A new type of oxygen analyzer utilizing a potentiostatic coulometric titration technique
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I.
INTRODUCTION
I T has been recognized that even trace amounts of the nonmetallic impurities, e.g., oxygen, sulfur, carbon, nitrogen, and hydrogen, exert significant effects on the quality of ferrous and nonferrous materials. 1.23 Advanced techniques to control and analyze the amounts of these impurities have become increasingly necessary. An inert gas fusion method has been used extensively for the rapid determination of the oxygen content in metals and alloys. A sample is reacted in a graphite crucible under an electrical arc. The CO gas evolved is carried away by a purified inert gas, e.g., argon or helium; the amount of CO gas is measured by infra-red absorption. The CO gas evolved may also be converted to CO, gas which is then measured by gas chromatography. This technique requires standard samples because only a relative value of the oxygen content is obtained. Additionally, blank conditions must be established. Accordingly, when the oxygen is present in trace amounts, this technique yields less reliable values. In a previous paper, 4 the electrical components passing through the cell: Pt, air/oxide electrolyte/O in liquid metals, during a potentiostatic pump-out experiment, were theoretically analyzed following Wagner's treatment. 5-~ On the basis of these theoretical calculations, a new type of oxygen analyzer was proposed which requires neither standard samples nor the establishment of blank conditions. The analytical principle overcomes the error due to the release of oxygen from the solid electrolyte, as was discussed previously,9.~0 ~1 as well as the error due to the electronic conduction in the electrolyte. The oxygen analyzer proposed appears to yield very precise values even when the oxygen content of the samples is very low. The objective of the present investigation is to demonstrate the feasibility of this new type of oxygen analyzer. Copper, indium, and tin pieces were used to test the performance of the apparatus.
II.
(a)
EXPERIMENTAL
A. The Oxygen Analyzer The oxygen analyzer of the present investigation is schematically shown in Figures 1(a) and (b). The analyzer conSHINYA OTSUKA, Instructor. and ZENSAKU KOZUKA, Professor, are with the Department of Metallurgical Engineermg, Faculty of Engineering, Osaka University. 2-1 Yamada-oka, Smta, Osaka 565, Japan. Manuscript submitted June 19, 1984. METALLURGICAL TRANSACTIONS B
(b) Fig. l - - A schematic drawing of the ox),gen analyzer used in this study. (a) the closed system with a circulating Ar + H2 gas mixture, (b) the electrical circuit VOLUME 16B MARCH 1985-- 113
tains an electrochemical oxygen pump: Pt, air/ZrO2 (+ Y203)/Ar + H2 + H20 gas mixture. Pt Oxygen pump (I) x =O
x =L
where x = O and x = L indicate the left-hand and righthand interfaces between the electrolyte and gas phases. The detailed theoretical basis for the oxygen analyzer has been given elsewhere, 4 so only a brief description of the experimental setup and precedure is given here. The ZrO_, (+ 8 mole pct Y203) solid electrolyte tube (21 mm o.d,, 17 mm i.d., and 500 m
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