Sulfides, antimonious precipitation

Hydrogen sulfide forms precipitates of several metal sulfides when passed through an aqueous solution of metal salts. Under acid conditions, several metals including arsenic, antimony, bismuth, cadmium, copper, lead, mercury, and tin are precipitated as their sulfide e.g. ... 

Stock solutions Nos. 1 and 2 contain those elements whose sulfides are precipitated from strongly acid solution. High-purity oxides of all these elements except antimony are readily soluble in hydrochloric acid. Antimony must be prepared as a separate stock solution because of its tendency to hydrolyze and precipitate as a basic chloride in dilute hydrochloric acid solution. This stock solution is stable for at least 30 days. 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

Iron Precipitation. Rich sulfide ore or Hquated antimony sulfide (cmde antimony) is reduced to metal by iron precipitation. This process, consisting essentially of heating molten antimony sulfide ia cmcibles with slightly more than the theoretical amount of fine iron scrap, depends on the abihty of iron to displace antimony from molten antimony sulfide. Sodium sulfate and carbon are added to produce sodium sulfide, or salt is added to form a light fusible matte with iron sulfide and to faciHtate separation of the metal. Because the metal so formed contains considerable iron and some sulfur, a second fusion with some Hquated antimony sulfide and salt foHows for purification. 

Hydrochloric acid digestion takes place at elevated temperatures and produces a solution of the mixed chlorides of cesium, aluminum, and other alkah metals separated from the sUiceous residue by filtration. The impure cesium chloride can be purified as cesium chloride double salts such as cesium antimony chloride [14590-08-0] 4CsCl SbCl, cesium iodine chloride [15605 2-2], CS2CI2I, or cesium hexachlorocerate [19153 4-7] Cs2[CeClg] (26). Such salts are recrystaUized and the purified double salts decomposed to cesium chloride by hydrolysis, or precipitated with hydrogen sulfide. Alternatively, solvent extraction of cesium chloride direct from the hydrochloric acid leach Hquor can be used. 

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

In the following year, however, he concluded that tire mineral contained neither bismuth sulfide nor antimony, that the gold was an essential constituent of it, and that it contained an unknown metal. In an investigation lasting three years and consisting of more than fifty tests, he determined the specific gravity of the mineral and noted the radish odor of the white smoke which passed off when the new metal was heated, the red color which the metal imparts to sulfuric acid, and the black precipitate which this solution gives when diluted with water (3). 

The bomb contents are digested with concentrated hydrochloric acid, and material still undissolved is then digested with potassium hydroxide and hydrogen peroxide. A crude separation is made by a sulfide precipitation from the combined digestion solutions. The sulfides are dissolved in aqua regia, the solution is evaporated, and antimony in the residue is reduced to antimony (III) with hydroxylamine hydrochloride. The sample, in ammonium thiocyanate-hydrochloric acid medium, is loaded onto a Dowex 2 column (SCN" form). Arsenic and other impurities are eluted with aliquots of more dilute ammonium thiocyanate-hydrochloric acid solutions. Antimony is eluated with sulfuric acid and fixed in solution by addition of hydrochloric acid. The activity of the solution caused by the 0.56 MeV y-ray of 2.8-day 122Sb is counted. 

Three and four-tenths grams of antimony trisulfide is prepared by passing hydrogen sulfide into a solution of 6.6g potassium antimonyl tartrate (4.5g of the trichloride or 2.9g of the trioxide may be used instead) in 2N hydrochloric acid until precipitation is complete. 

The antimony sulfide required as starting material is easily mada by dissolving either 46g of antimony trichloride or 29g of the trioxide in 2N hydrochloric acid and precipitating the trisulfide with hydrogen sulfide. The moist product may be used directly. 

Bismuth(lII) sulfide. Bi.Si. is precipitated by H2S from bismuth solutions. Complex sulfide ions form only slowly, so bismuth sulfide may be separated from the arsenic and antimony sulfides by this difference in properties. Lake the oxide, bismuth sulfide forms double compounds with tile sulfides of the oilier iuelids. 

During the fifteenth century the metals zinc, antimony, bismuth, and probably cobalt were discovered, together with many new reactions now used in quantitative analysis. For example, A. Libavius (1540-1616) noted how ammonia in water could be determined by the blue color formed with a copper salt. Robert Boyle (1627-1691) was the first to use a solution of hydrogen sulfide (which he made from flowers of sulfur, potash, and ammonium chloride) as an analytical reagent, and he noted the black precipitate it formed with lead, gold, and mercury. 

Radiochemical purification of the induced activity in the presence of carrier involves chemical operations such as precipitation, distillation, solvent extraction, chromatography, and ion exchange. While the chemistry performed on the carrier and sample should be designed to isolate the material in a pure state, a useful operation frequently carried out is scavenging. Strongly adsorptive precipitates such as ferric hydroxide, lanthanum fluoride, and antimony sulfide may be formed in the solution. These precipitates, by coprecipitation, occlusion, and surface adsorption can be used to remove unwanted traces of activity. A scavenging agent should be chosen that wdll not carry down appreciable amounts of the carrier from solution. An alternative method is to add hold-back carriers for the unwanted traces of activity and precipitate the required element in their presence. 

The second step is a precipitation of sulfide-forming cations mainly applied to separate molybdenum, but others like arsenic, antimony, bismuth, cobalt, etc. are precipitated, too. 

Separation. — From most of the metals, germanium may be separated by the formation of the sulfo-salts with ammonium sulfide. It may be separated from arsenic, antimony, and tin by exactly neutralizing the sulfo-salts with sulfuric acid and filtering after 12 hours. Evaporate to small bulk, add ammonia, ammonium sulfate, and sulfuric acid and saturate with H. GeS2 precipitates while the other metals remain in solution. 

The alkaline earth group as a whole stands in marked contrast to transition metals and post-transition metals. For example, most of the metals in the periodic table form insoluble precipitates with the sulfide ion (S2), with the result that sulfide ores of transition and post-transition metals are very common in Earths crust. Common examples of metal sulfides include galena (lead sulfide), cinnabar (mercury sulfide), gree-nockite (cadmium sulfide), acanthite (silver sulfide), cobaltite (cobalt arsenic sulfide), sphalerite (ZnS), stibnite (antimony sulfide), several copper sulfides, orpiment and realgar (both forms of arsenic sulfide), and pyrite (iron sulfide). None of the alkaline earths, however, are found as sulfides. 

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