Errors occurring in the determination of temperature-heat content curves

Abstract

In the determination of temperature-heat content curves errors may occur which are due to low transition rates, low diffusion rates and low rates of heat transfer. These errors are subsequently discussed. Liquid to solid transitions may be slow even when liquid and solid are in contact with each other at finite ratios. This causes appreciable differences between melting temperatures and solidification temperatures. The phenomenon is called two-phase undercooling. When two-phase undercooling occurs the cooling curve obtained may lead to erroneous conclusions, e.g. an impure substance may seem to be extremely pure. This is illustrated by experiments.

As far as is known two-phase superheating appears to be negligible under normal conditions. Therefore heating curves are preferred to cooling curves.

As stirring cannot promote diffusion in the solid phase it is doubted whether the stirrer method has advantages over the thin film method.

Errors due to limited heat transfer and occurring with the thin film method have been calculated. Formulae have been given relating the dimensions of a thin film apparatus and the rate of heating to the accuracy aimed at.

Application of an internal check on heating curves is recommended.