Effects of Dissolved Oxygen and Freezing Techniques on the Silver Freezing Point

Abstract

The freezing point of silver is being considered as a joining point between platinum resistance thermometry and optical pyrometry. Therefore the freezing points of high purity samples of silver have been investigated. An important impurity effect arises from the depression of the freezing point of pure Ag caused by residual dissolved oxygen contents in some samples. Melting range parameters were determined on both oxided and carefully deoxided samples to aid in the selection of the purest sample. In addition the presence of dissolved oxygen is indicated by a slope inversion on the melting curve. Evaluations of the freezing point depression for small oxygen contents are given. A graphite sample crucible, with graphite radiation and gettering dises interspersed in the thermal insulation of the thermometer well, is a convenient arrangement to prevent the contamination of the sample in the presence of traces of oxygen, but is not sufficient for purifying heavily oxided samples; such samples require a vacuum treatment at about 1050° C. The pressure effect on the freezing temperature of silver has been determined.