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.
An indirect amplification procedure for the determination of niobium by atomic-absorption spectroscopy
Niobium in the range 5 to 50 µg is determined by an amplification procedure in which molybdoniobophosphoric acid is formed and extracted into butanol. The molybdophosphoric acid, which is also formed, is first selectively extracted away from the molybdoniobophosphoric acid into isobutyl acetate. The eleven molybdate ions associated with each niobium atom are determined by direct atomic-absorption spectroscopy in a nitrous oxide-acetylene flame at 313·2 nm. Of the twenty-eight other ions studied, only titanium causes appreciable interference, although its presence at the same concentration as niobium can be tolerated. Large amounts of tantalum do not interfere.
THE IRON-ACETOCARMINE METHOD OF FIXING AND STAINING CHROMOSOMES
A short description of this procedure was given in Vol. 55 of the American Naturalist, pp. 573â€”574. The method has now been used almost daily for five years, and certain improvements have suggested themselves. The solution can be prepared by mixing about equal volumes of glacial acetic acid and water. To this is added powdered carmine in excess. The liquid is then â€ ? just brought to boil, cooled, and decanted. To the red solution there are then added a few drops of a solution of ferric hydrate in 50 per cent. acetic acid, to act as a mordant. The amount of iron varies with different objects. Too much produces a precipitate in a short time. The more