is very evident with a microscope of 4 powers. Oxide of antimony never forms octahedrons, but only prisms.

In employing this test, particular care must be taken to avoid soiling the sides of the tube in inserting the mixture, and that the substances operated on are perfectly dry; as unless this is attended to, the experiment does not succeed. The common plan is to introduce the mixture through a small paper funnel or tube extemporised for the purpose. The heat at first should be gentle, and merely sufficient to expel any adhering moisture from the mixture and the inner surface of the tube; after which (except where otherwise ordered) the upper portion of the mixture should be strongly heated, and then the bulb or bottom of the tube exposed to the full flame. After the operation is complete the bulb or lower portion of the tube is usually removed by a file, and the portion containing the deposit hermetically sealed, when it may be preserved, unaltered, for any length of time, ready to be produced as evidence if required.

This test is usually regarded as decisive; as we here actually obtain the arsenic in a solid form, recognisable by the most unequivocal characters.

_Reinsch's Test; Cupro-arsenical Test._ The suspected solution is strongly acidulated with hydrochloric acid (1 to 6 or 8), and after being raised to ebullition in a porcelain or glass vessel, a piece of bright and clean metallic copper about 1/2 inch long and 1/4 inch wide in the form of gauze or foil, but preferably the first, is added, and the whole boiled together. The time required for the ebullition varies according to the strength of the solution; when weak it should be continued for at least a quarter of an hour. When the quantity of arsenic in the suspected liquid is very small, at least half an hour should elapse before the removal of the copper. In solutions containing a notable quantity of arsenic, a few seconds is often sufficient to obtain a coating; but which, for safety sake, may be extended to two or three minutes, or even longer. Liquids rich in organic matter also require longer boiling than those nearly free from it. The coated copper, which has now acquired a characteristic iron-grey colour, is then taken from the liquid, carefully washed in distilled water, in alcohol, and (if greasy) in ether, next dried on blotting-paper, and then either cut into small pieces, or rolled into a small coil or cylinder. It is then heated in a reduction-tube over a spirit lamp, when the metallic arsenic forming the coating is volatilised, and yields a sublimate of minute octahedral crystals of arsenious anhydride; or, if the tube be very small, or any reducing agent be added, a bright metallic ring. When the coating on the copper is sufficiently thick, it may be scraped off with a knife, and heated separately in an arsenic-tube.

This test is invaluable as affording a certain and ready means of abstracting arsenic from its solution, whether pure or mixed with organic matter. The contents of the stomach or other viscera may thus be at once examined, without any tedious preliminary operations. In this way Dr Christison discovered the presence of arsenic upwards of four months after interment; and we have ourselves found it two years and eight months after interment. The coated copper may be preserved unharmed for years. Dr Taylor found that the 1-8th of an inch in one of these deposits that had been kept in paper nearly fourteen years gave a well-marked ring of octahedral crystals when heated.

_Sulphuretted Hydrogen Test; Sulphur Test._ This produces a bright yellow precipitate of trisulphide of arsenic (orpiment) in solutions containing a free acid; but acts slowly and imperfectly on pure and neutral solutions, and does not disturb those that possess an alkaline reaction. The suspected liquid should therefore be slightly acidulated with hydrochloric or acetic acid before applying this test, unless it be already acid, when it is better first to neutralise it with an alkali, and then to add the acid. The transmission of the gas through the liquid (see _engr._) should be continued for at least half an hour; when the end of the conducting tube, after being well rinsed in the liquid, is removed, and the glass, lightly covered with a piece of porous paper, set aside in a temperature of about 100° Fahr., until the odour of sulphuretted hydrogen is completely lost. The precipitate is now collected on a small filter, washed with pure water, and dried by a gentle heat. It is then placed in a watch-glass or small capsule, and redissolved in a little liquor of ammonia, which is then again expelled by heat; or it may be at once submitted to confirmatory tests. It is shown to contain arsenic by its ready and perfect solubility in ammonia, and in solutions of the fixed alkalies, their carbonates and bicarbonates, and in alkaline sulphides; by being nearly insoluble in hydrochloric acid, even when concentrated and boiling; and by yielding a metallic mirror when mixed with a flux and submitted to the reduction-test (which _see_).

Sulphuretted-hydrogen water and sulphydrate of ammonium act in a similar way to gaseous sulphuretted hydrogen; but much less effectively.

For accuracy, the sulphuretted hydrogen should be washed by passing it through a small bottle containing a little pure water, or dilute sulphuric acid, before allowing it to enter the arsenical liquor. The reduction of the newly precipitated sulphide is generally regarded as the most important part of the investigation, and requires great care and attention. An extremely elegant and sensitive method of effecting this is by heating the mixture in a stream of dry carbonic acid gas. This method has been followed by Drs Babo and Fresenius with the most satisfactory results, and is thus performed:--(_A_) is a capacious flask for the evolution of carbonic acid, half filled with rather large pieces of solid limestone or marble (not chalk). To one aperture of the doubly perforated cork, a funnel-tube (_a_) is adapted, which nearly reaches to the bottom of the vessel; to the other aperture a tube (_b_), by means of which the gas evolved is conducted into a flask of smaller size (_B_), in which it is washed and dried by concentrated sulphuric acid. The tube (_c_) conducts the carbonic acid into the reduction-tube (_C_), which is shortened in the _engr._, and must be made of difficultly fusible glass. When the apparatus is prepared, the sulphide of arsenic intended for reduction is rubbed in a small basin, previously heated in a water-bath, with about twelve parts of a well-dried mixture consisting of 3 parts of dry carbonate of sodium and 1 part of cyanide of potassium (prepared by Liebig's method). The mixed powder is then placed on a small strip of card-paper beat into the shape of a gutter, which is next pushed into the reduction-tube up to the point (_f_), and the tube is turned half round. In this manner the mixture is deposited without soiling any other part of the tube; after which the strip of card-paper is cautiously withdrawn. The reduction-tube is then, by means of the cork (_e_), fixed in its place; a moderate stream of carbonic acid gas is evolved by pouring hydrochloric acid into the funnel-tube (_a_), and the mixture carefully dried, by very moderately heating the tube along its whole length, by means of a small spirit lamp. When the gas-stream has become so low that the bubbles pass through the sulphuric acid at intervals of about a second, the spot (_k_) is heated to redness by means of a spirit lamp. When this point is attained another strong spirit-flame is applied to the mixture, progressing from (_d_) to (_f_), until all the arsenic is reduced and volatilised (the first flame at the same time continuing in action at (_k_)).

The reduced arsenic recondenses at the spot (_g_), forming a mirror, whilst an exceedingly small portion escapes at the capillary orifice (_h_), and fills the air with its garlic-like odour. The second spirit lamp is at last slowly advanced towards the other lamp, or the spot (_k_), so as to drive towards (_g_) all the arsenic which has adhered to the walls of the wider part of the tube. Both lamps are then removed, the tube closed at the point (_h_) by fusion, and heat applied, progressing from the point (_h_) towards (_g_), to contract the mirror on that side also, which increases its beauty and distinctness. The tube is then cut off at (_f_), and hermetically closed and sealed. In this state it becomes a permanent evidence which may be referred to in any future proceedings. Neither sulphide of antimony nor any other compound of antimony yields a metallic mirror or ring when treated in this way. Less than 1/300 gr. of trisulphide of arsenic thus gives a very distinct and beautiful mirror; and even 1/500 gr. a clearly perceptible one.

_Voltaic Test._ The wires from the opposite poles of a voltaic battery are immersed or brought in contact with a little of the arsenious solution placed in a capsule or on a piece of window glass. If arsenic be present it is developed at the negative pole; and if this be formed of copper wire, it becomes whitened and assumes the appearance of polished steel or silver, in consequence of the formation of arsenide of copper.

_Detection of Arsenic in Organic Mixtures._ Of the tests those which act by producing coloured precipitates are only applicable, with any degree of certainty, to perfectly limpid and colourless liquors. Those depending on the extrication of arseniuretted hydrogen are partially free from this inconvenience; but even here, if the suspected liquid be more than slightly charged with organic matter, so much frothing ensues, as to render the process nearly unmanageable. In this respect Reinsch's Test possesses advantages over all others, as it may be applied even to coloured liquids containing a considerable quantity of organic matter, without these being subjected to any preliminary process, and without danger of failure. In some cases also, as with liquids possessing only a slight degree of consistency or colour, the arsenic may be separated, after simple filtration and acidulation with hydrochloric acid, by a stream of sulphuretted hydrogen, in the usual manner. The reduction-test is only applicable to solid arsenious acid, or to compounds of arsenic obtained by means of other tests or processes. In toxicological examinations the poison is almost always to be sought for in mixtures loaded with organic matter, and under other conditions even more embarrassing. Soon after arsenic is swallowed it enters the circulation, contaminates the various tissues, localises itself in certain viscera, and is eliminated in the excretions. Hence it becomes necessary not only to examine the solids and liquids in which it is suspected the poison has been administered, the vomited matter, and the contents of the stomach and primæ viæ, but also, in fatal cases, the stomach itself, the liver, blood, muscles, and more especially the urine.[80] In such cases the stomach is the part first laid open, and a careful examination is made of its contents and coats in order to detect any undissolved particles of the poison, a pocket lens being employed, if necessary, in the search. If any particles, however minute, are found they are carefully collected and submitted to the reduction-test. If the reverse be the case, the stomach (cut into small pieces), together with its contents, is submitted to some further process, to obtain a solution suitable for the application of the usual tests. The liver, also some muscle, and any other portion of the body that may be selected, are likewise separately treated in the same manner. We have here both solid and liquid organic matter to operate on, and the problem for solution is the abstraction of their arsenic in the simplest and most certain manner, and in a form in which its presence may be demonstrated by tests. This subject has long engaged the attention of the most eminent chemists and toxicologists, and various plans have been proposed for the purpose, among which the following appear to be the most valuable and that usually adopted:--

[Footnote 80: Absorbed arsenic more particularly localises itself in the liver, in which it may generally be found in from 12 to 15 hours after administration. The liver also generally retains traces of arsenic long after it has been eliminated from the other viscera and the muscular tissues.]

(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.