Menadione and phenacetin systems were each prepared with a series of concentrations of materials of varying similarity to the host. The data indicate that the measured impurity is dependent upon not only the concentration but also the nature of the impurity. The determination of the actual beginning of melting by nuclear magnetic resonance results in more precise purity measurements.In the previous paper [1 ], it was shown that one of the attributed causes of error in the determination of purity by differential scanning calorimetry or differential thermal analysis-lack of thermal equilibrium-is not a principal source of error. Further, the utility of determination of the solidus by nuclear magnetic resonance [2] was established. The latter arises from the need to distribute the energy represented by the temperature decrement at the completion of melting over the span of the melting process. If the very start of melting is not established clearly, there may be substantial error when that energy is distributed from the inferred starting temperature to the end of the process.The difficulty of detection of the actual start of melting already exists in ideal systems at very low levels of impurity, because so small an amount of the eutectic composition needs be melted to dissolve the whole amount of impurity. As illustrated in Figure 1, the inability to detect the start of melting of relatively impure specimens may occur where solid solutions can form. As the temperature of a specimen of the indicated composition is heated through the solidus, it is only necessary to dissolve a fraction of the impurity because the major portion remains in solid solution.Whereas in any system of practical concern, the impurities present are materials that were able to remain with the host material through any purification steps, isomers, degradation products, or reactants used in making the host material, the inevitable chemical similarity to the host renders solid solubility more probable than total (ideal system) insolubility. That is, ideal behavior is improbable when a manufactured chemical compound is tested for purity.This investigation was undertaken to evaluate the errors from known components in systems of economic importance, namely menadione and phenyl salicylate.* The data are taken from a dissertation presented in partial fulfillment of the requirements for the Doctor of Philosophy degree.3". Thermal AnaL 25, 1982