Question

Iron(III) reacts with thiocyanate ions to form the red complex $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$. Sketch a photometric titration curve for Fe(III) with thiocyanate ion when a photometer with a green filter is used to collect data. Why is a green filter used?

Short answer

The titration curve for the titration of iron(III) with $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$is shown in the figure.

The green-colored light should be used for the titration purpose as the red color complex solution of $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$absorbs the green light.

Step-by-step solution

Step 1. Given information

Draw the sketch of the photometric titration curve for the titration of Iron(III) with $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$and also explain the reason for using the green filter.

Step 2. Photometric titration curve 

The photometric titration curve is the plot showing the relationship between the absorbance of the solution corrected for the particular volume changes versus the volume of the titrant present in the solution.

Step 3. The photometric titration curve for the titration of iron(III) with FeSCN2+

The absorbance of the solution increases linearly till the endpoint of the titration is reached in the solution. After the endpoint of the titration is reached, the increase in the value of the absorbance is almost stopped with the increase in the amount of titrant ${\mathrm{SCN}}^{-}$.

The photometric titration curve for the titration of iron(III) with $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$is shown below:

Step 4. Purpose of green coloured radiation

The green-colored radiation should be used for the titration purpose because the red color complex solution of $\mathrm{Fe}{\left(\mathrm{SCN}\right)}^{2+}$absorbs the green light easily.