G- Applications to Organic Materials

The enthalpic changes which occur in organic compounds are considerably less complex than those for organic polymers. However, they may exhibit various polymorphic changes which can be detected by DTA and DSC. The main sources of endothermic and exothermic enthalpic changes in organic compounds are fusion, vaporization, solid-solid transitions, sublimation, dehydration, decomposition, and combustion. 

The applications of these techniques to organic compounds have been extensively reviewed (2-8). Specific DTA applications are reviewed by Mitchell and Birnie (2), while DSC techniques are discussed by Gray (4). 

The DTA curve for oxalic acid dihydraie. the only acid studied containing water of hydration, had dehydration peaks with A7 n values ofl 10.120, and 125 C, respectively. All other curve peaks for the organic acids were caused by fusion and decomposition reactions. For example, the second endothermic peak in the succinic acid curve was probably caused by dehydration reaction. 

It has long been known that meconic acid (I) (3-hydroxy-4-oxo-4H-pyrane-2,6-dicarboxylic acid) can be heated in air to 120-220°C to form comenic acid (II) (00). DTA was used to establish the best preparative 

Curve (u) indicates that benzoic acid melts at about 122°C. Under 200 psig nitrogen pressure [curve (b) the melting-point endothermic peak remains unchanged while the boiling point is elevated to 378°C. In order to avoid sublimation and evaporation and to ensure equilibrium conditions, the samples were run in a small hermetically sealed aluminum pan which contained a small hole ( 0.002 in.) punched in the top to equalize the pressure. 

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