Thioarsenates

Arsenic pentasulfide (arsenic(V) sulfide), As S q, is stable in air up to 95°C, but at higher temperatures begins to dissociate into arsenous sulfide and sulfur. It is prepared by the fusion of arsenic with sulfur foUowed by extraction with ammonia and reprecipitation at low temperatures by addition of hydrochloric acid. Arsenic pentasulfide is precipitated at low temperatures from strongly acidic arsenate solutions by a rapid stream of hydrogen sulfide. It is hydrolyzed by boiling with water, yielding arsenous acid and sulfur. Salts derived from a number of thioarsenic acids are formed from arsenic pentasulfide and alkaH metal sulfides. 

Reacidification reprecipitates AS2S3 quantitatively. With alkali metal or ammonium polysulfides thioarsenates are formed which are virtually insoluble even in hot cone HCl ... 

Thio-ameisensiiure, /. thioformic acid, -anti-monsaure, /. thioantimonic acid, -arsenig-saure, /. thioarsenious acid, -arsenaaure, /. thioarsenic acid, -ather, m. thio ether, -car-baminsaure,/. thiocarbamic acid. 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

It oxidizes in air at elevated temperatures producing arsenic oxides, the products and yields of which depend on the air supply. In alkali metal sulfide solutions arsenic pentasulfide forms thioarsenate anion, [AsS4] and its alkali metal salts, e.g., NasAsS4. 

The reaction in polysulfide solution produces thioarsenate ion, AsS4. It is oxidized by common oxidants including nitric acid, hydrogen peroxide, ozone and permanganate undergoing vigorous to violent decomposition. 

The parent acid H3ASS4 has not been isolated, but the partly oxidized acids H3As02S2 and H3As03S have been isolated43 at low temperatures. The thioarsenate ion has the expected four-coordinate tetrahedral geometry. The (As04 S )3 anions are well known. 

In the thioarsenic halide anions [As2SC15] and [As Cl ]2- the As atoms are bridged by S and Cl.61 In the former the chlorine bridge forms an axial bond with each arsenic atom, which has trigonal pyramidal geometry (24). 

In alkaline solutions of arsenites, thioarsenates are formed, which may be crystallised out, while sulphite or bisulphite remains in solution, thus 4... 

A small quantity of arsenic is precipitated in each case. Sodium dihydrogen arsenite yields a considerable precipitate of arsenic and also of the red disulphide. The polythionates react similarly to thiosulphates, yielding sulphite, thioarsenate and arsenate.5 The per- 1 ites also cause oxidation to arsenate. Sodium hydrosulphite... 

If finely powdered realgar is heated with aqueous sodium sulphide in a sealed tube at 100° C., a thioarsenate is formed and arsenic, which may be contaminated with sulphur, is precipitated.12 Heated with arsenious oxide, metallic arsenic sublimes and sulphur dioxide is evolved.13... 

The salts are usually prepared by the interaction of arsenious sulphide with the metallic sulphide, hydrosulphide or carbonate, taking care to exclude air to prevent the formation of thioarsenates. Thus, Nilson12 obtained the salts of the alkali and alkaline earth metals by dissolving arsenious sulphide in the aqueous solutions of the respective hydrosulphides and concentrating in vacuo. 

A boiling dilute aqueous solution of an alkali thioarsenate on addition of hydrochloric acid yields a precipitate which contains arsenic penta-sulphide and may contain sulphur.2 Under ordinary conditions the precipitation is not quantitative 3 and the excess of sulphur is difficult to remove 4 for complete precipitation the liquid should be kept overnight before filtration.5... 

Arsenic pentasulphide acts as an acid thioanhydride and with metallic sulphides yields salts known as thioarsenates. These may be regarded as derivatives of the following hypothetical acids ... 

The alkali salts are generally formed when arsenic pentasulphide is dissolved in an aqueous solution of the hydrosulphide and the mixture evaporated or precipitated with alcohol, or when a solution of an arsenate is treated with hydrogen sulphide under suitable conditions. The thioarsenates of the heavy metals (except silver, see p. 279) may be formed by the action of an excess of an alkali thioarsenate on a salt of the heavy metal if the latter is in excess the sulphide is formed.4... 

The salts of the alkali and alkaline earth metals, as well as of gold, magnesium, beryllium and yttrium, are soluble in water, the solutions being colourless or pale yellow. They gradually decompose, however, when kept, with deposition of sulphur, arsenic and arsenic pentasulphide. The following thioarsenates have been described. 

Ammonium Thioarsenates. — Ammonium Orthothioarsenate, (NH4)3AsS4, separates as colourless prisms when alcohol is added to a hot solution containing arsenic pentasulphide and excess of ammonium hydrosulphide.5 The crystals gradually turn yellow in air and, when heated, melt and decompose, yielding ammonium sulphide, arsenious sulphide and sulphur. 

Antimony Thioarsenate.—When sodium orthothioarsenate is added to a solution of an antimonious salt, a brownish-yellow precipitate is produced.3... 

Beryllium Thioarsenate is formed in solution when arsenic pentasulphide and beryllium hydroxide are boiled with water for some time.3... 

Calcium Thioarsenates.—Calcium Orthothioarsenate, Ca3(AsS4)a. 20HaO, may be prepared by the methods described for the corresponding barium salt. It yields pale yellow rhombic crystals which are soluble in water. Unlike the barium salt, it decomposes when heated. 

Cerium Thioarsenates.—The addition of sodium orthothio-arsenate to an aqueous solution of a cerous salt produces a pale yellow precipitate of cerous orthothioarsenate.1 With sodium hydrogen ortho-thioarsenate the precipitate approximates in composition to cerous pyro-thioarsenate. Ceric salts also give pale yellow precipitates, probably ceric orthothioarsenate. Thioarsenates of other rare earth metals have not been described. 

Chromium Thioarsenate.—Sodium orthothioarsenate gives a yellow precipitate when added to an aqueous solution of a chromic salt.1... 

Cobalt Thioarsenate.—With solutions of cobalt salts, sodium orthothioarsenate gives a dark brown precipitate of cobalt pyrothio-arsenate, Co2As2S7, soluble in excess of the reagent.1... 

Copper Thioarsenates.—Cupric Orthothioarsenate, Cu3(AsS4)2, is formed as a dark brown precipitate when sodium orthothioarsenate is added to a solution of a cupric salt.2 The reaction, however, is complex, sulphides of copper and arsenic also being formed.3 A similar precipitate is formed when ammonium hydrosulphide or hydrogen sulphide is added to a solution of arsenic acid and a copper salt,4 and the proportion of sulphide and thio-salt in the precipitate varies with the concentration of the reactants. Copper hydroxide reacts with alkali thioarsenates to form copper sulphide and alkali arsenate, but some copper orthothioarsenate is formed and remains in solution in excess of alkali thioarsenate.5... 

Gold Thioarsenates.—Auric Orthothioarsenate, AuAsS4, and auric pyrothioarsenate, Au4(As2S7)3, are obtained as brown precipitates when solutions of gold salts are precipitated respectively with sodium ortho-and pyro-thioarsenates.6 Both salts are soluble in water. 

Iron Thioarsenates.—Both ferrous and ferric salts, when treated with a solution of sodium orthothioarsenate, yield brown precipitates which are soluble in excess of the reagent. The ferric salt is stable in air and may be heated to fusion without decomposition at a higher temperature sulphur is expelled. The ferrous salt decomposes on drying in the air, ferric hydroxide and thioarsenate being formed.6 Ferrous sulphide dissolves in an aqueous solution of an alkah thioarsenate.7... 

Lead Thioarsenates.—The ortho- and pyro-salts are obtained as red and deep brown precipitates when solutions of lead salts are treated respectively with sodium ortho- and pyro-thioarsenates.6 The mineral reniformite (p. 13), occurring in Japan, is probably a thioarsenate of composition Pb3(AsS4)2.2PbS.8... 

Lithium Thioarsenates.—Lithium Orthothioarsenate, Li3AsS4. nH20, is precipitated when alcohol is added to an aqueous solution of... 

Mercury Thioarsenates.—When an alkali orthothioarsenate is added to an aqueous solution of a mercurous salt, a black precipitate of mercurous sulphide separates and a thioarsenate is formed in solution. Berzelius 3 thought this to be the pyro-salt which, when obtained by evaporation and heated, lost mercury and formed the mercuric pyro-salt. Heubach,4 however, stated that mercurous orthothioarsenate was formed and that this was decomposed by excess of mercurous salt to mercurous sulphide and arsenic acid. The mercuric salts are more stable. 

Mercuric Orthothioarsenate, Hg3(AsS4)2, is formed as an orange-coloured precipitate by treating mercuric chloride solution with sodium orthothioarsenate.5 It may be dried at 100° C. and sublimed without decomposition. Mercuric pyrothioarsenate is obtained as a dark yellow precipitate when mercuric chloride solution is treated with sodium meta- or pyro-thioarsenate.8 The salt decomposes on heating with loss of arsenic and sulphur. 

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