Thioglycolic acid grafted onto silica gel and its properties in relation to extracting cations from ethanolic solution determined by calorimetric technique

Thioglycolic acid was immobilized onto silica gel surface using 3-aminopropyltrimethoxysilane as precursor silylating agent to yield silica. The amount of thioglycolic acid immobilized was 1.03 mmol per gram of silica. This new surface displayed a chelating moiety containing nitrogen, sulfur, and oxygen basic centers which are potentially capable of extracting cations from ethanolic solution, such as MCl 3 (M=Fe, Cr, and Mo). This process of extraction was carried out by the batch method when similar chemisorption isotherms were observed for all cations. The data were adjusted to a modified Langmuir equation. The sequence of the maximum retention capacity was Cr(III) > Mo(III) > Fe(III). The same adsorption was determined by calorimetric titration and the enthalpic values of −35.75±0.02, 32.90±0.15, and −84.08±0.12 kJ mol −1 for chromium, molybdenum, and iron, respectively, were obtained. From the calculated Gibbs free energy −23.4±0.2, −27.2±0.2, and −32.7±0.3 kJ mol −1, the variations in entropy obtained were 42±1, 201±1, and 172±1 J K −1 mol −1 for the same sequence. All thermodynamic values are in agreement with the spontaneity of the proposed cation–basic center interactions for these chelating processes.