Process by Hydrosulphuric Acid.—This method may be employed in two ways, according as the object is merely to prove the presence of oxide of arsenic, or to ascertain also its quantity.
a. If proof of its presence be all that is wanted, cut any soft solids into small pieces, add distilled water if necessary, boil for half an hour, let the decoction cool, and filter it. Add a little acetic acid to the filtered fluid, and if any precipitate form, filter again. Evaporate to dryness, first by ebullition, afterwards over the vapour-bath. Dissolve the residuum again in repeated portions of boiling distilled water, and filter the solution. If it be not acid to litmus-paper add more acetic acid, and transmit hydrosulphuric acid gas through the fluid until an excess be indicated by the sense of smell after agitation, Then expel the excess of gas by boiling; and if the precipitate of sulphuret of arsenic do not subside readily add a little of a strong solution of hydrochlorate of ammonia, which will facilitate subsidence. When the precipitate has fallen to the bottom, withdraw the supernatant fluid with the pipette, Fig. 8; and replace it with a little boiling distilled water. Lastly, collect the precipitate on a filter, and proceed as by the reduction process with soda-flux for oxide of arsenic, in a state of simple solution.
This method answers very well for ordinary cases where the quantity of arsenic is not extremely minute. But I have met with instances in medico-legal practice where the process of Reinsch, as well as that of Marsh, succeeded in detecting the poison in sources to which the method by hydrosulphuric acid had been applied without avail; because apparently the organic matter existing in solution prevented the action of the gas, or, as Orfila thinks, because boiling water will not in all circumstances remove oxide of arsenic from the textures of the animal body which are impregnated with it. In particular I doubt whether this method is sufficiently delicate to detect arsenic in those organs and textures into which it has been conveyed in cases of poisoning through absorption into the blood.—Another objection is its tediousness. The first filtration, if the substance to be examined be the stomach or its contents, may take two days; and one way or another the analysis can seldom be completed within four days. Reinsch’s process may be brought to a conclusion in two hours or less, even in the most difficult circumstances.
b. The last process to be mentioned, is one based, like the previous one, upon the precipitation of arsenic in the form of sulphuret, but with very material modifications, the purpose of which is to enable the analyst to separate the whole arsenic in a state of purity, so as to ascertain the exact amount of the poison in the mixture. This method has been recently proposed by Drs. Fresenius and von Babo.[[534]]
Cut any soft solids into small pieces, put the whole into a porcelain basin, add as much hydrochloric acid as equals the probable weight of the dry matter in the mixture, and then water enough to form a thin pulp. Heat the basin over the vapour-bath, adding every five minutes about half a drachm of chlorate of potass, and stirring frequently, until the liquid become clear-yellow, homogeneous, and thin. Add now two drachms more of the chlorate; filter through linen, washing the residuum on the filter with boiling water; concentrate to a pound; add a strong solution of sulphurous acid till its odour predominates, and expel the excess of it by heat. The liquid is now ready for the transmission of hydrosulphuric acid gas, which should be transmitted in a slow stream for twelve hours. Wash away any sulphuret adhering to the tube by means of ammonia, and add the solution to the principal liquid; which is next to be left at a gentle heat about 80° F., in a vessel covered with paper, till the sulphureous smell entirely disappear. The precipitate, which contains organic matter as well as sulphuret, is then to be collected on a paper filter, washed, and dried with the filter over the vapour-bath. The animal matter is next destroyed, and the sulphuret converted into arsenic acid, by dropping on it fuming nitrous acid till the whole is moistened, drying the product thoroughly over the vapour-bath, moistening the residuum with concentrated sulphuric acid, heating the mixture again in the vapour-bath for two or three hours, and raising the heat afterwards gradually in a sand-bath to 300° F., till a charred brittle mass be obtained. This is to be heated over the vapour-bath with twenty parts of distilled water, filtered, and washed with boiling water on the filter till what passes through ceases to redden litmus. The solution, which ought to be colourless, is next acidulated with hydrochloric acid, and treated as formerly with hydrosulphuric acid gas. When the sulphuret has been collected on a small filter, diluted ammonia is to be sent through the filter as long as it dissolves any sulphuret, and is to be received in a weighed porcelain basin, in which the ammonia and water are to be driven off at a temperature not exceeding 212°. The sulphuret which is alone left may now be weighed by again weighing the basin; and one grain of sulphuret is equivalent to 0·803 of a grain of oxide of arsenic.—The authors add an elaborate process for obtaining from this the whole arsenic by reduction. But such a proceeding is unnecessary. It is sufficient in medico-legal inquiries to ascertain by the simpler method given above [p. [204]], that it does yield by reduction with soda-flux a true arsenical crust, and that this yields by oxidation white, sparkling crystals with triangular facettes.
After a comparative trial of the most esteemed process, Drs. Fresenius and von Babo state that they found the one now described as delicate as any other, and the only method by which the quantity of oxide of arsenic can be ascertained with accuracy.—The hydrochloric acid used at the commencement enables the water to dissolve compounds of arsenic which water alone will not act on; and it farther facilitates solution by breaking up or dissolving organic textures. The addition of chlorate of potash prevents the escape of oxide of arsenic during the subsequent evaporation; which is apt to happen when hydrochloric acid is present. The subsequent addition of sulphuric acid converts arsenic acid into arsenious acid, in which shape the sulphuret of arsenic is more readily formed by the action of hydrosulphuric acid gas, when organic matter co-exists in the solution. The steps for destroying organic matter thrown down with the sulphuret at its first formation require no further commentary: They are the most important particulars in the process for its main object,—the determination of the quantity of pure sesqui-sulphuret, and, through it, of the sesquioxide originally in the subject of analysis.
Of certain alleged Fallacies in the case of Organic Mixtures.
Before taking leave of the detection of arsenic in organic mixtures, it is necessary to notice certain alleged fallacies in the way of every process, arising from arsenic obtaining admission into the subject of analysis through other means than its intentional addition or its introduction as a poison into the body. This topic, one of paramount importance in medico-legal chemistry, has lately undergone careful investigation during and since the notorious trial of Madame Lafarge. The results are the following:—
It has been alleged that arsenic may obtain accidental admission into the subject of analysis, 1, because the reagents used in the processes may be adulterated with arsenic; 2, because the material of the apparatus may contain it; 3, because it may have existed in antidotes administered during life; 4, because it sometimes forms a constituent part of the human body in the natural state; and 5, because it exists in the soil of some churchyards.
1. Arsenic may exist as an adulteration in some reagents.—It must be apt to occur in sulphuric acid, when that substance is prepared with pyritic sulphur, which commonly contains some sulphuret of arsenic; and it has actually been found in abundance in the acid by various experimentalists, and in England for the first time by Dr. Rees.[[535]] It may be detected by transmitting hydrosulphuric acid gas through the diluted acid; and it may be effectually removed in the same way,[[536]] the acid being afterwards filtered in a funnel whose throat is filled with asbestus, and the excess of gas being expelled by heat.—Hydrochloric acid may contain arsenic, because it may have been prepared with an arsenicated sulphuric acid. The impurity may be detected and removed in the same way as in that substance. Nitric acid seems not apt to be similarly adulterated;[[537]] but it may be tested by Marsh’s process, after neutralizing the acid with potash, and adding more sulphuric acid than is required to decompose the nitre thus formed. Zinc occasionally contains a little arsenic, which will be evolved in Marsh’s process. Dr. Clark of Aberdeen says zinc is scarcely ever free of a trace of arsenic; and it has been occasionally detected by others. Orfila, however, very seldom found so much as to be discoverable by Marsh’s test applied continuously for a great length of time.[[538]] A committee of the French Institute came to the same conclusion.[[539]] M. Jaquelain, acting under the directions of Professor Dumas, could not detect an atom in any French specimen of zinc, or its carbonate or silicated oxide, as met with in commerce.[[540]] Lastly, Mr. Brett satisfied himself that no British or foreign zinc he could obtain indicated the presence of arsenic by a process capable of detecting a 5000th of that metal in zinc.[[541]] It is an obvious inference from all these inquiries that no difficulty can be experienced in obtaining zinc so pure as to exhibit not a trace of arsenic by Marsh’s method. Neither is there any difficulty in obtaining sulphuric, muriatic, and nitric acid free of that adulteration.