Estimation of Tungsten

The most convenient form in which to weigh tungsten is the trioxide WO₃, which does not volatilise even at high temperatures. For the analysis of tungsten ores there are three methods of procedure in common use: (1) A solution is obtained by digesting the finely powdered mineral with equal parts of hydrochloric and hydrofluoric acids. If tin is present it remains behind as stannic oxide. The solution is evaporated with excess of hydrochloric acid, and then boiled with a mixture of hydrochloric and nitric acids until all the hydrofluoric acid is expelled and the tungsten separates out as tungstic acid. This is dissolved on the filter by means of a little warm dilute ammonia, the solution being received in a weighed platinum dish. After evaporation on a water-bath the residue is ignited and weighed as WO₃. (2) The powdered ore is fused with alkali as already described and the mass extracted with boiling water. The solution contains alkali tungstate and any alkali silicate and stannate (if tin is present). After evaporation to dryness with excess of nitric acid, the residue is heated in an oven at 120° C. and then treated with a dilute solution of ammonium nitrate slightly acid with nitric acid. The residue is ignited in a platinum crucible with free access of air, when tungstic anhydride remains. The residue may contain silica, which can be removed by means of hydrofluoric acid, and a trace of stannic oxide, which may be volatilised as stannic chloride 4 by ignition with ammonium chloride. (3) The finely powdered mineral is heated, but not boiled, with aqua regia, the liquid evaporated to about one-third of its bulk and then diluted with water. After standing, the solution is decanted and the residue washed by decantation with hot water acidulated with hydrochloric acid. The residue is next treated with a solution of ammonia containing a little ammonium chloride until all the yellow tungstic acid is dissolved and a white residue of silica remains. The ammoniacal solution is evaporated to dryness in a platinum dish, the residue is gently heated to expel ammonium salts, and then strongly ignited. After cooling, it is moistened with hydrofluoric acid, evaporated to dryness and again ignited, and finally weighed as WO₃.

Many modifications of the above methods have been described.1 If arsenic is present in the ore it may be removed by treating the aqueous extract - after fusion with sodium carbonate and sodium peroxide - with phosphoric acid. This forms a complex with the tungsten, and the arsenic may be distilled off. It has been suggested that the formation of complex silicotungstates may be a source of error in the above methods of analysis. Tungstates may conveniently be estimated by precipitating the mercurous salt and converting it to the trioxide by ignition. Use has been made of the fact that tungstic acid volatilises when heated in a current of carbon dioxide saturated with carbon tetrachloride vapour. By collecting the product, evaporating with nitric acid and igniting, the pure anhydride is obtained. According to Gutbier and Weise, tungstic acid in acetic acid solution is quantitatively separated by means of Busch's "nitron" acetate reagent. Organic precipitants have also been employed.

Reliable volumetric methods for the estimation of tungstic acid are not known. An indirect method recently suggested for use with low-grade ores consists in precipitating tungstic acid from a slightly acid solution by means of cinchonine hydrochloride, dissolving the precipitate in ammonium acetate solution, and adding an excess of a standard solution of lead acetate. After standing half an hour the excess of lead acetate is titrated with ammonium molybdate. Another volumetric process employs benzidine hydrochloride as precipitant. The washed precipitate is mixed with water and titrated at 60° C. with decinormal sodium hydroxide, with phenolphthalein as indicator. It is necessary to boil at the end point in order to keep the solution free from carbon dioxide. The benzidine tungstate is readily hydrolysed, so that the result gives the total WO₃ present.

Colorimetric methods of determination have been suggested. For the determination of the tungsten in commercial forms of the metal, gravimetric methods similar to the above are available. The metal, when in the finely powdered condition, readily dissolves in a mixture of ammonia and hydrogen peroxide, or in ammonium persulphate; when massive, it is best dissolved electrolytically, using ammonia containing a little ammonium persulphate in a platinum dish as cathode, but small pieces will also dissolve readily in fused alkali nitrate.

Lottermoser has described two methods for estimating the amount of colloidal tungsten in commercial tungsten powders. In the first method the powder is shaken with water and the mixture allowed to sediment for two days. The supernatant liquid is removed and replaced by water, and the shaking and sedimentation repeated as long as there is any measurable quantity of tungsten in the liquid. The sediment, when dried and weighed, gives the non-colloidal portion. The second method consists in examining the solutions from which the powder has sedimented. The quantity of colloidal tungsten present is estimated by means of its absorption of light from a quartz mercury lamp, which passes through the solution on to a potassium photoelectric cell. The absorption is measured by deflection of a galvanometer.

In the analysis of tungsten steels the sample may be dissolved in hydrochloric acid or dilute sulphuric acid in absence of air. The tungsten remains undissolved and is ignited, fused with sodium carbonate, and estimated by ordinary methods.

Ferrotungsten alloys decompose completely when fused with ammonium sulphate and concentrated sulphuric acid. If the mass is extracted with water the iron may be separated from the solution by means of ammonia; the tungstic acid remains in the residue, and any going into solution may be recovered by addition of hydrochloric acid. A method of analysis which appears suitable for rapid industrial control of the proportion of tungsten in iron alloys and tool steels employs a 50 per cent, solution of nitric acid, containing a little hydrofluoric acid in order to dissolve the sample. Subsequent digesting with concentrated sulphuric acid eliminates hydrofluoric acid and causes complete precipitation of tungstic acid, which may be ignited and weighed in the usual way.