Abstract:

A rapid microwave digestion technology for the determination of silicon dioxide in crude sodium fluoride by potassium fluorosilicate titrimetric method with HNO₃ and HF mixed acid as the digestion agent was studied.Saturated potassium chloride solution was selected to instead of solid potassium chloride as precipitant,and the effect of digestion acid system,saturated potassium chloride dosage,precipitation temperature,precipitation time,interference of coexisting elements and selection of indicators on the determination were investigated and optimized.The optimal test condition was determined that the precipitate was filtrated and washed with 50 g/L potassium chloride-50% ethanol solution after placing for 15 min and the precipitation temperature was controlled at 20~25 ℃ with 8 mL nitric acid+2 mL hydrofluoric acid as digestion solvent and 8 mL saturated potassium chloride solution as the precipitant.The color of the endpoint of the titration changed obviously by using bromothymol blue and phenol red as a mixed indicator. This method was applied to the determination of the silicon dioxide content in the recovered crude sodium fluoride products,which could greatly shorten the time to decompose the sample,and effectively reduce the detection cost.The recovery rate was within the range of 99.15%~100.63% and the relative standard deviation（RSD,n=8） was less than 1%.The results were consistent with those obtained by traditional high-temperature alkali fusion method.The method showed high accuracy and precision.

Key words: microwave digestion, potassium fluosilicate titrimetric method, sodium fluoride, silicon dioxide