THE FREEZING POINTS OF HIGH PURITY METALS AS PRECISION TEMPERATURE STANDARDS:V. THERMAL ANALYSES ON 10 SAMPLES OF TIN WITH PURITIES GREATER THAN 99.99+%

Abstract

Extensive thermal analyses have been made on 10 samples (suppliers' analyzed impurity contents <0.2 to <100 p.p.m.) of high purity tin, including zone-refined metal; liquidus points have been intercompared with a precision of about 0.0002 °C and alloy melting ranges have been examined following different types of freezing with and without overnight anneals near the solidus temperature. Samples of nominal 99.9999% purity tin were found to have such narrow alloy melting ranges that any ambiguity, arising from unknown impurity concentrations, in specifying the liquidus point of pure tin is well inside 0.001 °C; a value of 231.913 °C (Int. 1948) was found for the standard liquidus point of pure tin. An account is given of the supercooling that was observed on the bulk samples and of anomalous structures that were found on melting curves. An appendix gives the results of long-term intercomparisons of the temperatures realized in four water triple point cells.