Melting-point purity determinations: Limitations as evidenced by calorimetric studies in the melting region

Typical melting curves, chosen from the results of more than 125 melting-point studies, are presented to illustrate both the reliability and the inherent limitations of the calorimetric method of purity determination. It is shown that this method usually is one of the best means of accurately determining small concentrations of impurity. However, analysis of anomalous melting curves leads to the conclusion that pseudo-equilibrium curves are often obtained as a result of inhomogeneous distribution of impurity in the liquid phase or of non-equilibrium distribution of impurity between the solid and liquid phases. Evidence is given to support the contention that solidsolutions were formed in as many as half of the samples studied and that for this reason, impurity values calculated for perhaps $\text{1}\text{3}$ of the samples are in error by 200% or more. Although these large uncertainties in impurity values usually correspond to less than 0.1 % uncertainty in purity values, modified procedures are proposed to minimize such errors.