The problems concerned with non-proportionality between the absorbance and the vapor density and its dependence on the vapor temperature, which are inherently encountered by vacuum-sealed quartz cell/atomic absorption spectrophotometry combination, were solved through model calculations in this study. Model calculations revealed that "colligated analytical-curve" is useful. Activities of Bi and In in the Bi-In liquid alloy were measured over the entire composition range at the temperature from 850 to 1050 K. An alloy was vacuum-sealed in a quartz cell and heated at the temperature of interest. The absorption for Bi 307 nm radiation from Bi lamp was measured for Bi atom vapor in the cell. By heating a pure metal as a standard and measuring the absorbance as a function of the temperature, a colligated analytical-curve for Bi atom vapor was constructed and used for conversion of the absorbance to the vapor density. Bi activity was determined as the ratio of the Bi atom vapor density over the alloy to that over a pure metal. The same procedure was applied to the In 304 nm radiation from In lamp and In activity was determined independently of Bi. Thermodynamic behavior of the Bi-In liquid alloy was optimized with a sub-regular solution model by taking into account activity data obtained in this study. The agreement between activities optimized in this study and those in the literature was fairly good. The model also well predicted the liquidus curve on the Bi side and the heat of mixing in Bi-In binary, both of which are comparable with the literature values. Finally it was concluded that by constructing colligated analytical-curves the vacuum-sealed quartz cell/atomic absorption spectrophotometer combination was established as a useful technique to measure the activities of elements in alloy systems.
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