Determination of Ultra-micro Amounts of Oxygen in Zone-refined Pure Iron by the Vacuum Fusion Method Using Silicon for Decrease of Blank Value

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  • 空試験値低減にシリコンを用いた真空融解法によるゾーン精製純鉄中の超微量酸素定量
  • カラシケンチ テイゲン ニ シリコン オ モチイタ シンクウ ユウカイホウ ニ

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For the determination of the amounts of oxygen up to the level of 1 ppm in zone-refined iron, the vacuum fusion method using Si was studied.<BR>When 0.07-0.15 grams of pure Si were added into the graphite crucible at 2073 K, the oxygen blank values from the crucibles were reduced under 0.5 μg/600 s, and consequently the oxygen in pure iron was able to be rapidly determined. The procedures were as follows: After degassing the graphite crucible for 7.2 ks at 2473 K, the temperature of the crucible was redued to 2023 K and Si was added into the crucible, degassing being made for 1.2 ks at this temperature. The electropolished samples of 5-10 g were next added into the crucible and gases were collected for 600 s.<BR>The usable amount of Si for the determination of oxygen in pure iron must not exceed 3 mass% because of the influences of Si on the determination of oxygen. In addition, large samples (surface area: less than 10×10−4 m2) were necessary to be used in order to minimize the sample contamination. Thus the crucibles were improved: The size of the inner crucible was 25 mm in diameter, 70 mm in height and 3 mm in thickness and the size of inlet of the crucible funnel was 18 mm in diameter.<BR>This method was applied to the determination of oxygen in pure iron refined under the several zone conditions. The oxygen contents was found to be varied according to the zone-refined conditions and the location of zone-refined iron. The precision of this method was 7.2% with the relative standard deviation at the oxygen concentration of 2.2 ppm.

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