Arsenious acid Solubility

The solubility of AS2O3 in water, and the species present in solution, depend markedly on pH. In pure water at 25°C the solubility is 2.16 g per lOOg this diminishes in dilute HCl to a minimum of 1.56g per lOOg at about 3 m HCl and then increases, presumably due to the formation of chloro-complexes. In neutral or acid solutions the main species is probably pyramidal As(OH)3, arsenious acid , though this compound has never been isolated either from solution or otherwise (cf. carbonic acid, p. 310). The solubility is much greater in basic solutions and spectroscopic evidence points to... 

By the action of nascent hydrogen upon soluble arsenic compounds, as by the introduction of arsenious acid into on apparatus evolving hydrogen —... 

Arsenious oxide is soluble in a number of organic liquids. Thus, 100 parts of absolute alcohol dissolve at 15° C. 0-025 part, and at boiling point 3-402 parts of the octahedral form. The solubility is increased by the addition of water. The vitreous modification dissolves to the extent of 1-060 parts per 100 at 15° C. and addition of water decreases the solubility. Esters of arsenious acid may be obtained by heating, with stirring, a mixture of the oxide and an alcohol in the presence of a hydrocarbon such as benzene, toluene or xylene.7... 

Barium Metarsenite, Ba(As02)3, may be obtained by warming barium chloride with a solution of ammonium arsenite to which acetic acid has been added until arsenious acid is on the point of precipitation. The precipitate is then dried at 100° C.2 It is a white powder, easily soluble in water, but it can also be obtained as a gelatinous mass 3 when a mixture of barium chloride and potassium metarsenite in solution is left to stand for a few hours. On strongly heating it decomposes to form arsenate and free arsenic, but to a much less extent than is the case with the orthoarsenite.4... 

Bismuth Arsenite.—When arsenious acid acts upon a solution of sodium bismuth chloride, a white precipitate of approximate composition BiAsO3.5H20 is formed.6 The substance has not been obtained pure. It is not decomposed by boiling aqueous alkali 7 it is soluble in nitric acid.8... 

Calcium Pyroarsenite, Ca2As205, is a white powder obtained by slowly precipitating a solution of arsenious acid with excess of lime-water, or by adding calcium chloride or sulphate to aqueous ammonium arsenite, and heating the precipitate to 105° C.7 If the product is dried in the air at the ordinary temperature, the monohydrate is obtained. At red heat calcium arsenate is formed. The pyroarsenite is only slightly soluble in water, 100 parts dissolving 0-025 to 0-030 part of the salt. It is more soluble in the presence of alkali chlorides and some ammonium salts, such as the nitrate, sulphate, acetate and succinate.8 It also dissolves in dilute acids. 

Tin Arsenites.—Stannous Orthoarsenite, Sn3(As03)2.H20, was obtained by Stavenhagen 3 by adding a solution of arsenious oxide to a solution containing potassium ehlorostannite and potassium chloride. It may also be obtained 4 by the action of sodium metarsenite or potassium tetrarsenite on a solution of stannous chloride. It is a white amorphous powder, sparingly soluble in water, but readily dissolved by dilute acids and alkalis with separation of metallic arsenic. It is also soluble in excess of arsenious acid. 

The solubility is greater in concentrated arsenious acid solution and there is evidence that the tetroxide exists in the form of arsenious arsenate,7 AsAs04. 

The time required for a definite small amount of iodate to be consumed will be measured by determining the time required for the iodine produced by the reaction (as I3) to oxidize a definite amount of a reducing agent, arsenious acid, added at the beginning of the experiment. Under the conditions of the experiment, arsenious acid does not react directly with iodate at a significant rate but reacts with iodine as quickly as it is formed. When the arsenious acid has been completely consumed, free iodine is liberated, which produces a blue color with a small amount of soluble starch that is present. Since the blue color appears rather suddenly after a reproducible period of time, this series of reactions is commonly known as the iodine clock reaction. 

Benzophenone-4 -arsenious acid, C6H5.CO.CeH4.As(OH)2, results when the foregoing oxide is boiled for several hours witlr a large bulk of water. It forms fine crystals, which have a similar solubility to the oxide. 

Realgar is insoluble in water, but when it is boiled in water, it changes to As S3 generating hydrogen sulphide gas. When it is treated v/ith dilute bromine water, it becomes arsenious acid, and with iodine solution in carbon disulphide, Asl. V/hen it is boiled in nitric acid, it become arsenic acid and sulphuric acid. With dilute ammonia solution no change occurs, but with cone, ammonia solution the crystal surface is attacked. It is only slightly soluble in carbon disulphide or benzene. 

Garrett, A. B., Holmes, O., and Laube, A., 1940, The solubility of arsenious oxide in dilute solutions of hydrochloric acid and sodium hydroxide. The character of the ions of tri valent arsenic. Evidence for polymerization of arsenious acid Journal of the American Chemical Society, v. 62, p. 2024-2028. 

Pure arsenious acid is easily recognised, as a heavy white powder, volatile without smell, sparingly soluble in water, and, when heated with charcoal, black flux, formiate of soda, or cyanide of potassium, yielding a volatile crust of metallic arsenic, the vapours of which have the odour of garlic. 

Figure 1 Oxidation states of volatile and soluble arsenic transformed by biological systems. pKi represents the equilibrium between the protonated cation and neutral forms of the hydroxyl group of dimethylarsinic acid. pKj represents the equilibrium between the neutral form and anionic form (2). Occasionally referred to as arsenous acid (2). HO—As=0 has also been referred to as arsenious acid (52). The term arsenite often refers to any of the As(lll) oxides. The species will be a function of the solute and the pH. Given that the pKa of arsenious acid is 9.2 (52), the protonated form is most likely the predominant form in aqueous environmental and physiological samples below pH 9. Structure commonly used when discussing arsenite in biological transformations, but is most likely not the predominant form in aqueous solutions. Monosodium methane arsonic acid (MSMA) is the commercially available sodium salt.

Hydrolyzes with formation of water soluble salts of dephenyl-arsenious acids. 

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