18 19This paper gives an introduction to MTDATA, Phase Equilibrium Software from the National Physical Laboratory (NPL), and 20 describes the latest advances in the development of a comprehensive database of thermodynamic parameters to underpin 21 calculations of phase equilibria in large oxide, sulphide and fluoride systems of industrial interest. The database, MTOX, has 22

Uncertainty estimation for the effect of impurities on fixed points requires an accurate assay of the fixed-point material. To apply a correction also needs a knowledge of the liquidus slope of the solute binary system. Two methods are presented here for giving a realistic uncertainty and potential for correction without detailed knowledge of the impurities present. Both methods are based on the Scheil equation. The first method uses the gradient at about 50 % solid; and where some limited knowledge of impurities is available, this method can, in cases where the bulk of impurities segregate preferentially into the liquid phase, be used to apply a correction. To apply an uncertainty simply knowing typical impurities for a particular metal may be considered sufficient. The second method involves a best fit for the four variables in the Scheil equation. It is shown that this second method can work even where multiple impurities are present, but that when applied to real data, problems arise due to deviations from Scheil behavior. This deviation is thought to be due to difficulties in maintaining a uniform solid/liquid interface at the end of a freeze.

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The International Temperature Scale (ITS-90) is defined in part by a series of metal freezing points between 156 • C and 1,084 • C. These freezing-point cells provide reference temperatures with an uncertainty of realization claimed to be in the range of several tenths of a millikelvin. The impurities in the nominally 99.9999 % pure metals make a major contribution to the uncertainty of realization of the fixedpoint temperature. Recently, a new method to correct for the influence of the impurities by summing the individual contributions of each impurity has been suggested. This method is referred to as the "sum of individual estimates" (SIE). NPL is a partner in a Euromet project to improve the realization of ITS-90 metal fixed points. As part of this project, NPL is particularly interested in the fixed-point cells of tin (231.928 • C) and aluminum (660.323 • C). This article describes the use of a thermodynamic model, embodied in NPL's MTDATA software, to estimate the initial drop in the freezing temperature and the temperature decrease during freezing using both equilibrium and "Scheil" approaches. Calculations of this type establish the effect of single elements, and with the chemical analysis of the metal, enable an estimation of the whole freezing curve. This has been done for a sample of tin from this laboratory, and thereafter, the theoretical curves are compared with previously published experimental data on impurity-doped aluminum, with good agreement, e.g., better than 1 mK over most of the curve for 76 µg · g −1 Ag in Al.

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