Acid-Base Properties of Thiopyrine in the Aqueous Dimethylformamide and Aqueous Dimethylsulfoxide Solvents

The acid-base properties of thiopyrine (TP) were studied in the water–dimethylformamide and water–dimethylsulfoxide solvents containing 25, 50, and 75 vol % DMF and DMSO at 298 K and ionic strength of 0.1 mol/L (NaClO4). The pK values of thiopyrine were determined over a wide range of pH values. Areas with different forms of thiopyrine were identified depending on the dimethylformamide and dimethylsulfoxide contents. It was established that pK of thiopyrine increases with the content of the organic solvent. Based on the obtained pK values it was found that in aqueous dimethylformamide and dimethylsulfoxide solutions, thiopyrine exhibits the properties of a weak base.