Is it reliable to determine the stoichiometry of extraction complexes using the classic slope analysis model in acidic extraction systems?†

Although the classic slope analysis model has become a standard tool for determining the stoichiometry of extraction complexes in acidic extraction systems, its application limitation has not been taken seriously yet. Specifically, this model simplifies the activity coefficients to 1 and constant extractant contents—these assumptions may fail under certain conditions. In this study, a general modified fitting model applicable to acidic extractants was proposed, incorporating corrections for activity coefficients and extractant mass balance. Results reveal that the stoichiometries determined by both the slope analysis model and the modified fitting model align closely for high extractant-to-metal ratios. However, at low extractant-to-metal ratios, the slope analysis model tends to overestimate the stoichiometry (extractant–metal ratio) in the extraction complex relative to the modified fitting model. This discrepancy highlights the need to carefully review the applicability and reliability of the slope analysis model in extraction experiments.