Decreasing the pH of a redox reaction system containing Fe3+ and SCN- ions means that the concentration of H+ ions will increase. The reaction between Fe3+ and SCN- ions can be represented as follows:Fe3+ + SCN- <=> FeSCN2+When the pH decreases, the concentration of H+ ions increases, which can lead to the protonation of SCN- ions:SCN- + H+ <=> HSCNThis reaction will consume some of the SCN- ions, decreasing their concentration in the reaction system. According to Le Chatelier's principle, when the concentration of a reactant decreases, the equilibrium will shift towards the side of the reactants to compensate for the change. In this case, the equilibrium of the Fe3+ and SCN- reaction will shift to the left, favoring the formation of more Fe3+ and SCN- ions and decreasing the concentration of FeSCN2+ ions.In summary, decreasing the pH of the redox reaction system containing Fe3+ and SCN- ions will shift the equilibrium position towards the reactants, resulting in a decrease in the concentration of FeSCN2+ ions.