Activities of oxygen in liquid Bi, Sn, and Ge from electrochemical measurements
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A modified coulometric titration method proposed by two of us 1,2is capable of precise measurements of the oxygen concentration in liquid metals, because any errors due to the electronic conduction in a solid electrolyte and the release of oxygen from the electrolyte are avoided. Since our recent work on Pb-O and Sb-O systems, 2 this experimental technique has been successfully applied to determine the activity coefficients of the dilute solution of oxygen in several liquid metals 3,4 and alloys: ,6 Also, we have recently tried to arrange the experimental values on the thermodynamic properties of oxygen dissolved in liquid elements of groups Ib ~ VIb and find a limited number of empirical correlations. 4,7 Such an investigation is very important in predicting the thermodynamic properties of oxygen in liquid elements where data are not available. Reliable data have not yet been reported on the activity coefficients of oxygen in liquid bismuth and germanium. Also, the published values 8-~~for liquid tin are in significant disagreement. Therefore, as part of such a research program, the activity coefficients of oxygen in liquid bismuth, tin and germanium were precisely measured in this investigation by a superior modified coulometric titration method. Recently, Fitzner carried out a similar coulometric titration,~,~2 referring to our previous investigations on Pb-O and Sb-O systems. 2:3 However, his experimental techniques led to the extremely scattered results. Through the present paper, it will be reconfirmed that
the modified coulometric titration method proposed by two of us produces remarkably reproducible and reliable data. EXPERIMENTAL PRINCIPLE The details of the experimental principle have been presented previously TM and only a brief description is given here. The galvanic cell used in the present experiments is similar to that in previous papers 3,4 and described schematically by O in liquid metal/ZrO2 ( + CaO)/Air, Pt.
After the initial open-circuit emf, E~(r) is recorded, a voltage is applied by use of a potentiostat between(~) and ( ~ wires so that the emf between Q and G wires has a preselected value, E2(m ), (see Fig. 1 in Ref. 3). Thus, oxygen in the liquid metal is removed into the air of the reference electrode. The total quantity of electrical charge contributed by the ionic current passing through the cell is estimated from the decay curve of electric current 2 and then used for the calculation described below. If the oxygen in liquid metal obeys Henry's law, an apparent initial oxygen concentration in the liquid metal, C l(app), can be calculated according toZ4 C~(app) = lOOy •
SHINYA OTSUKAis Research Associate, and ZENSAKU KOZUKA is Professor, Department of Metallurgical Engineering, Faculty of Engineering, Osaka University, Osaka, Japan. TOYOKAZU SANO, formerly a Graduate Student, Osaka University, is now with Kobe Steel Ltd., Kobe, Japan. Manuscript submitted July 15, 1980.
[Cell I]
(
(y + 1) + ( y -
1) e x p k ~ ] j [1]
where y = (Qio,/2F) • (Mm/Wm) ; Ct(app)is the apparent initial o
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