STUDIES ON THE METHOD OF THERMAL ANALYSIS OF ORGANIC MEDICINALS. IV. DOUBLE MELTING POINT OF ORGANIC BINARY MIXTURES.

Abstract

In order to improve the dependability of the thaw-melt method, the double melting point phenomenon which has hitherto been neglected or overlooked was investigated on five kinds of organic binary systems with one or two molecular compounds. The systems were those of aminopyrine and barbital, aminopyrine and sulfisoxazole, benzoic acid and isonicotinamide, nicotinamide and sulfamerazine, and urea and N-sulfanilyl-3, 4-xylamide. The metastable or the first melting point occurs either by the delayed peritectic reaction in a stable mixture or by the suspended compound formation in a metastable mixture. In the former case, the metastable melting points appear on the upper extension of the middle branch of the stable liquidus. In the latter, they are observed on either or both of the extensions of the right and the left branch of liquidus. The occurrence of this phenomenon depends not only on the nature of the system but also on the conditions of measurement, such as agitation and heating rate. The submerged maximum which was thought to be hypothetical was clearly observed with the system of aminopyrine and sulfisoxazole and that of urea and N-sulfanilyl-3, 4-xylamide. The isolated compound of the former was examined by the differential thermal analysis. Heat effect was noted only at the metastable melting point of this compound. If the sample contains no supercooled liquid, the occurrence of double melting point in the heating procedure is actually limited to those cases described above. It will therefore, be very useful for the detection of molecular compound. Also the knowledge of this phenomenon will help to construct a correct phase diagram.