91Ó°ÊÓ

Given the information that $$ \begin{array}{ll} \mathrm{Fe}^{3+}+\mathrm{Y}^{4-} \rightleftharpoons \mathrm{FeY} & K_{\mathrm{f}}=1.0 \times 10^{25} \\ \mathrm{Cu}^{2+}+\mathrm{Y}^{4-} \rightleftharpoons \mathrm{CuY}^{2-} & K_{\mathrm{f}}=6.3 \times 10^{18} \end{array} $$ and the further information that, among the several reactants and products, only \(\mathrm{CuY}^{2-}\) absorbs radiation at \(750 \mathrm{~nm}\), describe how \(\mathrm{Cu}(\mathrm{II})\) could be used as an indicator for the photometric titration of \(\mathrm{Fe}(\mathrm{III})\) with \(\mathrm{H}_{2} \mathrm{Y}^{2}\). Reaction: \(\mathrm{Fe}^{3+}+\mathrm{H}_{2} \mathrm{Y}^{2-} \rightarrow \mathrm{FeY}+2 \mathrm{H}^{*}\)

The chelate \(\mathrm{CuA}_{2}{ }^{2-}\) exhibits maximum absorption at \(480 \mathrm{~nm}\). When the chelating reagent is present in at least a tenfold excess, the absorbance depends only on the analytical concentration of \(\mathrm{Cu}(\mathrm{II})\) and conforms to Beer's law over a wide range. A solution in which the analytical concentration of \(\mathrm{Cu}^{2+}\) is \(2.15 \times 10^{-4} \mathrm{M}\) and that for \(\mathrm{A}^{2-}\) is \(9.00 \times 10^{-3} \mathrm{M}\) has an absorbance of \(0.759\) when measured in a \(1.00-\mathrm{cm}\) cell at \(480 \mathrm{~nm}\). A solution in which the analytical concentrations of \(\mathrm{Cu}^{2+}\) and \(\mathrm{A}^{2-}\) are \(2.15 \times 10^{-4} \mathrm{M}\) and \(4.00 \times 10^{-4} \mathrm{M}\), respectively, has an absorbance of \(0.654\) when measured under the same conditions. Use thi? information to calculate the formation constant \(K_{\mathrm{f}}\) for the process $$ \mathrm{Cu}^{2+}+2 \mathrm{~A}^{2-} \rightleftharpoons \mathrm{Cu} \mathrm{A}_{2}^{2-} $$