Testing Trace Bismuth in High Purity Ammonium Metatungstate -- Co-Precipitation-HG-AFS Method
Ammonium metatungstate (AMT) is an important intermediate compound of tungsten wet smelting process, with appearance of white and little yellow crystal. In recent years, as the requirement of tungsten product purity being higher and higher, for controlling and accurate detecting of impurity content in tungsten products, the required analysis of impurity element types gas been more and more. A study has put forward a method of co-precipitation-HG-AFS to testing trace bismuth in high purity ammonium metatungstate; HG-AFS refers to hydride generation atomic fluorescence spectrophotometry, which has shown to have advantages of high sensitivity, good selectivity and small matrix interference. 
Steps of co-precipitation-HG-AFS method to detect trace bismuth in high purity ammonium metatungstate are as follows:
1. Weigh 0.125g (Accurate to 0001g) of high purity (99.995%) ammonium metatungstate sample in 50mL beaker, add a small amount of water, suitable amount of coprecipitation agent and KOH (The standard addition test requires adding standard solution in advance);
2. Carry out filtration after heated for 15min at low temperature, and wash the precipitation and the beaker twice by using KOH solution, again to wash twice with water;
3. Then goes on operation of coprecipitation twice, transfer to 25mL volumetric flask and add 5.0mL HCI, 2mL tartaric acid solution after the precipitation dissolved with hydrochloric acid solution (HCl), then diluted with water to scale, shake well to get mixture solution under test;
4. Test the ready solution by atomic fluorescence spectrometer;
5. Prepare series of different concentrations of bismuth standard solution, test the fluorescence intensity according to running program, and process data with computer, draw standard curve using monadic linear regression equation to calculate the content of bismuth.
The aim of choosing hydrochloric acid (HCl) as Current-carrying medium is to obtain bismuth fluorescence with high strength and stability, while, when the solution acidity is changing among 1~6mol/L, bismuth fluorescence value changed little, however the increasing acidity will help to eliminate the interference.
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