Critical evaluation of the gran method for free acidity determination in sulfuric solutions containing interfering ions

The Gran method, developed in the mid-20th century, remains a widely used technique for potentiometric determination of equivalence points in acid–base titrations. This study critically evaluates its performance for determining free acidity in sulfuric acid liquors, particularly under interference from Fe³⁺, Fe²⁺, and Al³⁺ ions. Experimental results demonstrated that Fe³⁺ concentrations above 8 g L-1 cause endpoint distortion, limiting the method's applicability in complex leachates. For comparison, the oxalate method was also evaluated and exhibited superior applicability limits, since the method is susceptible to interferents in the presence of trivalent cations. While the Gran method retained high precision in strongly acidic solutions (>1 N H₂SO₄), its accuracy declined under low acidity or high Fe³⁺ levels. These findings support the continued use of the Gran method in hydrometallurgical processes, if sample composition is controlled and validated. Practical guidelines and comparative criteria are provided to assist in method selection for industrial monitoring.