Determination of Silica in Geological Samples by Silicon-Molybdenum Blue Spectrophotometry Using High-pressure Acid Digestion

Abstract

BACKGROUND Polymerized silicic acid cannot be quantitatively complexed with molybdate in geological sample, therefore the silicon is determined by silicon-molybdenum blue colorimetric spectrophotometry. The key of this method is how to digest the solid sample to prepare a solution and ensure that the silicic acid occurs as a single molecule. OBJECTIVES To completely digest the geological samples by high-pressure acid digestion without loss of silicon and polymerization of silicic acid. To improve the stability of silicon-molybdenum yellow by adding a stabilizing agent. METHODS Hydrofluoric acid-nitric acid was used to digest the geological samples, so that the silicon in the sample was completely converted into fluorosilicic acid and was stably present in the solution. Boric acid and ammonium molybdate were added to make the excess hydrofluoric acid and boric acid to form stable BF4- dosing ion, which replaced the evaporation and removal of hydrofluoric acid, thus avoiding the volatilization loss of silicon and fluoride ions converted into gaseous SiF4 during the acid removal process. Under the action of ammonium molybdate, silicon was fully converted into silicon molybdenum heteropolyacids. In the color development process, the stability of silicon-molybdenum yellow was significantly improved by adding acetone, and the absorbance was increased to some extent, thereby improving the color development effect of silicon by silicon-molybdenum blue pectrophotometry. RESULTS The method avoided the problem that the silicic acid was easy to be polymerized during the acidification process when using the conventional alkali fusion digestion. A large amount of flux was introduced to cause a high blank. The validity of the method was evaluated by analyses of rock, soil, sediment and quartzite reference materials. The results of SiO2 were in good agreement with certified values and the relative standard deviation (n=10) was less than 1%. CONCLUSIONS This method is suitable for the rapid and accurate analysis of silica in common geological samples.

Keywords

• rock
• soil
• sediment
• silica
• hf-hno3 dissolution
• high-pressure digestion
• spectrophotometry