TABLE OF CONTENTS Measurements of the depression of the melting point [1] of a sample are often used to determine its purity [2]. Melting endotherms, recorded using differential scanning calorimetry (DSC), are routinely used to recognize the occurrence of melting and to measure the melting temperature of the sample. With a bit more effort, as discussed below, it is possible to determine the purity of the sample by analyzing, in detail, the shape of the melting endotherm. There is no need to have a high-purity sample of the substance under investigation for comparison, although a sample of any very pure material, such as indium metal, is needed to determine the thermal performance of the particular instrument being used. Because such materials are used, in any case, to calibrate the instrument for temperature and enthalpy measurements, this last requirement is readily achievable. Calculations are based on the assumptions that solid solutions are not formed and that the melt is an ideal solution. Melting must not be accompanied by decomposition or sublimation. The assumptions made apply only to relatively pure (>98%) materials.
The practical aim of purity determinations is usually to decide whether or not the sample meets certain specifications, determined by the intended further uses of the sample. Special Technical Publication 838 of the ASTM [3] is an important source of information on purity determination. In it, a review by Brennan and co-workers [4] cutlines the history of the DSC method and emphasizes A.P. Gray's pioneering work in...
