On the value of recent microscopic methods of identification used for organic substances

Abstract

The value of recent methods of identification of organic substances by means of the micro melting point apparatus is investigated, using 3 groups of closely related substances (benzoic acid derivatives, barbituric acid derivatives, and sulphonamids). The experiments show that a combination of the melting point determination with an observation of sublimation and conversion during heating and, moreover, with the observation of the conversion of crystallised melting drops etc. very markedly increases the value of the determination. The preparation of derivatives from quantities below l mg can be performed more rapidly and more simply than the preparation of larger amounts. The melting point determination of the derivatives is of the greatest value for the identification of the respective substances. On the other hand, the melting point determination on eutectic mixtures of the substance in question with a test substance is found to be of much lower value as a method of identification, if there is a possibility for the occurrence of a number of closely related substances having the same melting point ; such substances often have the same eutectic melting point with the same test substance. The determination of the index of refraction of molten substances (i.e., the determination of the temperature interval within which the molten mass has the same index of refraction as a glass powder of a known index of refraction) is an excellent method of identification, since the scattering of the temperature intervals found for related substances is of the same magnitude, and for not related substances is considerably higher, than the scattering of their melting points.