Direct Determination of Fluoride in Tungsten Using the Fluoride Ion Activity Electrode Bruce A. Raby and William E. Sunderland Lawrence Radiation Laboratory, Uniuersity of California, Livermore, Califo
THECOMMON METHOD for determining fluoride in tungsten has been pyrohydrolysis (1, 2). The method has some disadvantages: the expense of a platinum furnace tube; the possibility of fluoride loss in or contamination by furnace tubes which are not platinum; and the volatility of WOz+z derivatives which interfere because of reaction with the reagents. When other methods are used to decompose the sample, fluoride must be separated from many elements to avoid their interferences. The usual separation methods, such as distillation (3) and anion exchange (4,are generally difficult. These considerations motivated the development of a simple fusion decomposition which circumvents the uncertainties of pyrohydrolysis. Use of the fluoride ion activity electrode allows direct measurement of fluoride without a separation step. EXPERIMENTAL
Procedure for Dissolution of the Sample. Weigh a 0.25to 1.5-gram sample of tungsten into a 50-ml platinum crucible. Cover the sample with 0.6 gram of sodium nitrate plus 1.0-1.2 grams of sodium hydroxide. Cover the crucible with a platinum lid and place it in a furnace whose temperature is less than 250" C. Over a 30-minute period, raise the temperature to 550" C ; hold at that temperature for 5 or 10 minutes. Remove the crucible from the furnace and cool it for about 5 minutes. Remove the lid and rinse any spattered material into the crucible. Rinse the inside and rim of the crucible, filling it about half full with water. Stir with a Teflon-covered stirring bar until the melt is dissolved. It is unnecessary t o remove the brownish platinum oxide which will be present. If the sample is a water-soluble salt, dissolve the sample in water. Choose a sample size which contains a n amount of metal in the range indicated above. Apparatus for pF Determination of Fluoride in Tungsten. A model 94-09 fluoride ion activity electrode (Orion Research, Inc.) is used with an expanded-scale pH meter. Very dilute solutions of fluoride (
