Distribution Coefficients of Impurities in Metals

Abstract

Impurities dissolved in very pure metals at the level of parts per million often cause an elevation or depression of the freezing temperature of the order of millikelvins. This represents a significant contribution to the uncertainty of standard platinum resistance thermometer calibrations. An important parameter for characterizing the behavior of impurities is the distribution coefficient $$k$$ , which is the ratio of the solid solubility to liquid solubility. A knowledge of $$k$$ for a given binary system is essential for contemporary methods of evaluating or correcting for the effect of impurities, and it is therefore of universal interest to have the most complete set of values possible. A survey of equilibrium values of $$k$$ (in the low concentration limit) reported in the literature for the International Temperature Scale of 1990 fixed points of Hg, Ga, In, Sn, Zn, Al, Au, Ag, and Cu is presented. In addition, thermodynamic calculations of $$k$$ using MTDATA are presented for $${>}$$ 170 binary systems. In total, the combined values of $$k$$ from all available sources for $${>}$$ 430 binary systems are presented. In addition, by considering all available values of $$k$$ for impurities in 25 different metal solvents ( $${>}$$ 1300 binary systems) enough data are available to characterize patterns in the value of $$k$$ for a given impurity as a function of its position in the periodic table. This enables prediction of $$k$$ for a significant number of binary systems for which data and calculations are unavailable. By combining data from many sources, values of $$k$$ for solutes (atomic number from 1 to 94) in ITS-90 fixed points from Hg to Cu are suggested, together with some tentative predicted values where literature data and calculations are unavailable.