Calibration of capillary melting-point apparatus to the international temperature scale of 1990 (ITS-90) by use of fluxed highly pure metals

Abstract

Capillary melting-point measurements have always lacked traceability to internationally accepted or precisely determined temperature standards. Highly pure (99.999%) indium, tin, bismuth, and cadmium or lead, when covered with stearic anhydride acting as a flux, gallium covered with 3 M HCl and zinc covered with m-( m- phenoxyphenoxy)benzene containing triphenylboron, give sharp visual melting points which correspond to primary or secondary “fixed points” on the International Temperature Scale of 1990 (ITS-90). Melting is observed to begin with collapse and coalescence to a mush. It ends with the formation of a mercury-like spheroidal droplet, taken as the melting point; the entire range is less than 0.5°C. Readings may be interpolated either graphically or computationally to give tabular corrections yielding true temperatures. The performance of thermometers cannot safely be predicted from information engraved thereon, as > 10° errors were observed. It is recommended that organic chemical publications insist on the use of the phrase “The m.p. apparatus was metal calibrated to ITS-90”.