Analysis of Tungsten by Visible Absorption Spectrophotometry (USGS laboratories)
1 g of sample is heated at 100°C overnight with 5 ml conc nitric acid and 15 ml conc HF. Two 5-ml aliquots of 80% HCl are added. The sample is centrifuged, and 5 ml of the solution is heated with 5 ml of 30% stannous chloride. 1 ml of 1.5% zinc dithiol solution is added. After heating for 30 min, 4 ml heptane is added, and the sample is shaken. The absorption of the organic layer at 630 nm is then measured with a visible-light absorption spectrophotometer. Primary standards prepared from sodium tungstate are used for calibration. Details may be found in O'Leary (1990) and Welsch (1983).