Objection 3. Chromate of potass produces with nitrate of silver a yellow precipitate, which, when placed side by side with one produced by arsenious acid, cannot be distinguished by colour or appearance from it. This fact has lately been announced by Dr. Porter, of the University of South Carolina (Silliman’s Journal, iii. 355); but as the presence of Chromate of Potass can never be suspected in any research after arsenic, in cases of forensic interest, it is unnecessary to enter into any details respecting it.
We have stated above, that in consequence of the inability of arsenious acid to decompose nitrate of silver by simple elective attraction, the presence of some alkali becomes indispensable in the examination; and for this purpose Dr. Marcet suggested the superior advantages which would attend the application of ammonia, in all those cases where the arsenic had not been previously combined with a fixed alkali; since the former does not, when added singly, decompose nitrate of silver; a circumstance which, in using the fixed alkalies, is very liable to occasion fallacy. This led Mr. Hume to improve his original plan, by forming at once a compound,[[254]] which he calls the Ammoniaco-nitrate of silver, but which may with more propriety be designated, as an ammoniuret of that metal.
b. The Ammoniuret of Silver. This is an improvement of considerable value; for, while it obviates the necessity of ascertaining the exact proportion[[255]] of alkali required in each experiment, it possesses the desirable property of not in the least disturbing the solution of phosphate of soda.
c. Sulphate of Copper. This test of arsenic is the one discovered by Scheele; when added to the arsenite of potass a beautiful green precipitate (constituting a pigment known by the name of Scheele’s green) is produced; “so decidedly,” says Dr. Bostock, “does this phenomenon indicate the presence of arsenic, that I thought it desirable to ascertain, as exactly as possible, what were the best proportions in which the ingredients should be employed, and in what way they should be mixed, so as to exhibit the effect in the most obvious manner. After a number of trials, in which the substances were employed in various quantities, and under different circumstances, I am disposed to recommend that the proportions of the arsenic, the potass, and the sulphate of copper, should be to each other as the numbers one, three, and five, respectively; for instance, if one grain of arsenic and three grains of potass, be dissolved in two drachms of water; and, in another equal quantity of water, five grains of sulphate be dissolved, we have two solutions, which are transparent, and nearly colourless; but upon mixing them together, the whole is converted into the most beautiful grass-green, from which a copious precipitate of the same hue slowly subsides, leaving the supernatant fluid nearly without colour. If the same materials are employed, in the same manner, but without the arsenic, a delicate sky-blue is formed, which is so decidedly different from the former colour as not to admit of the possibility of error.” In this experiment then, as well as in that with the nitrate of silver, it is necessary that the arsenious acid should be combined with an alkaline base; and for the same reason, in order to bring the double elective attractions into play; Mr. Hume has accordingly availed himself of the property of ammonia, to form an ammoniuret of copper, which is to be made according to the formula already given for the preparation of the silver test.
d. Ammoniuret of Copper. In using this test care must be taken that it be not too highly concentrated, for in that state it will not produce precipitation.
Notwithstanding the confidence with which Dr. Bostock has supported the pretensions of the Sulphate of Copper, as an infallible test for arsenic, its validity has been lately called in question, and it has been stated that a decoction of onions has the property of imparting to the copper precipitate, produced by a fixed alkali, a green colour and appearance completely analogous to that which is occasioned by the presence of arsenic. This opinion was boldly advanced, and supported, on the trial of Donnall, before alluded to, and of which we have given a very ample report in the Appendix. Since this event an opportunity occurred which enabled the author to examine this alleged fact, by a fair and appropriate series of experiments,[[256]] the result of which has satisfactorily proved that the opinion was grounded on an optical fallacy, arising from the blue precipitate assuming a green colour, in consequence of having been viewed through a yellow medium.[[257]] The phosphoric salts may also, under similar circumstances, be mistaken for arsenic; for the intense blue colour of the phosphate of copper will, when viewed through a yellow medium, necessarily appear green. Such instances of optical fallacy are by no means uncommon in the history of chemical reagents; thus corrosive sublimate has been said to possess alkaline characters, in consequence of appearing to turn the syrup of violets green, whereas this apparent change is to be solely attributed to the optical combination of the yellow hue of the sublimate with the blue colour of the violet.
Whenever therefore such a source of fallacy can be suspected, the operator would do well to repeat his experiment on white paper, in the manner we have already pointed out, when treating of the silver test; and let it be remembered that the results, when obtained in glasses, should always be examined by day light, and viewed by reflected, and not by transmitted light. Dr. Bostock observes, that a weak solution of the sulphate of copper, without any addition, when held between the eye and the window, frequently presents a greenish tinge. It should be also known that the usual reaction of the ammoniuret of copper, upon a diluted solution of arsenic, is prevented by the presence of tannin; strong tea may therefore render the test inefficient.
e. Sulphuretted hydrogen. This is a very delicate test for arsenic, producing with its solution a beautiful golden coloured liquor, which, after a short time, lets fall a precipitate, and which will take place sooner if a small quantity of acetic acid be added. By this re-agent so small a quantity as 1/100000 may be detected in solution. The test, however, is not, says Dr. Bostock, sufficiently discriminative to be depended upon alone; since tartarized antimony and some other bodies, will produce phenomena that may be mistaken for the effects of arsenic. It has, however, the merit of not being affected by tannin, and may therefore be conveniently employed for precipitating arsenious acid, when dissolved in tea.
f. Lime water produces with the solution of arsenic a beautiful white precipitate of arsenite of lime, which easily dissolves in an excess of arsenious acid.
The precipitates occasioned by the foregoing reagents, should be carefully collected, and treated with black flux, in a glass tube, for the purpose of obtaining the metallic sublimate, as above described.