Читать книгу Treatise on Poisons - Robert Sir Christison - Страница 25

Sulphuric acid is known as a poison chiefly in the form of the concentrated commercial acid. But a few cases of poisoning have also been produced by blue-liquor or the solution of indigo in strong sulphuric acid; and one instance^[233] has been recorded of poisoning with the aromatic sulphuric acid of the Pharmacopœias, which is an infusion of aromatics in a mixture of sulphuric acid, ether and alcohol. In the following remarks on its tests, it will be sufficient to consider it first in the concentrated form,—secondly, in a state of simple dilution,—and thirdly, when mixed with various impurities, more especially with vegetable and animal matter. The acid solution of indigo may be known by the tests for the concentrated acid, and its blue colour, removable by a solution of chlorine; and the aromatic sulphuric acid may be distinguished by its odour and the tests for the diluted acid.

1. When concentrated it is oily-looking, colourless, or brownish from having acted on organic particles, without odour, much heavier than water, and capable of quickly corroding animal substances. If from these properties, and its effect in reddening litmus, its exact nature be not considered obvious, it may be heated with a few chips of copper; when sulphurous acid is disengaged and may be readily recognised by its odour.

2. When diluted, it may be distinguished from all ordinary acids by solution of nitrate of baryta occasioning a heavy white precipitate of sulphate of baryta, which is insoluble in nitric acid. Selenic and sulphurous acids, however, and also, as Mr. Alfred Taylor informs me he has lately found, the fluo-silicic acid, are similarly acted on in all respects. But selenic and fluo-silicic acids in all forms, and sulphurous acid in a state of solution, are so seldom met with, being known only in the laboratory of the scientific chemist, that they can scarcely be considered sources of fallacy. Sulphuric acid may at once be distinguished from sulphurous acid, by the latter possessing a peculiar pungent odour. From the two other acids it may be distinguished by collecting and drying the barytic precipitate, mixing this with charcoal, converting it into sulphuret of barium by heating it in a platinum spoon before the blowpipe, and then adding diluted muriatic acid to the sulphuret, so as to disengage sulphuretted hydrogen gas,—which again is easily known by its odour, or its property of blackening paper dipped in solution of acetate of lead. A much more important source of fallacy than these is the possible presence of a bisulphate in solution, or a neutral sulphate along with any other free acid; for these substances will present the same reactions with litmus and barytic salts as free sulphuric acid itself. Much has been published lately upon this point; but the difficulty has not yet been satisfactorily overcome. It may be got rid of indeed by proving, that no bisulphate or neutral sulphate is present. Their absence may be shown by no solid residuum being left on evaporating the suspected fluid, or at least no more than a mere haziness, owing to the sulphate of lead which commercial sulphuric acid always contains in small quantity. Or as Orfila suggests, we may establish their absence still better by concentrating the fluid, and finding that neither carbonate of soda, which would cause a precipitate with earthy or metallic bases, nor chloride of platinum, which would do so with potash or ammonia in combination, nor fluo-silicic acid, which precipitates soda salts, has any effect when applied to separate portions of the subject of inquiry. But suppose it appears in the course of these trials that one or more bases are actually present, how is it to be settled whether the sulphuric acid, indicated by litmus and a salt of baryta, is really free or not? To this question I must reply, that no method has yet been proposed, which is at once satisfactory and easily available. Mr. Alfred Taylor proposes to concentrate the fluid, and agitate it with alcohol, in the hope that the alcohol will remove sulphuric acid, and not a sulphate, from the water.^[234] But it removes sulphuric acid from a bisulphate even when dry, and still more when a little water is present. Orfila^[235] proposes, in the case of sulphuric acid in vinegar,—where there is both a vegetable acid and a neutral sulphate of lime,—to concentrate to a sixth, and agitate the residuum with four times its volume of sulphuric ether, in the expectation that this fluid will remove the free acid alone, and separate it from sulphates. But notwithstanding the authority of his name for the fact, pure ether will not remove sulphuric acid from a watery fluid; and etherized alcohol, which does remove it, takes it away also, like alcohol itself, from bisulphates. These results I have observed in some careful trials made along with Dr. Douglas Maclagan. I suspect, therefore, that where sulphates or bisulphates do exist, there is no absolutely satisfactory way of determining whether free sulphuric acid also co-exists, except by a quantitative analysis, for ascertaining whether the amount of acid and of bases corresponds with this supposition or not. And it is scarcely necessary to add, that so operose a method is scarcely applicable to ordinary medico-legal investigations.

3. It is seldom that the medical jurist is called on to search for sulphuric acid in either of the states already mentioned. Much more generally it has mingled with and acted on various organic substances. The circumstances in which it has usually to be sought for in the practice of medical jurisprudence are twofold,—on the one hand, in stains on clothes,—and on the other, in vomited matter, the contents of the stomach, or organic mixtures generally.

Process for analyzing stains on clothes.—When sulphuric acid is thrown upon your clothes, it produces a permanent red, reddish-brown, or yellowish stain, destroys the cloth entirely or renders it brittle, and in consequence of its strong attraction for water keeps the stain long in a moist state. In the course of the decomposition of the cloth a part of the acid is itself decomposed, sulphurous acid being disengaged. But it is an important medico-legal fact, that after a time the change either goes on very slowly, or is arrested altogether, possibly by the dilution of the acid with moisture from the atmosphere; and that consequently it may be discovered in a free state in stains after a much longer interval than would à priori be expected. In the case of Macmillan formerly alluded to, Dr. Turner and I, who were employed by the crown to examine the different injured articles of dress, found on a man’s hat, stock, shirt-collar and coat many discoloured and corroded spots, which were sour to the taste fourteen days after the crime was committed; in the subsequent case of Mrs. Humphrey I discovered six-tenths of a grain of free sulphuric acid in two small spots on a blanket seven weeks after the crime; and from an express experiment on the same blanket with two drops of acid of known strength, it appeared that only one-half of the acid disappeared in seven weeks. It may therefore be inferred, that, in every instance where stains have been produced by concentrated sulphuric acid on clothes, at least on woollen clothes, and no attempt has been made to remove the remaining acid by washing or neutralization, a sufficient quantity will be present even after several weeks to admit of being satisfactorily detected by chemical analysis.

The following are the steps of the process which appear to me the most delicate and equivocal. Cut away the stained spots; boil them for a minute or two in several successive small portions of distilled water; and filter if necessary. Next prove the acidity of the fluid by litmus, and likewise by the taste if the quantity of solution is large enough to allow of so coarse a test being used; and with a few drops ascertain the existence of sulphuric acid in one form or another by nitrate of baryta and nitric acid, as mentioned in the process for the pure diluted acid. If no precipitate appears, the search for sulphuric acid is at an end. But if a precipitate is produced, ascertain the absence of bisulphates and sulphates by proving the absence of bases, according to the method described in the process for the simple diluted acid. If, however, bases be found in material proportion to the acid, the analysis is subject to all the difficulties mentioned above in speaking of the detection of the diluted acid in similar circumstances.

Process for the contents of the stomach and other complex mixtures.—When sulphuric acid has been mixed with various mineral and organic substances, it may in no long time cease to exist in the free state. Part may be decomposed by organic matter in the way formerly mentioned. Or the whole may be neutralized at once by earthy or alkaline carbonates, administered purposely as antidotes. Or it may also be neutralized more slowly by the gradual development of ammonia in consequence of the decay of the animal matter co-existing in the mixture. Thus in a case mentioned by Mertzdorff of a child killed in twelve hours with sulphuric acid, the contents of the stomach did not redden litmus, but on the contrary had an ammoniacal odour; and they contained a considerable quantity of a soluble sulphate, probably the sulphate of ammonia.^[236] In like manner MM. Orfila and Lesueur found that when this acid was left some months in a mixture which contained putrefying azotized matter, it was gradually neutralized by ammonia.^[237] It appears from Orfila’s latest researches,^[238] that in most cases of acute poisoning with this substance some free acid will be found in the contents or tissues of the stomach, provided alkalis or earths were not given as antidotes, and the examination of the body be made before decay sets in.

The detection of sulphuric acid in complex organic mixtures, simple though it appears at first sight, is one of the most difficult problems in medico-legal chemistry. The difficulty arises from a variety of sources,—from the probable presence of neutral sulphates along with free hydrochloric, acetic, or some other acid,—the possible presence of a bisulphate,—the occasional neutralization of the sulphuric acid by antidotes given during life, or ammonia evolved during decay after death,—or its neutralization, together with the development of a different free acid, by its having displaced this acid from a salt existing in the mixture.

The subject was investigated in most of its relations in the last edition of the present work, and a process proposed which overcame some difficulties, but left others untouched. The inquiry has been since undertaken also by M. Devergie and Professor Orfila, but with most success in Germany by Dr. Simon.^[239] The result of all these researches is, that a satisfactory process for detecting sulphuric acid in organic mixtures still remains to be discovered. Meanwhile the most eligible method appears to me to be the following.

a. If the mixture be acid, add distilled water, if necessary, boil, filter, and test a few drops of the fluid with nitrate of baryta, followed by nitric acid. If there be no precipitate, the search for sulphuric acid is at an end. If a precipitate form, distil the fluid from a muriate of lime or oil bath, at a temperature not above 240°, till the residuum acquire a thick syrupy consistence; and preserve apart the last sixth of the distilled liquor. In this liquor test for hydrochloric acid by litmus-paper and nitrate of silver, and for acetic acid by litmus-paper, and the odour and taste of the liquid. If these acids be not in the distilled fluid, they are not in the residuum. In a portion of this residuum search for nitric acid, and in another portion for oxalic acid, by the processes for these poisons in complex mixtures. If all these acids be thus proved to be absent, it is most unlikely that the acidity of the mixture is owing to any other but sulphuric acid, especially in the case of the contents or textures of the stomach.

Dilute now what remains of the syrupy extract, and add nitrate of baryta with nitric acid. If a precipitate arise, there is a strong presumption that the acidity of the mixture was owing either to a bisulphate or to free sulphuric acid. And between these the question may be almost settled, first by the probability or improbability of a bisulphate having come in the way, and secondly, by the symptoms and morbid appearances. The result however cannot justify more than a presumptive opinion.—But if hydrochloric, acetic or nitric acid be indicated in the subject of analysis, or an acid sulphate, the whole process is vitiated, and it is scarcely possible to arrive at any trustworthy conclusion.

The difficulties adverted to above have been made the ground-work of various processes; which however seem to me all imperfect.—It has been proposed to divide the mixture into two equal parts, to precipitate one directly by a barytic salt, to do the same with the other after drying and incinerating it, to compare the weight of the precipitates, and to infer the presence of free sulphuric acid if the former is more than double the latter. Various objections however may be brought against this check, not the least serious being its difficulty in ordinary hands, whenever the precipitates are none of them considerable.—Simon proposes to exhaust the residuum of evaporation with absolute alcohol, in the hope that free sulphuric acid will alone be taken up;^[240] but he himself found that neutral sulphates are dissolved partially; and besides, alcohol removes sulphuric acid from bisulphates.—Orfila proposes to remove free sulphuric acid by agitating the concentrated liquor with sulphuric ether, and separating and evaporating off the ether; for he holds that all neutral and acid salts of sulphuric acid are insoluble in ether.^[241] This proposal is unaccountable. Simon stated in his paper three years before, that ether does not remove sulphuric acid from watery fluids containing it. And Dr. Douglas Maclagan and I, on inquiring into the matter, found that we could not, by means of ether, separate a particle of sulphuric acid from an ounce of rice soup and mucilage to which ten drops of the acid had been added. The process of Orfila for establishing the absence of bases in a simple watery solution is applicable to organic mixtures also, after incineration. But if bases be present in material quantity, all the difficulties now in question remain in full force.

b. When the mixture is neutral, sulphuric acid may be detected in it by the first steps of the preceding process. But the inference, that it once existed free can only be drawn when the subject of examination is not in a state of decay, when the quantity of sulphate of baryta obtained is considerable, when the administration of an antidote is proved, and when the ashes after incineration contain the antacid base which is said to have been administered. Even then the inference is only presumptive.