Some New Standard Melting Points at High Temperatures

Abstract

Melting points of salts for use as standard temperatures for thermocouples, 400° to 1100°C.—After purification by crystallization from solution, a gram of the salt is placed in a Pt crucible 18 mm by 10 mm in diam., the junction is placed in the axis and 3 to 4 mm from the bottom, and the temperature is raised about 5° per minute. For most pure substances and certain eutectics the heating curve becomes quite flat at the melting point and hence the form of curve obtained is a sensitive test of purity. The couples used were carefully calibrated with standard metals, and gave the following results, accurate to ± 0.5°: potassium dichromate 397.5°; ($30.5 NaCl+69.5\mathrm{} {\mathrm{Na}}_{2}S{O}_4$) 627.0°; $\mathrm{KCl}$ 770.3°; $\mathrm{NaCl}$ 800.4°; ${\mathrm{Na}}_{2}S{O}_4$ 884.7°; $K_{2}S{O}_4$ inversion point 583±1°, melting point 1069.1°; also ${\mathrm{Ca}}_2{B}_2{O}_5$ 1304±5°. Detailed suggestions as to technique are given, also a convenient method of using pure metal strips or wires in calibrating couples.