Manganese Arsenates

Manganous Orthoarsenate, Mn₃(AsO₄)₂.H₂O, is obtained as brown needles when manganese sulphate is heated with sodium orthoarsenate in a sealed tube at 175° C.

Commercial manganese arsenate, consisting mainly of the normal salt, Mn₃(AsO₄)₂, is sometimes used, with or without diluents, for combating plant pests. It is obtained by the reaction between a manganous salt and normal sodium arsenate, by heating intimate mixtures of manganese compounds with arsenious oxide in the presence of an oxidising agent, or by treating arsenious oxide with manganese compounds having an oxidising action in the presence of water.

H₂O.Manganous Monohydrogen Orthoarsenate, MnHAsO₄.H₂O, is produced by saturating aqueous arsenic acid with manganese carbonate; it separates from solution as sparingly soluble rectangular tablets, and is more readily precipitated by addition of alcohol. When heated to 140° C. it loses its water of crystallisation, and at a higher temperature it forms the pyroarsenate. The solubility in water at the ordinary temperature is 0.013 gram of salt in 100 c.c. of water. Hot water converts it to manganic arsenate. When heated in hydrogen or carbon monoxide, arsenic is volatilised and oxide of manganese remains. It readily dissolves in arsenic acid to form manganous dihydrogen orthoarsenate, Mn(H₂AsO₄)₂, which crystallises in rectangular plates. The latter may also be obtained from a solution of manganese carbonate in the theoretical quantity of arsenic acid. The crystals deliquesce in air. The monohydrate, Mn(H₂AsO₄)₂.H₂O, is formed in the presence of excess of arsenic acid, although if the latter is present in large excess the acid salt, Mn(H₂AsO₄)₂.H₃AsO₄, separates in micro-crystals. Both salts are hydrolysed by water and are stable only in the presence of an excess of arsenic acid.

Colloidal manganese arsenate may be obtained in an analogous way to that described for cadmium and cobalt arsenates. If concentrated solutions of manganous chloride and potassium dihydrogen arsenate are mixed at the ordinary temperature, a solid jelly is formed, which remains stable for weeks; but finally, rose-coloured crystals separate and the jelly partly liquefies. The essential constituent of the jelly appears to be the monohydrogen arsenate, MnHAsO₄, and the crystals are the monohydrate, MnHAsO₄. H₂O.

Manganous Pyroarsenate, Mn₂As₂O₇, results when manganous monohydrogen orthoarsenate or manganese ammonium arsenate is strongly ignited, or when the oxide or carbonate is dissolved in fused potassium dihydrogen orthoarsenate. The tabular crystals are colourless, but when fused a red glass is formed.

Manganic Orthoarsenate, Mn₂(AsO₄)₂.2H₂O, may be prepared by treating a hot aqueous solution of arsenic acid with a concentrated solution of manganese nitrate. It forms a dark grey powder which dissolves slowly in hydrochloric acid but is insoluble in nitric acid. When heated to redness it forms manganous pyroarsenate.

When manganous salts undergo oxidation in the presence of arsenic acid, a violet colour develops, due to the formation of triarsenato-manganic acid, H₆[Mn(AsO₄)₃].3H₂O. This may be obtained in the form of violet-red crystals by dissolving manganic hydroxide or acetate in cold concentrated arsenic acid solution and allowing to evaporate, or by adding potassium permanganate to a solution of manganous dihydrogen arsenate in aqueous arsenic acid. The product may be recrystallised from aqueous acetic acid. Prolonged contact with water causes hydrolysis to arsenic acid and manganic orthoarsenate.

Mangani-arsenic Acid, Mn(H₂AsO₄)₄, containing quadrivalent manganese, may be prepared by dissolving hydrated manganese dioxide in concentrated aqueous arsenic acid, or, more conveniently, by the action of arsenic acid on a permanganate, or by the interaction of arsenic acid with a mixture of manganous nitrate and a permanganate. It is a flesh-coloured microcrystalline powder, insoluble in water, but hydrolysed by prolonged contact to arsenic acid and hydrated manganese dioxide. An ammonium mangani-arsenate, of composition (NH₄)₂H₂MnAs₂O₉ or O:Mn:[AsO₄(NH₄)H]₂, has been obtained by adding aqueous ammonia to a suspension of the above acid in water.

Double salts with the alkali metals have been obtained, and a number of mixed arsenates with calcium, magnesium, lead, iron and other metals occur in Nature, as also do several basic arsenates. An acid arsenate of composition 5MnO.2As₂O₅.6H₂O has been obtained in the crystalline form by stirring manganous carbonate with aqueous arsenic acid.