Question

Given the information that

$$\begin{gathered} {\mathrm{Fe}}^{3+}+{\mathrm{Y}}^{4-}\rightleftharpoons {\mathrm{FeY}}^{-}{\mathrm{K}}_{\mathrm{f}}=1.0\times {10}^{25} \\ {\mathrm{Cu}}^{2+}+{\mathrm{Y}}^{4-}\rightleftharpoons {\mathrm{CuY}}^{2-}{\mathrm{K}}_{\mathrm{f}}=6.3\times {10}^{18} \end{gathered}$$

and the further information that, among the several reactants and products, only CuY^2-absorbs radiation at 750 nm, describe how Cu(II) could be used as an indicator for the photometric titration of Fe(III) with H₂Y^2-. Reaction:${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{Y}}^{2-}\to {\mathrm{FeY}}^{-}+2{\mathrm{H}}^{+}$

Short answer

Few drops of Cu(II) solution is added to the Fe(III) solution which is to be titrated with the EDTA solution. The absorbance at 750 nm is measured for every drop of EDTA added to Fe(III) solution. Since FeY^-stable complex does not absorb radiation at 750 nm, no significant absorbance is observed. The first drop added after the equivalence point will increase the absorbance significantly as CuY^2- begins to form and absorbs radiations at 750 nm.

Step-by-step solution

Given Information

$$\begin{gathered} {\mathrm{Fe}}^{3+}+{\mathrm{Y}}^{4-}\rightleftharpoons {\mathrm{FeY}}^{-}{\mathrm{K}}_{\mathrm{f}}=1.0\times {10}^{25} \\ {\mathrm{Cu}}^{2+}+{\mathrm{Y}}^{4-}\rightleftharpoons {\mathrm{CuY}}^{2-}{\mathrm{K}}_{\mathrm{f}}=6.3\times {10}^{18} \end{gathered}$$

${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{Y}}^{2-}\to {\mathrm{FeY}}^{-}+2{\mathrm{H}}^{+}$

Only CuY^2- absorbs radiation at 750 nm.

Explanation

Formation constant of FeY- is higher than that of CuY^2-. So Fe³⁺ forms a stable complex with H₂Y^2- than Cu²⁺. So, we can add small amount of Cu²⁺ to the unknown solution of Fe³⁺ and titrate with a standardized solution of H₂Y^2-. Absorbance of the solution is measured at 750 nm for every drop of titrant added. No absorbance is observed until all the Fe³⁺ is used up by H₂Y^2-. After the equivalence point, absorbance of the solution will increase gradually as Cu²⁺ starts to complex with H₂Y^2-. The point which the absorbance has a significant increase can be used as the end point.

Conclusion

Few drops of Cu(II) solution is added to the Fe(III) solution which is to be titrated with the EDTA solution. The absorbance at 750 nm is measured for every drop of EDTA added to Fe(III) solution. Since FeY^- stable complex does not absorb radiation at 750 nm, no significant absorbance is observed. The first drop added after the equivalence point will increase the absorbance significantly as CuY^2- begins to form and absorbs radiations at 750 nm.