Thermal studies on the sodium salts of aminosalicylic acids

Abstract

The effect on the stability of the isomers of aminosalicylic acid of formation of their sodium salts has been studied by use of differential scanning calorimetry and thermogravimetry, coupled with evolved gas analysis by Fourier transform infrared spectroscopy. X-ray powder diffraction and infrared spectroscopy provided complementary information. The DSC curves for the sodium salts of all of the isomers showed complex dehydration/decomposition endotherms. From the initial mass losses of the TG curves, the amounts of water per mole of salt were estimated as 0.5, 2.4 and 1.4 moles for the sodium salts of 3-aminosalicylic acid, 4-aminosalicylic acid and 5-aminosalicylic acid, respectively. TG-FTIR results for the sodium salt of 3-aminosalicylic acid showed the evolution of carbon dioxide in three stages: below 150°C, between 200 and 300°C and continuous formation up to 500°C. This behaviour differs from that of 3-aminosalicylic acid itself, which forms CO2 between 225 and 290°C. For the sodium salt of 4-aminosalicylic acid, the formation of carbon dioxide starts from 250°C and is still being formed at about 650°C. 4-aminosalicylic acid decarboxylates above 150°C. 5-aminosalicylic acid and its sodium salt showed no evolution of carbon dioxide below 600°C.