The effect of changing the pH on the equilibrium concentration of the complex ion Fe CN 6 4- can be understood by examining the formation of the complex and the factors that influence its equilibrium. The complex ion Fe CN 6 4- is formed by the reaction between Fe3+ and CN- ions:Fe3+ + 6CN- <=> Fe CN 6 4-To predict the effect of pH on the equilibrium concentration of Fe CN 6 4-, we need to consider the Le Chatelier's principle, which states that when a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust itself to counteract the change and restore a new equilibrium.In this case, the pH change will affect the concentration of CN- ions, which are derived from the dissociation of HCN:HCN <=> H+ + CN-When the pH decreases more acidic conditions , the concentration of H+ ions increases. According to Le Chatelier's principle, the equilibrium of the HCN dissociation will shift to the left, decreasing the concentration of CN- ions. As a result, the equilibrium of the complex formation will also shift to the left, decreasing the concentration of Fe CN 6 4- ions and increasing the concentration of Fe3+ ions.On the other hand, when the pH increases more basic conditions , the concentration of H+ ions decreases. This will shift the equilibrium of the HCN dissociation to the right, increasing the concentration of CN- ions. Consequently, the equilibrium of the complex formation will shift to the right, increasing the concentration of Fe CN 6 4- ions and decreasing the concentration of Fe3+ ions.In summary, the effect of changing the pH on the equilibrium concentration of the complex ion Fe CN 6 4- depends on the direction of the pH change. A decrease in pH more acidic conditions will decrease the concentration of Fe CN 6 4-, while an increase in pH more basic conditions will increase the concentration of Fe CN 6 4-. This can be explained by the Le Chatelier's principle and the influence of pH on the concentration of CN- ions, which are involved in the formation of the complex ion.