Tungsten

TUNGSTEN (wolfram), chemical symbol W, molecular weight 184.0, a hard, lustrous white metal, specific gravity 19.1, melting point about 3350°. In nature it occurs in the form of wolframite [(Fe, Mn)WO₄], scheelite (CaW0₄), and others. Tungsten is used for the manufacture of high-grade steel and filaments for electric lamps. When heated in air, tungsten oxidizes slowly; when dissolved in aqua regia it forms the anhydride of tungstic acid, W0₃, a yellow fusible powder soluble in alkalis, forming salts of tungstic acid—tungstates. Tungstic acid dissolves in acids, forming complex acids; among them phosphotungstic and silicotungstic acids are used as reagents for alkaloids. Phosphotungstic acid is prepared by adding to a solution of 100 parts of sodium tungstate (Na₂WO₄+2H₂O) 500 parts of water, a solution of 70 parts of disodium hydrogen phosphate (Na₂HP0₄) in 500 parts of water, and then acidifying the solution with HN0₃. To obtain silicotungstic acid, a solution of sodium tungstate is boiled with freshly precipitated, thoroughly washed gel of silicic acid. Calcium and magnesium tungstates (CaW0₄ and MgW0₄) in the metallic form are used for the manufacture of screens for fluoroscopy. Tungstic acid, obtained by mixing equal volumes of a 10% solution of sodium tungstate and 2/3 normal sulfuric acid, is widely used as a precipitant for proteins in all kinds of biochemical analyses, in particular, in the analysis of kobs.