Namely: 62. WET ASSAYS. 1. Gravimetric assay.-0.5 gramme (7.71 grains) of antimony sulphide is dissolved in aqua regia, some tartaric acid is added, and the solution is filtered. The filtrate is saturated with ammonia and an excess of yellow ammonium sulphide, and digested for some time on the water-bath. It is then filtered, but without bringing the precipitate upon the filter. This is again digested with ammonium sulphide and then filtered. The antimony. sulphide is thrown down out of the filtrate by the addition of diluted hydrochloric acid. The sulphuretted hydrogen is driven off on the water-bath, and the liquid then filtered upon a filter, previously dried at 100° C. (212° F.), and weighed, washed with sulphuretted hydrogen water, the precipitate dried at 100° C. (212° F.) until the weight remains constant. A part of the contents of the filter is then weighed off in a small porcelain boat, placed in a glass tube, and heated in a current of carbonic acid gas (Fig. 58, p. 230), in order to remove the sulphur in excess, until no more of the latter escapes. The Sb,S3, containing 71.8 per cent. Sb, is weighed, and from this the antimony contained in the entire mass in the filter is calculated. 2. Volumetric method.-The antimony sulphide is dissolved in hydrochloric acid, and heated until the odor of sulphuretted hydrogen can no longer be detected. Tartaric acid, or potassium-sodium tartrate, is added, and the solution supersaturated with a cold, saturated solution of sodium bicarbonate in the proportion of about 20 cubic centimeters (1.22 cubic inches) to 0.1 gramme (1.54 grains) of Sb2O3. The result will be a solution, which, on the addition of iodine, will be converted into antimonic acid and hydriodic acid, SbHO2+21+H2O=SbHO3+2HI. Starch solution is now added, and a sufficient quantity of titrated solution of iodine to color the fluid blue is allowed to drop into the solution from a burette. A decinormal solution of iodine is prepared by dissolving 12.7 grammes (196 grains) of iodine in potassium iodide, and diluting this to the bulk of 1 liter (1.76 pint). 2 atoms of consumed iodine correspond to 1 molecule of antimony oxide, or 1 cubic centimeter (0.061 cubic inch) of solution of iodine to 0.0061 gramme (0.09 grain) of antimony. The standard of the solution of iodine is determined by titration with tartar emetic, observing the same conditions regarding the concentration and quantity of the reagents (tartaric acid, sodium hydrocarbonate, etc.) in the subsequent titration. See "XVIII., Sulphur," concerning the indirect determination of antimony in Sb,S, by the quantity of sulphuretted hydrogen evolved thereform. XIV. ARSENIC. 63. ORES. Native arsenic, mispickel, FeAsS, with 46 As; leucopyrite, Fe,As, with 66.8 As; nickel and cobalt ores, etc. 64. FIRE ASSAYS. The object of these is to determine the quantity of arsenic, or of its compounds, which may be obtained from the ores. A. Native arsenic.-300 to 500 grammes (4629.70 to 7716.17 grains) of mispickel or leucopyrite (when arsenious acid is present the ores are mixed with 16 to 20 per cent. powdered charcoal, and with potassium carbonate in case metallic sulphides should be contained in the ore) are introduced into a clay tube, one end of which is closed. A spiral of sheet iron is placed in the open end, and the tube is closed by a sheet-metal cap, which is loosely luted on. The tube is then brought into the furnace, and the charge is gradually heated for one to one and a half hours to a red heat, the end with the spiral projecting from the furnace. When the operation is finished, the tube is allowed to cool, and taken out. The sheet-iron spiral is then taken out and unrolled, and, if the operation has been conducted at the proper temperatures, scales of white, flaky arsenic and some gray powder, both allotropic modifications of arsenic, will fall off. The product is then weighed (FeAsS = FeS+ As and Fe, As3=2Fe As+As). White metallic scales will be the result, when the receiver is small and has nearly the same temperature as that of the arsenical vapors ; and gray powder, when the receiver is large or has a decidedly lower temperature than that of the arsenical vapor, and when it is evolved along with other heated gases (as in the reduction of arsenious acid by charcoal). B. Arsenious acid.-2 to 5 grammes (30.87 to 77.16 grains) of ore are placed in the open end of a refractory glass tube, the other end of which, somewhat drawn out and bent in a right angle, projects into a large Woulf bottle. The end of the tube containing the ore is laid so as to incline somewhat upwards, in a combustion furnace (Fig. 58, p. 230), and heated. The Woulff bottle is connected with an aspirator, when the arsenious acid will deposit itself in the straight and curved part of the tube, and in the Woulff bottle. It is driven by heating, from the horizontal to the bent part of the tube; this is cut off, the arsenious acid is wiped out with a feather, and then weighed together with that contained in the bottle. C. Realgar (red orpiment), with 70.03 As, and yellow orpiment, AsS3, with 60.9 As.-The object of the docimastic tests is to determine the quantity of orpiments which can be prepared from the raw materials in question, or to ascertain the proportions of these to be added in certain technical processes. 1. Assays for the determination of realgar. - Iron pyrites (7FeS2-FeS2+6FeS+6S=23 per cent. S) and arsenical pyrites (FeASS=FeS + As=46 per cent. As) in different proportions (20 to 30 grammes=308.64 to 462.97 grains, or more) are heated in a glass tube closed at one end, or in a glass retort, whereby realgar is sublimed. This is fused in a porcelain crucible, and its color examined. Lighter or darker shades can be given to the color by adding, respectively, sulphur or arsenic. 2. Assays for the determination of yellow orpiment.A mixture of arsenious acid with sulphur (together 10 to 20 grammes, 154.32 to 308.64 grains) in different proportions is heated (generally with 6 to 12 per cent. of sulphur, or less) in a glass retort at a gradually rising temperature, until the sublimation of the yellow product is complete. While the native yellow orpiment is a combination of As,S,, difficult to dissolve in acids, the artificial product consists principally of arsenious acid colored by a few per cent. of arsenic sulphide, and is consequently very poisonous. These may be— 65. WET ASSAYS. A. Gravimetric assays. 1. Wet assay.to 1 gramme (7.71 to 15.43 grains) of ore is ignited in a porcelain crucible at a strong red heat, with 4 to 5 times the quantity of saltpetre, and 13 times the quantity of calcined sodium carbonate, with a thick covering of these fluxes. When the crucible is cold, the potassium arseniate is extracted by lixiviation with hot water, and evaporated to dryness with nitric acid. The dry mass is treated with water, the silicic acid filtered off; the filtrate is treated with ammonia in excess, then with a solution of magnesium sulphate (or at once with magnesia mixture prepared from 110 parts of crystallized magnesium chloride, 700 parts concentrated ammonia, 140 parts ammonium chloride, and 1300 parts water). The liquid is allowed to stand for 12 hours, the ammonium-magnesium arseniate, 2(MgNH4. AsO4)+H2O, is filtered off upon a filter previously dried at 100° C. (212° F.), and weighed; 100 parts of salt dried at 100° C. (212° F.) and weighed, will contain 60.51 parts of arsenic acid, corresponding to 65.21 per cent. of arsenic and 86.08 per cent. of arsenious acid. By igniting very carefully and not too quickly, the magnesium salt passes into Mg, As,O,. Or the sample is digested in strong nitric acid with the addition of a few crystals of potassium chlorate. The solution is diluted with water and filtered. Some lead nitrate in solution is added to the acid liquid, when lead sulphate will be separated, lead arseniate remaining in solution. The precipitate of lead sulphate is filtered off, the filtrate saturated with soda, when the lead arseniate will be separated. This is filtered off, washed, dried, and weighed. 100 parts arseniate=22.2 parts of metallic arsenic, or 29 parts of arsenious acid. 2. Wet method combined with the dry-1 to 1 grammes (15.43 to 23 51 grains) of the substance is fused in the manner indicated under 1, with saltpetre and sodium carbonate, in order to obtain a solution of alkaline 1 Plattner-Richter's Lōthrohrprobirkunst, 1878, p. 651. |