Determination of silica in silicates by formation of 12-molybdosilicic acid and redox titration of molybdenum(III)

An indirect titrimetric method for the determination of silica in silicates is described. A boron trioxide—lithium carbonate (10:1) mixture is used as flux and the melt is dissolved in an acidic molybdate solution to form 12-molybdosilicic acid. The yellow complex is extracted with methyl isobutyl ketone (MIBK) and, after removal of the co-extracted molybdate reagent, is decomposed with alkali. The resultant aqueous phase is acidified and passed through a Jones reductor to convert the molybdenum(VI) into molybdenum(III), which is then titrated with potassium permanganate. The method has high precision and accuracy. Phosphorus, titanium, zirconium and vanadium, in amounts that may occur in silicate rocks, do not interfere; at least 5% P 2O 5, 5% TiO 2, 2% ZrO 2 and 0.5% V 2O 5 are permissible. Fluorine (at least 2%) can also be tolerated when lithium metaborate is used as flux.