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(2)O(5), 5% TiO(2), 2% ZrO(2) and 0.5% V(2)O(5) are permissible. Fluorine (at least 2%) can also be tolerated when lithium metaborate is used as flux.
Differential spectrophotometric determination of silica in rocks as α-molybdosilicic acid in presence of phosphate and other interferences
Abstract High concentrations of silica in rocks are determined by means of the stable α-12-molybdosilicic acid formed at around pH 4.2 in acetate buffer. Phosphate interference is avoided by acidification with sulphuric acid. Silica is quantified by differential spectrophotometry at 400 nm against standard rock solutions. Considerable amounts of phosphorus, arsenic and titanium and small amounts of germanium and vanadium do not interfere. The standard deviation is about 0.3%. The method is rapid and suitable for routine analysis of silicate and phosphate rocks.
Highly enantio- and diastereoselective synthesis of β-methyl-γ-monofluoromethyl-substituted alcohols.
Enanatiopure β-methyl-γ-monofluoromethyl alcohols were prepared from the allylic alkylation between fluorobis(phenylsulfonyl)methane with Morita-Baylis-Hillman carbonates. The reaction was catalyzed by using the Cinchona alkaloid derivative, (DHQD)(2)AQN. The origin of the stereoselectivity was verified by DFT methods. Calculated geometries and relative energies of various transition states strongly support the observed stereoselectivity.