Question

The metal ion indicator xylenol orange (Table 12-3) is yellow at$\mathbf{pH}\mathbf{6}\mathbf{(}{\mathbf{\lambda }}_{\mathbf{max}}\mathbf{=}\mathbf{439}\mathbf{nm}\mathbf{)}$.The spectral changes that occur as${\mathbf{VO}}^{\mathbf{2}\mathbf{+}}$is added to the indicator atpH6 are shown here. The mole ratio${\mathbf{VO}}^{\mathbf{2}\mathbf{+}}$/xylenol orange at each point is

Suggest a sequence of chemical reactions to explain the spectral changes, especially the isosbestic points at 457 and 528nm.

Absorption spectra for the reaction of xylenol orange with${\mathbf{VO}}^{\mathbf{2}\mathbf{+}}$atpH 6.0

Short answer

A sequence of chemical reactions to explain the spectral changes, especially the isosbestic points at 457 and 528nm is

$$\begin{gathered} \mathrm{X}\left({\mathrm{VO}}^{2+}\right)+\mathrm{Y}\left(\mathrm{xylenol}\mathrm{orange}\right)\to \mathrm{XY} \\ \mathrm{XY}+\mathrm{X}\to {\mathrm{X}}_2\mathrm{Y} \end{gathered}$$

Step-by-step solution

Define isosbestic point:

During the physical change or chemical change, the total absorbance sample will not change and it will have a specific wavelength or frequency is called isosbestic points.

Determine the sequence of chemical reactions from the given isosbestic points:

Hence, when${\mathrm{VO}}^{2+}$is added, corresponding to the traces1-9,the peak at 439nm decreases while one arises at 485nm with isosbestic points at 457nm.

Observe the traces 10-16, the peak at 485nm decreases while one arises at 566 nm with isosbestic point at 528nm.

Thus, we write the reaction as,

$$\begin{gathered} \mathrm{X}\left({\mathrm{VO}}^{2+}\right)+\mathrm{Y}\left(\mathrm{xylenol}\mathrm{orange}\right)\to \mathrm{XY} \\ \mathrm{XY}+\mathrm{X}\to {\mathrm{X}}_2\mathrm{Y} \end{gathered}$$

Therefore, the sequences of chemical reaction about the spectral changes are,

$$\begin{gathered} \mathrm{X}\left({\mathrm{VO}}^{2+}\right)+\mathrm{Y}\left(\mathrm{xylenol}\mathrm{orange}\right)\to \mathrm{XY} \\ \mathrm{XY}+\mathrm{X}\to {\mathrm{X}}_2\mathrm{Y} \end{gathered}$$