Thermal and spectroscopic study to investigate p-aminobenzoic acid, sodium p-aminobenzoate and its compounds with some lighter trivalent lanthanides

• The p-aminobenzoic acid melts followed partial evaporation.
• The stoichiometry of compounds was established by TG, EA and complexometry.
• The TG–DTA curves provided previously unreported information about thermal behavior.

The characterization, thermal stability and thermal decomposition of some lighter trivalent lanthanide p-aminobenzoates, Ln(C7H6NO2)3·H2O (Ln = La, Ce, Pr, Nd, Sm), as well as the thermal behavior and spectroscopic study of p-aminobenzoic acid C7H7NO2 and its sodium salt were investigated. The following methods were utilized: simultaneous thermogravimetry and differential thermal analysis (TG–DTA) in dynamic dry air and nitrogen atmospheres; differential scanning calorimetry (DSC); middle (MIR) and near (NIR) infrared region spectroscopy; evolved gas analysis (EGA); elemental analysis; complexometry; X-ray diffraction (XRD); and diffuse reflectance spectroscopy (DR) in the ultraviolet and visible regions. All the compounds were obtained monohydrated and the thermal decomposition occurred in two, three or four steps in an air atmosphere, and three or four steps in N2 atmosphere. In both atmospheres (air and N2) the final residues were CeO2, Pr6O11, Ln2O3 (Ln = La, Nd, Sm). The results also provided information concerning the coordination mode and thermal behavior, as well as the identification of the gaseous products which evolved during the thermal decomposition of these compounds. The DR and NIR spectra provided information about the ligand absorption bands and the f–f transitions of the Nd3+, Pr3+ and Sm3+ ions.