Effects of trace impurities on metal fixed points: antimony and bismuth intin

The unavoidable presence of impurities in metals generally leads to an offs et of the fusion interval towards higher, or lower temperatures depending o n the nature of the (metal + impurity) phase behaviour. In this paper, the effect of trace impurities on metal fixed points is considered both theoret ically and experimentally. A thermodynamic analysis of the effect of impuri ties on the fractional fusion behaviour, and on the heat capacity in the fu sion range shows that all impurities can be classified into three categorie s: eutectic-type, peritectic-type, and nearly-degenerate type. The conseque nces of impurities in metals that are used for fixed-point determinations a re discussed in the light of the accuracy needed in the temperature determi nations. The theoretical considerations are supplemented by an experimental study of the fusion characteristics of high-purity tin samples contaminate d with a Bi mole fraction of 271 . 10(-6) and Sb mole fractions of (113 and 370) 10(-6). The {(1 - x)Sn + xBi} system exemplifies a eutectic-type beha viour, and the {(1 - x)Sn + xSb} system a peritectic-type behaviour. The in fluence of other impurities is also briefly considered. The evaluation of t he purity of a compound from fractional fusion characteristics is discussed . (C) 1999 Academic Press.