Comprehensive Investigation of the Labile Ferric‐Thiosulfate Complex: Stability and Kinetics Through Visible Spectrophotometric Analysis

ABSTRACT The extensive and long‐term use of cyanide in gold leaching has raised serious environmental and public health concerns, prompting extensive efforts towards identifying safer alternatives. Among the various non‐cyanide lixiviants explored, thiosulfates have emerged as a promising candidate owing to their non‐toxicity and good performance in processing copper‐bearing gold ores. This study presents a comprehensive investigation into the kinetics and stability of one such system—the labile ferric‐thiosulfate complex. Key aspects of the complex such as stability across varying pH conditions, empirical modeling of pH‐dependent stability, decomposition kinetics, the influence of ion concentration on complexation, and complex stoichiometry have been determined using visible spectrophotometric analysis. The complex exhibits λ max at 520 nm. Job's method of continuous variation confirmed 1:1 (Fe 3+ :S 2 O 3 2− ) stoichiometry for the complex. Kinetic studies affirm that complex decomposes faster at higher pH, and hydrogen ion concentration of 0.16 M (pH = 0.69) is optimal for its maximum stability. An empirical equation that quantitatively relates absorbance to concentration of H + ions and time has been found out to be A = 6.214[ H + ] −1.186 (time) −0.827 . Kinetics of decomposition of complex follows a second‐order rate law, with a straight‐line plot of ln(rate) versus ln[complex] (slope ≈ 2). Absorbance decreased with time, fitting to power‐law plots ( y = mx n ). Variation in ferric and thiosulfate ion concentrations resulted in Gaussian‐shaped absorbance versus concentration curves, suggesting that when present in deficit, ferric or thiosulfate ions act as the limiting reagent, leading to reduced complexation and lower absorbance. Maximum absorbance is observed only when both ions are present in the optimal 1:1 stoichiometric ratio allowing for complete complexation and greater stability. The findings of this work advance our understanding of the Fe 3+ ‐S 2 O 3 2− complex, its stability, and kinetics, opening avenues for its potential applications in catalysis, energy storage and gold‐leaching.