Thermal Stabilities of Metal Hydrogen Maleates

Abstract

The thermal stability of sodium, potassium, calcium, barium, zinc, cadmium, lead and silver hydrogen maleates in solid state was studied by the methods of TGA and DTA. All the metal salts exhibited an endothermic process accompanying a considerable loss in weight between 150C and 30C. During this process maleic acid was liberated and the resultant residue was the metal maleate except the cases of alkali and alkaline earth salts. Accordingly, it is concluded that the endothermic process corresponds to an ion exchange reaction between a metal ion and a hydrogen ion in the metal hydrogen maleate. The ion exchange took place perfectly for the heavy metal salts, but imperfectly for the alkali and alkaline earth salts. Particularly, in the case of alkali metal salts carbon dioxide was also formed partly by the decarboxylation of the carboxyl group. The mechanism of the ion exchange reaction in solid state is discussed from the viewpoint of the crystal structure of the metal hydrogen maleate. In the case of alkaline earth metal salts, the residues are stable intermediates, which include hydrogen maleate ions and maleate ions at a fixed ratio and give the characteristic X-ray diffraction patterns.