An assessment of barium and strontium carbonates as temperature and enthalpy standards

Abstract

A study of the thermal behavior of both barium and strontium carbonates was performed in dynamic argon atmosphere over a wide range of temperatures. Both barium and strontium carbonates are white crystalline powders at room temperature. These compounds undergo reversible polymorphic transitions from the orthorhombic to hexagonal crystal forms when heated to temperatures above 800 °C. On further heating, the barium carbonate undergoes an additional polymorphic transition where the hexagonal form transforms to a face-centered cubic form near 976 °C. On continued heating, this cubic form of barium carbonate decomposes to barium oxide liberating carbon dioxide gas. Strontium carbonate, on the other hand, tends to be thermally less stable than barium carbonate. It is reported to lose mass over a broad range of temperatures beginning below the observed polymorphic transition (Charsley et al. in J Therm Anal Calorim 40:1415–1422, 1993). Our studies, using both differential thermal analysis (DTA) and thermogravimetric analysis techniques, attempt to answer questions of uncertainty of the reported temperatures of transition, as well as heats of transition, for these industrially important compounds. As a result of these findings, recommendations are given for the use of these compounds as reference standards for calibration of DTA instruments.