Free Acidity in Hydrometallurgy is not pH: A Critical Review of Measurement Methods, Errors, and Process Consequences

Abstract : Free acidity is a key control parameter in hydrometallurgical processes, governing dissolution, precipitation, solvent extraction, neutralization, and corrosion behavior, yet its determination in real process liquors remains poorly standardized and often misunderstood. In industrial solutions, the distinction between total acidity, free acidity, and active acidity is frequently obscured by high ionic strength, metal hydrolysis, ion pairing, and complexation reactions, making simple pH measurement or conventional titration unreliable. This review critically examines the main analytical approaches used to determine free acidity in hydrometallurgical liquors, including classical acid–base titration, potentiometric and Gran methods, oxalate and complexation-based procedures, Ba(OH)₂ and other selective titrations, inline monitoring techniques, and speciation-based modeling calculations. Particular emphasis is given to the effect of interfering species commonly present in industrial circuits, such as Fe³⁺, Al³⁺, SO₄²⁻, Cl⁻, and F⁻, which strongly influence endpoint detection, hydrogen ion activity, and the apparent acid balance. The review shows that discrepancies between analytical methods are not only experimental but also thermodynamic, arising from differences between proton concentration, activity, and chemically bound acidity. These inconsistencies can lead to significant errors in process control, affecting reagent consumption, precipitation efficiency, scaling, solvent extraction stability, and environmental performance. By comparing reported methods and industrial practices, this work identifies the limitations of current procedures and highlights the need for standardized definitions, speciation-aware analysis, and improved inline monitoring strategies for reliable acidity control in modern hydrometallurgical operations.