Sodium Arsenites

It is difficult to obtain arsenites of sodium in the crystalline state and there is some doubt as to the possibility of preparing them in a pure form. A study of the ternary system Na₂O-As₂O₃-H₂O at 30° C. suggests the existence of solid arsenites of composition NaAsO₂, Na₄As₂O₅ and Na₁₀As₄O₁₁.26H₂O. As the proportion of alkali in the solution increases, the solubility of the arsenious oxide increases rapidly. By digesting the latter with excess of alcoholic sodium hydroxide and extracting with alcohol, a product approximating in composition to sodium orthoarsenite, Na₃NaO₃, has been obtained, and Vanzetti, in an attempt to obtain this compound in the absence of water, dissolved arsenious oxide in a methyl alcohol solution of sodium methoxide, using quantities in accordance with the reaction

As₂O₃ + 6NaOCH₃ = 2Na₃NaO₃ + 3(CH₃)₂O

The excess of alcohol was evaporated off to give a syrup, which was then left to crystallise in a vacuum desiccator containing calcium chloride. The crystals formed were highly hygroscopic and varied in composition, the arsenic content diminishing from about 80 per cent, in the first fractions to about 20 per cent, in the last of that required by the formula Na₃NaO₃. Very little ether was apparently liberated during the reaction, which did not go to completion. The crystals were white, dissolved readily in water, and the solution with silver nitrate gave a precipitate of yellow silver orthoarsenite.

The equivalent electrical conductivity of the aqueous solution has been determined by several investigators. Walden, using solutions containing one-third of a mole of Na₃NaO₃ in v litres, obtained the following values:

v	32	64	128	256	512	1024
Λ	158.3	160.1	161.1	160.6	156.7	154.3

Using the dihydrogen salt, NaH₂NaO₃, von Zawidzki determined the equivalent conductivity in the presence of N/32 arsenious acid in order to diminish hydrolytic dissociation, and concluded that it resembled the salt of a monobasic acid. At extreme dilutions the equivalent conductivity increased, apparently owing to hydrolysis and oxidation. This view that arsenious acid is essentially a feeble monobasic acid is supported by Thomsen's thermochemical values for the heats of neutralisation of the acid.

The aqueous solution of sodium arsenite undergoes slow oxidation when exposed to the air; but according to Reinders and Vies, this can only proceed in the presence of a catalyst. Suitable catalysts are finely divided copper and copper salts; the former dissolves in a cold solution of sodium arsenite in the presence of oxygen. Oxidation may readily be effected at the ordinary temperature by passing air through a solution of the arsenite containing in suspension copper powder, cuprous oxide, cuprous chloride, zinc dust or yellow phosphorus. The oxidation is more readily effected in the presence of a second easily oxidisable substance, such as stannous chloride, manganous or cobaltous hydroxide, or sodium sulphite; with the last-named the oxidation of the arsenite is not induced if the solution is strongly alkaline. The oxidation of the added substance may be retarded by the presence of the sodium arsenite; this is the case with stannous salts, sulphites, phosphorus, chloroform and certain aldehydes. The arsenite is also oxidised in aqueous solution by nitric oxide,

Na₃NaO₃ + 2NO = Na₃AsO₄ + N₂O

and by hydroxylamine,

Nitrogen is also evolved due to simultaneous decomposition of hydroxylamine as follows:

3NH₂OH = NH₃ + N₂ + 3H₂O

With sodium thiosulphate the arsenite forms oxythioarsenates, as it also does with tri- and tetra-thionates; sodium dithionate does not react either in cold or boiling solution. Sodium tellurate causes oxidation to arsenate. An ammoniacal solution of silver azide is reduced to silver by sodium arsenite; other metallic azides do not react.

Sodium orthoarsenite is converted largely to arsenate and free arsenic when heated in an inert atmosphere; slow heating below 300° C. causes loss of arsenious oxide. Fusion with sodium hydroxide causes conversion to arsenate.

Schreinemakers and de Baat's study of the system Na₂O-As₂O₃-H₂O afforded evidence of the formation of sodium pyroarsenile, Na₄As₂O₅, both in the anhydrous state and as the hydrated salt, Na₄As₂O₅.9H₂O, but the isolation of these compounds in a pure form is difficult.

Sodium Metarsenite, NaAsO₂, may be prepared by boiling an aqueous solution of sodium carbonate with an excess of arsenious oxide and evaporating the solution to dryness. It is a white powder which, when heated, yields arsenic and sodium arsenate. It undergoes oxidation by atmospheric oxygen under pressure, and determinations of the velocity of the reaction at 100° to 250° C. show it to be a reaction of the first order. The addition of sodium metarsenite to excess of a solution of a metallic salt generally causes precipitation, but the composition of the precipitate varies with the nature of the salt. Thus the orthoarsenite is obtained with nickel and lead nitrates, zinc sulphate and stannous chloride. With stannic chloride a basic arsenite, 7SnO₂.As₂O₃, is formed. Cobalt nitrate and cadmium sulphate yield the corresponding pyroarsenites; metarsenites do not appear to be precipitated by this means.