Copper Arsenites

A natural arsenite known as trippkeite, of composition nCuO.As₂O₃, is found in the form of bluish-green tetragonal crystals (a:c = 1:0.9160) associated with cuprite in Chile. By arsenious oxide under various conditions products are obtained which contain more or less pure arsenites.

Copper Orthoarsenite, Cu₃(NaO₃)₂.2H₂O, was obtained by Sharpies by boiling a solution of copper sulphate with arsenious oxide and sodium carbonate, and drying the precipitate at 100° C. At 150° C. it still retains water. It is soluble in dilute acids and alkalis and in ammonia, but is decomposed by concentrated alkali. The anhydrous salt was described by Stavenhagen as resulting when alcoholic solutions of cupric chloride and potassium orthoarsenite were mixed, the precipitate being dried at 100° C. The product darkened in colour in contact with water and its purity and identity are doubtful.

The pigment first prepared by Scheele in 1778, and known as Scheele's green, is essentially cupric orthoarsenite containing excess of cupric oxide, the composition varying with the mode of preparation. It is best obtained by a method similar to that used by the discoverer. Hot aqueous solutions of arsenious oxide and copper sulphate are mixed and to the mixture is added, in small successive portions, a solution of potassium carbonate, until the precipitate attains its maximum colour intensity. After filtration, the precipitate is washed with hot water and dried at a moderate temperature. Similar products are obtained by treating aqueous arsenious acid or potassium hydrogen di-arsenite with a hot aqueous or ammoniacal solution of a copper salt. The use of this pigment, owing to its poisonous nature, is prohibited. It resembles Emerald green, but is in every way inferior to that pigment. Pigments prepared by slightly modified processes, but of essentially the same composition, are known under many designations, including Swedish green, Vienna green, Mitis green and Veronese green. Bloxam stated that when the green precipitate was dried in the air, its composition corresponded with copper pyroarsenite, Cu₂As₂O₅ or 2CuO.As₂O₃, with 2.89H₂O, and when dried at 100° C. to yield Scheele's green it was CuHNaO₃ (copper monohydrogen arsenite). Other investigators maintain that the composition approximates more closely to the normal orthoarsenite; but it is undoubtedly variable and the amount of cupric oxide present increases with the amount of alkali used in its preparation. The compound decomposes on heating, giving off arsenious oxide and water and leaving a residue of copper arsenide and arsenate.

Efforts to prepare an improved Scheele's green resulted in 1814 in the production of Emerald green, cupric aceto-arsenite, the colour of which is a nearly normal green, slightly verging upon bluish-green. It is brighter and more opaque than Scheele's green, and like it is extremely poisonous. It is generally prepared by adding a solution of verdigris in acetic acid to a boiling solution of white arsenic; on continued ebullition the Emerald green is deposited. A mixture of copper sulphate, acetic acid and potassium arsenite may also be employed. The product must be washed free of soluble salts with boiling water. The pigment is known also under the names Schweinfurt green and Paris green. The composition approximates very nearly to 3Cu(AsO₂)₂.Cu(C₂H₃O₂)₂. The pigment dissolves in boiling dilute mineral acids. After boiling for some time with a large excess of arsenious oxide solution, the substance suddenly becomes paler in colour and is converted to anhydrous copper metarsenite, Cu(AsO₂)₂, a grey crystalline powder. The dihydrate, Cu(AsO₂)₂.2H₂O, was obtained by Stavenhagen as a pale green amorphous precipitate by adding a solution of arsenious oxide in 50 per cent, aqueous alcohol to a slightly acid solution of cupric chloride in alcohol. The precipitate was dried over sulphuric acid. At 100° C. it lost 1 molecule of water; above this temperature arsenious oxide was also liberated.