This method affords a separation from iron, aluminium, chromium, manganese, nickel, cobalt, and zinc, and is applicable in the presence of phosphates and silicates. Small quantities of titanium (2-50 mg) may be readily determined.
The titanium content of the solution should not exceed O.lg of TiO2, and the titanium should be present as the sulphate or chloride. Add dilute ammonia to the solution until the odour persists, then (cautiously) lOmL of concentrated sulphuric acid and 40 mL of 10 per cent tannic acid solution. Dilute to 400 mL, stir thoroughly, and cool. Introduce a 20 per cent aqueous solution of'phenazone' (antipyrine; 2,3-dimethyl-l-phenyl-5-pyrazolone) with constant stirring until an orange-red flocculent precipitate is obtained. Stop the stirring, and continue the addition of the phenazone solution until a white, cheese-like precipitate (produced by the interaction of tannic acid and phenazone) is formed in addition to the red precipitate. Boil the mixture, remove the flame, add 40 g ammonium sulphate, and allow to cool with occasional stirring. Filter the bulky precipitate through a Whatman No. 41 or 541 filter paper, supported on a Whatman filter cone (hardened, No. 51), with slight suction, and wash with a solution of 100 mL water, 3 mL concentrated sulphuric acid, 10 g ammonium sulphate, and 1 g phenazone. Dry the precipitate at 100 °C, transfer to a weighed crucible, heat gently at first, and then ignite at 700-800 °C to constant weight. Weigh as TiO2-
Note. If the wet precipitate is heated directly, caking occurs which renders the complete oxidation of the carbonaceous matter very slow. If alkali metals were originally present, the ignited oxide must be washed with hot water, filtered, and re-ignited to constant weight.