チオシアン酸による鉄鋼中のモリブデン吸光光度分析における銅およびタングステンのマスキングについて

概要

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A method for the masking of copper and tungsten in the photometric determination of molybdenum in iron and steel with thiocyanate has been studied. The interfering copper is masked with thiourea solution and tungsten with phosphoric acid, although the former is unsuccessful in cases of stannous chloride reduction. The established method using L-ascorbic acid as a reducing agent is as follows. The sample (0.10.5g) is decomposed by treatment with perchloric acid, and evaporated to vigorous fumes. After cooling, it is diluted exactly to 100 ml with water, and an aliquot containing not more than 300μg of molybdenum is transfered to a 50 ml of volumetric flask. To it are added 10 ml of perchloric acid, 2 ml of sodium thiocyanate solution (30%), 10 ml of L-ascorbic acid solution (5%) and 5 ml of thiourea solution (10%) and it is diluted exactly to 50 ml with water. The absorbance is measured at 460 mμ against water. Samples containing large amounts of tungsten (for example, high speed steel) are decomposed by using 20 ml of aqua regia, 5 ml of phosphoric acid and 0.5 g of ammonium bifluoride, and evaporated to fumes with 20 ml of perchloric acid.
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