Tungsten Bronzes

The compounds known as tungsten bronzes are reduction products of the tungstates of the alkali and alkaline earth metals. Their exact constitution is not known, but it is generally recognised that the molecule contains several hexavalent tungsten atoms and one tetravalent tungsten atom, and may be represented by the formula R^•₂O.(WO₃)_x.WO₂. The empirical formula accordingly becomes R^•₂.(WO₃)_x+1.

The substances have been designated as tungsten bronzes because, owing to their remarkable properties, they may be used as substitutes for bronze powders. They all possess extremely vivid colours and are quite insoluble in water. Strong acids, including hydrofluoric acid, do not attack them; even hot aqua regia has only slight action. They are decomposed, however, by fusion with alkali, sulphur, ammonium persulphate, or ammonium hydrogen sulphate.

The bronzes may be obtained by the following methods:

1. Reduction of an acid tungstate by heating in hydrogen4 or coal gas, or by fusion with metallic tin, zinc, or iron.
2. Electrolytic reduction of a fused acid tungstate; or of a fused mixture of tungstic acid with the requisite quantity of a metallic carbonate; or of a solution of tungstic acid in a fused mixture of alkali chlorides, the ratio of tungstic acid to chloride being greater than 1:2.
3. Synthesis by fusing together a metallic tungstate, either acid or normal, with tungsten dioxide, in absence of air.

The bronzes are obtained in the crystalline form, generally as cubes or needles, and exhibit colours ranging from golden yellow to violet and dark blue, a metallic lustre also being sometimes evident. The colour sometimes varies according to whether the bronze is dry or moist, and suspensions of a finely powdered bronze often show by transmitted light the colour which is complementary to its usual colour. The bronzes show metallic properties in being of high density, 6.5 to 7.5, and excellent conductors of electricity. The difficulties which are encountered in the analysis of the tungstates are enhanced in the case of the bronzes by their insolubility and resistance to ordinary reagents. The difficulty of determining whether a product is homogeneous or not, and the fact that the same method of preparation will sometimes give totally different products when repeated, lead to doubt as to whether many of the bronzes described in the literature are definite compounds. For example, von Knorre, by fusing together equal molecular proportions of sodium and potassium paratungstates, and heating the mass to redness in hydrogen, obtained on one occasion a purple- red sodium-potassium bronze to which he ascribed the composition 2Na₄W₅O₁₅.5K₂W₄O₁₂, and on another occasion a darker red bronze of composition 2Na₂W₃O₉.3K₂W₄O₁₂.

The bronzes are generally analysed by first oxidising completely to tungstate and then determining the amount of metal and tungstic acid in the product. It is therefore necessary that the oxidation should be really complete and not merely superficial as when the bronze is heated in the air - a method which has sometimes been employed. Wohler suggested heating the bronze with sulphur, decomposing the mass with aqua regia, and estimating the alkali metal and tungstic acid in separate portions of the liquid. A more satisfactory method is due to Philipp.2 The finely powdered bronze is boiled with a large excess of ammoniacal silver nitrate solution, and silver is precipitated which is ignited and weighed. The silver always contains a little tungstic acid, which is left as a residue on treating with nitric acid and can be weighed. The tungsten and sodium are estimated in the filtrate from the silver. In this reaction the quantity of silver precipitated is always proportional to the amount of oxygen necessary for the complete oxidation of the bronze, so that the ratio WO₂:WO₃ is accurately obtained. If the bronze is rich in tungsten it is necessary to heat the mixture to about 120° C. in order to obtain complete decomposition. This reduction of silver nitrate by bronzes is remarkable in view of their stability towards ordinary oxidising agents.