Abstract：The kaolin sample is hardly decomposed. During the determination of silicon dioxide in kaolin by silicon molybdenum blue spectrophotometry, there are some problems such as easy polymerization of silicic acid in acid solution and bad stability of silicon molybdenum yellow, etc. The sample was firstly wetted with little ethanol. Then the kaolin sample was decomposed by fusion using sodium hydroxide-sodium peroxide as mixed flux. After fusion decomposing of sample, a large-volume container was used to avoid the polymerization of silicon by hydrochloric acid inverse acidification. 5 mL of anhydrous ethanol and ammonium molybdate solution were successively added into 0.10-0.20 mol/L hydrochloric acid system to improve the stability of silicon molybdenum yellow. Then oxalic acid-sulfuric acid mixture was added to eliminate the interference of phosphorus and arsenic. After reducing silicon molybdenum yellow to silicon molybdenum blue, the solution was determined at 660 nm. Consequently, the determination method of silicon dioxide content in kaolin by silicon molybdenum blue spectrophotometry was established. The results indicated that Beer’s law was obeyed for silicon dioxide with mass concentration in range of 1.00-10.00 μg/mL. The correlation coefficient was 1.000 and the detection limit of silicon dioxide was 0.033 μg/mL. The interference tests of main component (aluminum oxide) and impurity components (ironic oxide, titanium dioxide, calcium oxide, magnesium oxide, potassium oxide, sodium oxide, phosphorus and arsenic) in kaolin were conducted. The results showed that these components had no interference with the determination. The proposed method was applied for the determination of silicon dioxide in two certified reference materials of kaolin, and the found results were basically consistent with the certified values. The relative standard deviations (RSD, n=6) were 0.29% and 0.36%, respectively. The content of silicon dioxide in six actual samples of kaolin was determined according to the experimental method, and the found results were basically consistent with those obtained by secondary hydrochloric acid dehydration gravimetric method in national standard method (GB/T 14563-2008).
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