Standard Reference Material 1751: Gallium Melting-Point Standard

The melting point of gallium (29.7646 °C) is a defining thermometric fixed point of the International Temperature Scale of 1990 (ITS-90). Realization of this melting point is performed using a fixedpoint cell containing high-purity (≥99.999 99 wt. % pure) gallium. A 30 kg single lot of gallium (99.999 9956 wt. % pure) constituting Standard Reference Material (SRM) 1751 was evaluated, and certified as suitable for use in the realization of the melting-point temperature of gallium for the ITS-90. Based on results obtained with three fixed-point cells containing random samples of SRM 1751, the expanded uncertainty (k=2) assigned to the melting-point temperature of the metal is 0.07 mK. In this document, the methods used for the fabrication of the gallium melting-point cells and for the evaluation of SRM 1751 are described.