DEVELOPMENT OF SULFENTRAZONE EXTRACTION PROCEDURE FROM COMMERCIAL HERBICIDE FORMULATION AND DETERMINATION OF ITS IONIZATION CONSTANT (pKa) BY POTENTIOMETRIC TITRATION

Abstract

The herbicide sulfentrazone (SFZ) is widely used in soybean and sugarcane crops, although it is classified as very dangerous to the environment. To minimize barriers to the development of research with this herbicide due to the high cost of acquisition of its analytical standard and the availability of few distributors to acquire the standard in the country, a procedure of extraction and purification of SFZ was developed from a commercial herbicide formulation available in Brazil. Using the most suitable experimental conditions established for the extraction procedure, the SFZ content in the resulting solid after extraction and drying was 97.5% (m/m), which value was determined by acid-base potentiometric titrations. To characterize the solid obtained in this procedure, the material was studied by thermal analysis (thermogravimetry and differential scanning calorimetry) and Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) to confirm the presence of functional groups present in the SFZ molecule. Thermal analysis of the solid resulting from the extraction indicated processes of melting, thermal degradation and volatilization of the SFZ during its heating. By potentiometric titrations of the purified solid dissolved in water/acetonitrile (ACN) binary mixtures, it was possible to determine the ionization constant (pKa) of SFZ in water at 25.0ºC. By this study, extrapolated value of pKa in water is 6.45 obtained from the linear correlation of pKa versus molar ACN fraction plot. The value obtained at 25.0 °C by potentiometry in this study is quite close to the literature value of 6.56 determined at 20 °C by the spectrophotometric technique. However, the pKa value determined by potentiometric titrations of dilute solutions of the SFZ commercial formulations was much higher than that of 6.56, indicating that the degree of ionization of SFZ under herbicide application conditions can be quite distinct in the presence of possible adjuvants present in the commercial formulation of the herbicide.