32. Sulphurets of arsenic.
Arsenic may be combined with sulphur by exposing a mixture of the metal and sulphur or of the white oxide and sulphur, to a heat approaching to redness. In the latter case more sulphur is required, because the oxygen is carried off in the shape of sulphurous acid. Three parts of arsenic with two, three or more of sulphur may be used; the heat should be less if a greater proportion of sulphur is intended to be united. As both the elements are volatile in a moderate heat, and that in unequal degrees, considerable difficulty occurs in ascertaining by the synthetic mode, the proportions of the elements combined; if too little heat be used, only a mechanical mixture is obtained, of any proportions we please; if too much heat be used, part of the arsenic as well as part of the sulphur sublimes, and the sulphuret itself sublimes at a heat not much exceeding that required for their union. Hence in a great measure we have the discordant results of those who have taken the synthetic method. The analytic method is to be preferred, and those who have taken it have succeeded the best; but even this is attended with greater difficulties than with most of the other sulphurets.
The artificial sulphurets of arsenic constitute two varieties chiefly, and these are also found native in various parts of the earth.
Protosulphuret. Native sulphuret of arsenic, called orpiment, is found in Turkey and elsewhere in considerable masses; when broken it exhibits a foliated structure, somewhat flexible, and of a brilliant golden yellow colour. Its specific gravity is usually about 3.2; at least that was the case with the specimen I used. When heated so as to be near melting, its surface reddens, probably by the loss of sulphur. The same sulphuret is procured artificially in the humid way whenever a solution of the oxide of arsenic in water, &c. is treated with sulphuretted hydrogen, or a hydrosulphuret, and afterwards with an acid; or when this or any other species of sulphuret of arsenic is dissolved in an alkali and the solution treated with an acid. Kirwan in 1796 states, that it is generally thought to consist of 100 arsenic and 11 sulphur, but that Westrumb says it contains 100 arsenic and 400 sulphur, which Kirwan thinks more probable; they are both however very wide of the truth. Thenard, in the 59 Vol. of the An. de Chimie, 1806, asserts that it consists of 100 arsenic and 75 sulphur; but he does not point out the experiments on which this result rests; and it is not very near the truth. Laugier in the same An. Vol. 85, for 1813, in a paper of great merit, finds the native orpiment to contain 38 per cent. of sulphur; his method is to dissolve the orpiment in warm dilute nitric acid; to precipitate the sulphuric acid by nitrate of barytes, and from the sulphate of barytes infer the sulphur; the rest he considers as arsenic, not knowing how to detach the arsenic acid from the nitric acid so as to determine the arsenic by experiment. I have pursued this method with the advantage of being able to determine the arsenic as well as the sulphur: Ten grains of orpiment in fine powder were dissolved in 100 measures of 1.346 nitric acid diluted with as much water, by digesting in a heat so as to keep a constant moderate effervescence for about 2 hours. The liquid obtained, being diluted, yielded 536 measures of 1.061. By carefully and gradually dropping in muriate of barytes I found 150 measures of 1.162 just sufficient to saturate the sulphuric acid, and the sulphate of barytes produced dry was 28 grains, the loss I estimated 1 grain: now one third part being sulphuric acid, and ⅖ of the acid being sulphur, we have ²/₁₅ of 29 = 3.87, or 3.9 for sulphur. The residuary liquid was then treated with lime water till an excess was manifest, and produced no farther precipitate; the arseniate of lime was collected and dried, and gave 16 grains. Now I had determined by experiments hereafter to be related, that ⁴/₇ of arseniate of lime are acid and ⅔ of the acid are arsenic; hence ⁸/₂₁ of 16 = 6.1 for the arsenic, which added to 3.9 sulphur, make up the 10 grains of orpiment.
When this orpiment is treated with caustic alkali, it is completely dissolved; it is thrown down by acids I find unaltered. If 61 arsenic combine with 39 sulphur, 100 must take 64 nearly; which corresponds with 1 atom of each, or 21 arsenic + 13 or 14 sulphur.
Subprotosulphuret. Sulphur and arsenic are found native in certain places, combined in masses of a brownish red or orange colour and glassy fracture: this combination is called realgar, and is also manufactured in large quantities in Saxony, chiefly for the use of calico-printers. Its constitution and specific gravity vary considerably, owing chiefly I imagine to the greater or less heat to which it is exposed, and to the proportions of the elements in the first mixture. I have specimens of 3.3 and 3.7 sp. gr.; and it is probable these are not the extremes; the heaviest is the darkest colour. Of course the heaviest contains the most arsenic, and I have reason to believe that the sp. gr. is nearly as good a test of the proportions of the elements as chemical analysis. Realga when pulverized is of an orange colour: it is much sooner dissolved in dilute nitric acid and requires less, than the same weight of orpiment. Caustic alkali dissolves it partially, taking up the protosulphuret and leaving the excess of arsenic, the quantity of which may hence be ascertained. Ten grains of realgar took 80 measures of 1.347 nitric acid, diluted with as much water; digested in a heat of about 150° it was all dissolved in 1½ hour, and yielded 536 liquid of 1.05 sp. gravity. This treated as before gave 24 sulphate of barytes = 3.2 sulphur, and 18 arseniate of lime = 6.9 arsenic. This result agrees nearly with Laugier’s in regard to the sulphur in native realgar: but the artificial realgar, which he made by combining arsenic and sulphur, yielded him 40 per cent. sulphur by my estimation and 42 by his own: the sp. gravity of his artificial realgar is not given. Westrumb estimates realgar at 100 arsenic and 25 sulphur, and Thenard at 100 arsenic and 33 sulphur. But from the above it must be concluded to contain 100 arsenic and 45 to 50 of sulphur. One hundred parts of the same realgar heated in caustic potash were resolved into 78 orpiment taken up by the liquid and 22 arsenic precipitated.
It appears to me most probable that a true subsulphuret would be most convenient for the printers’ use, or one containing 100 arsenic and 32 sulphur, that is, 2 atoms arsenic and 1 sulphur. The object being to deoxidize indigo and obtain it in solution in a green state, we may suppose that 1 atom arsenic takes the oxygen from the indigo and then forms arseniate of lime which precipitates, whilst the other atom in union with the sulphur, takes the green indigo and unites it to the potash, making a quadruple compound of arsenic, sulphur, green indigo and potash in solution. If this view be right the heaviest and darkest coloured realgar of commerce must be the most advantageous for this purpose. Some printers however prefer the protosulphuret.
Deutosulphuret. Proust, by heating 100 arsenic with 300 sulphur in one instance got 222 parts, and in another 234 parts of a transparent deep greenish yellow sulphuret, (Jour. de Phys. 59—p. 406. 1804). Now it is very remarkable that if we take the atom of sulphur at 13 and that of arsenic 21, one of this and two of the former will be found as 100 to 124, together 224; but if sulphur be 14, then the proportion will be 100 to 133, together 233. It seems more than probable that Proust had accidentally used that degree of heat in the combination which is requisite for forming the deutosulphuret. It is probable too that Laugier always used a higher heat, as he uniformly obtained the same (lower) sulphuret whatever were the proportions, the excess of either being sublimed or separated by the heat.
Trisulphuret, quadrisulphuret, &c. When a solution of the oxide of arsenic is treated with quadrisulphuret of lime, little precipitate appears; but if muriatic acid be dropped in, a fine yellow precipitate is formed. This I have reason to think is sometimes a trisulphuret, and at other times a quadrisulphuret or higher; but it is difficult to investigate these compounds, and on that account I speak with some uncertainty.