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Additional info for Analytical Chemistry of Niobium and Tantalum
Filter through a 9 cm close texture filter paper and wash 8 times with a cold 4 per cent hydrochloric acid solution. Char and ash the paper and precipitate in a platinum crucible at 600° until the carbon has completely disappeared. Moisten the residue in the crucible with water, add 3 ml of 50 per cent sulfuric acid and 3 ml of concentrated hydrofluoric acid, and heat moderately until solution is complete. Raise the temperature and heat until sulfuric acid fumes are evolved. Continue the fuming for 10 min.
Preserve the filter paper for the final filtration of the pure niobium precipitate, after washing it with water. The washings recovered in a beaker have a volume of about 200 ml and should contain 6-8 ml of 33 per cent sulfuric acid. Boil the solution to remove hydrogen peroxide, cool, and repeat the precipitation with ammonium persulfate as described above. After filtration and washing the precipitate may be ignited and treated for the spectrophotometric determination of the niobium by the thiocyanate-acetone method.
When the ratio is less, two precipitations are required to give equally accurate results. DISSOLUTION OF THE SAMPLE 35 TANTALUM, NIOBIUM AND TITANIUM GRAVIMETRIC D E T E R M I N A T I O N BY T A N N I N , S E P A R A T I O N O F TANTALUM FROM NIOBIUM AND COLORIMETRIC DETERMINATION OF TITANIUM IN EACH— BHATTACHARYAÜ9) Mixed oxides containing tantalum and titanium, niobium and titanium, or tantalum, niobium and titanium can be analyzed by this procedure. Procedure. Apply the complete modified tannin procedure of Bhattacharya <14) for the mixed oxides of tantalum and niobium as described above.