By Ross W. Moshier
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Extra resources for Analytical Chemistry of Niobium and Tantalum
5 ml of 10 per cent oxalic acid dihydrate solution, and immediately 25 ml of 10 per cent selenous acid solution. Dilute the mixture to 150 ml with water. Cover the beaker and digest on a steam bath for 3 hr, add filter paper pulp, and filter through a dense paper. Wash the precipitate with a hot 10 per cent hydrochloric acid solution. Char and ignite the paper and precipitate at 900°, cool and weigh. Repeat the entire procedure given above on the ignited residue, once if the ignited residue weighs no more than 15 mg, and twice if the weight is greater than 15 mg.
Treat the hot solution with a fresh solution containing 1 g of magnesium sulfate, 2 g of ammonium chloride, 4 drops of ammonium hydroxide in 25 ml of water, and digest on a water bath for 30 min. Filter off the flocculent precipitate which contains the niobium, tantalum, and any titanium and zirconium present in the solution, and wash the precipitate with one quarter-saturated ammonium chloride solution. Rinse the precipitate from the filter into the original beaker with the quartersaturated ammonium chloride washing solution.
The ignited precipitate must be reprecipitated once for samples containing up to 15 mg tantalum pentoxide, and two reprecipitations are necessary when larger amounts are present. Up to 30 mg Ce, Zr, V, Mo, W, Fe, Sn, Sb, and Bi do not interfere. For very accurate results, because of coprecipitation of traces of niobium and titanium, these elements should be determined colorimetrically after the final weighing, and a weight correction made. The determination of niobium by the thiocyanate method of Freund and Levitt, and titanium by the hydrogen peroxide method are recommended by them.
Analytical Chemistry of Niobium and Tantalum by Ross W. Moshier