Arsenic trisulphide

Arsenic. The presence of arsenie in an organie eompound is generally revealed by the formation of a dull grey mirror of arsenic on the walls of the test-tube when the eompound is fused with sodium in the Lassaigne teat. Usually sufficient arsenic is found in the fusion solution to give a yellow precipitate of arsenic trisulphide when the solution is acidified with hydrochloric acid and treated with hydrogen sulphide. 

Wilson and Dickenson observed no exchange, over a period of three hours at 100 °C, between arsenate and arsenite ions in media ranging from aqueous acid to aqueous alkali. Martin et al have found similar results for the exchange between arsenate and thioarsenite ions in aqueous media. However, in liquid ammonia exchange occurred between ammonium arsenate and arsenic trisulphide. The isotopic method was used -... 

Various authors have mentioned the particularly dangerous character of the arsenic trisulphide/hydrogen peroxide interaction without giving any details regarding their appearances. 

The trivaleney of the complex ion, [Pt(NH3)5Cl] , is confirmed by conductivity measurements, the chloride having conductivity 404 at 1000 litres dilution, and by the power of the salts to coagulate colloidal solutions of arsenic trisulphide. 

Solutions of arsenic trisulphide in methylene iodide. (See Bryant, 1932.)... 

Vanadium pentoxide sols can be employed to bring about coagulation of positively charged colloids for example, ferric hydroxide and aluminium hydroxide. The amount necessary for the coagulation of a given quantity of the positive colloid is very small in comparison with the required quantities of arsenic trisulphide, antimony trisulphide, and other negative colloids. It appears, therefore, that the colloidal... 

Arsenious oxide dissolves in the boiling trichloride to form arsenic oxychloride, AsOCl (seep. 109). Arsenic trisulphide yields thiochlorides.16... 

Many other methods of preparation have been employed. For example, the triiodide is formed when arsenious oxide,5 or a mixture of this oxide with sulphur,6 is heated in iodine vapour or when arsenious oxide is heated with iodine,7 hydriodic acid,8 a mixture of potassium iodide and acetic acid,9 or a mixture of potassium iodide and potassium hydrogen sulphate.10 When arsenic disulphide and iodine, in the proportions 1 As Ss 3la, are heated together, arsenic triiodide is formed.11 When arsenic trisulphide is fused with an excess of iodine, the product is soluble in carbon disulphide and the solution on evaporation deposits arsenic triiodide, then a sulphiodide and finally sulphur with excess of sulphide the product is the sulphiodide, AsS2I. If a solution of iodine in carbon disulphide is added to arsenic di- or tri-sulphide, the triiodide and sulphur are formed. The triiodide is also produced when a mixture of the trisulphide and mercuric iodide is heated 12. when hydriodic... 

Dry hydrogen sulphide in the cold does not react with arsenic triiodide. At 200° C. some of the latter volatilises and is carried away by the gas stream, while the remainder is converted 6 to a crystalline iodosulphide, A 2S5I. The aqueous solution with hydrogen sulphide yields a precipitate of arsenic trisulphide.7... 

Fusion with sodium thiosulphate yields arsenic trisulphide.7... 

The reaction of arsenic acid solutions with hydrogen sulphide has been the subject of much investigation and it is found that arsenic pentasulphide or arsenic trisulphide may be precipitated as the main product, according to conditions.12 Thus, in an aqueous solution of the pentoxide at the ordinary temperature and in the presence of 8 to 14 per cent, of hydrochloric acid, the precipitate is the pentasulphide as the concentration of hydrochloric acid is increased, the trisulphide and sulphur are also precipitated in increasing proportion, and at 32 to 33... 

Sodium hydrosulphite reduces arsenates and arsenites, yielding precipitates containing sulphides the composition of which varies with conditions. In strongly acid solutions arsenic trisulphide is the main product.2 Sodium thiosulphate also precipitates arsenic as the trisulphide from acid solutions,3 but the amount of precipitation depends on the nature and concentration of the acid present. Thus, with hydrochloric, perchloric or sulphuric acid, the precipitation reaches a maximum of 50 to 80 per cent, for 0-1N acid,4 and above this concentration the amount of precipitation falls to zero with hydrochloric acid but passes through a minimum with perchloric acid at N concentration and with sulphuric acid at 2 to 3N concentration. At still higher acid concentrations precipitation becomes almost quantitative. 

If arsenopyrite is allowed to stand in aqueous hydrochloric acid for some time, the formation of arsenic trisulphide may be observed.5... 

Physical Properties.—Arsenic trisulphide crystallises in short octahedral prisms which were at first described as rhombic 8 but which Groth described as monoclinic.9 The crystals are greasy and possess a lustre which is nacreous at the plane of cleavage. The cleavage on the (OlO)-face is perfect, while that on the (100)-faee shows in traces. The (OOl)-face is a gliding plane.10 The optical character is negative.11 The... 

Arsenic trisulphide does not conduct electricity at the ordinary temperature,5 but if heated above 60° C. conductivity becomes appreciable. The incidence of light appears to be without effect.6 The sulphide exhibits no fluorescence in ultraviolet light 7 it is opaque to X-rays.8... 

Chemical Properties.—Arsenic trisulphide may be reduced to arsenic by heating in a stream of hydrogen the arsenic sublimes. The reaction begins at about 300° C.,10 but proceeds more readily if the sulphide is first fused with an alkali carbonate.11 A similar reduction occurs when the sulphide is heated with a mixture of charcoal and alkali carbonate12 or lime when heated with potassium cyanide,13 an oxalate14 or with a metal such as silver or iron the latter if in excess yields arsenide. 

In ammoniacal solution, arsenic trisulphide is oxidised by hydrogen peroxide to arsenic and sulphuric acids.2... 

When arsenic trisulphide is exposed to dry hydrogen chloride or hydrogen bromide, it liquefies at the ordinary temperature and on heating complete volatilisation occurs.9 It is not readily attacked by halogen acids. When boiled with concentrated hydrochloric acid it is decomposed, but with great difficulty, and the hydrogen sulphide and arsenious chloride evolved reproduce arsenious sulphide in the receiver.10 A similar reaction occurs when heated with a chloride in the presence of concentrated sulphuric acid, but the decomposition is incomplete.11 The reaction is facilitated by the presence of cuprous chloride or ferric chloride. Only a slight reaction is observed with dilute acid,12 and the... 

Arsenic trisulphide is not dissolved by dry liquid hydrogen sulphide.8 The precipitated sulphide obtained by the action of hydrogen sulphide on solutions of arsenious oxide in aqueous hydrochloric or acetic acid is found to contain an amount of sulphur in excess of that required by the formula As2S3, but which cannot be extracted with carbon disulphide. This has been attributed to the formation of a hydrosulphide since, if the precipitate is dried in a vacuum and then heated at 115° C., hydrogen sulphide is evolved. The products vary in composition, but that produced in presence of hydrochloric acid approximates to 16As2S3.H2S and that from acetic acid solutions to 8As2S3.H,S.9... 

Arsenic trisulphide is insoluble in benzene or carbon disulphide it is soluble in an aqueous solution of citric acid or of an alkali citrate.7 An aqueous solution of borax (2 per cent.) dissolves the sulphide slowly in the cold, more rapidly on heating.8... 

According to Gazzi,11 the most highly purified sols contain an excess of arsenious oxide since, on analysis of the hydrosol, the quantity of oxide obtained is always much greater than that which could result from the complete hydrolysis of soluble arsenic trisulphide. Gazzi found the solubility of the precipitated trisulphide to be 0-5166 mg. 

BEHAVIOUR OF MIXED SOLS OF ARSENIC TRISULPHIDE AND FERRIC HYDROXIDE. 

The amount of a positive sol necessary for complete precipitation is not the chemical equivalent of the arsenic trisulphide present and the requisite amounts of various positive sols differ considerably. This is shown in the following list of optimum quantities required for the precipitation of a hydrosol containing 24 mg. of arsenic trisulphide 4... 

---