Abstract
It was formerly believed that when a binary compound dissociates extensively on fusion, parts of the solidus and liquidus of the system become horizontal and therefore parallel. Since this is unacceptable in terms of the phase rule, thermal analysis was carried out with the picric acid-m-hydroxybenzaldehyde and naphthalene-m-dinitrobenzene systems which were hitherto considered as typical examples of such systems. The phase diagrams reconstructed by the improved thaw-melt method showed two eutectic lines, closely adjacent but distinctly different. In both cases, the melting curves consisted of three parts and the central ones were gently curved but had maximums at equimolar composition. Thus, it is certain that these systems belong to the category forming a congruently melting compound with a combining ratio of 1 : 1, and that the so-called parallelism resulted from a lack of experimental accuracy and/or rather arbitrary drawing of the phase diagrams in the past. In addition to the visual method, measurements by differential thermal analysis (DTA) and differential scanning calorimetry were done with the latter system. It was found that the melting point could not be determined in several cases; however, differentiation of the two close eutectic points was possible at a sufficiently slow rate of heating. Also, a small heat effect due to the metastable eutectic liquefaction was often recorded on the DTA curves. These findings show that the instrumental methods are useful for detecting the binary compound but by themselves are unsuitable for constructing the phase diagram of such a system.
