<p>During gold and silver leaching with thiourea, the presence of the oxidizing ions Cu²⁺ and Fe³⁺ promotes thiourea oxidation, which reduces process efficiency if their concentration in the system is not adequately controlled. Therefore, several alternatives have been proposed, such as pretreatment of minerals, including flotation, acidic leaching, bioleaching, and others, to reduce the concentration of these ions and prevent the formation of stable complexes with them, using oxalate in the thiourea leaching process. However, this strategy introduces a new complication due to the interference of oxalate during thiourea quantification by permanganate titration. This work presents the effect of masking agents, such as zinc acetate and calcium chloride, on the quantification of thiourea by permanganate titration in an acidic medium. In this context, calcium chloride can mask oxalate due to the formation of CaC₂O₄, which must be filtered out before the titration.</p> Graphical abstract <p></p>