Treatise on Poisons by Sir Robert Christison

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. But at the same time it is equally obvious, that in medico-legal analyses, unless the reagents used be previously known to be free of arsenic, they ought invariably to be subjected in the first instance to the process, whatever it may be, which the analyst proposes to employ for detecting arsenic in a suspected substance.