Some New Standard Melting Points at High Temperatures

Abstract

Melting points of salts for use as standard temperatures for thermocouples, 400\ifmmode^\circ\else\textdegree\fi{} to 1100\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{} 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 \ifmmode\pm\else\textpm\fi{} 0.5\ifmmode^\circ\else\textdegree\fi{}: potassium dichromate 397.5\ifmmode^\circ\else\textdegree\fi{}; ($30.5 NaCl+69.5 {\mathrm{Na}}_{2}S{O}_{4}$) 627.0\ifmmode^\circ\else\textdegree\fi{}; $\mathrm{KCl}$ 770.3\ifmmode^\circ\else\textdegree\fi{}; $\mathrm{NaCl}$ 800.4\ifmmode^\circ\else\textdegree\fi{}; ${\mathrm{Na}}_{2}S{O}_{4}$ 884.7\ifmmode^\circ\else\textdegree\fi{}; ${K}_{2}S{O}_{4}$ inversion point 583\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}, melting point 1069.1\ifmmode^\circ\else\textdegree\fi{}; also ${\mathrm{Ca}}_{2}{B}_{2}{O}_{5}$ 1304\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}. Detailed suggestions as to technique are given, also a convenient method of using pure metal strips or wires in calibrating couples.