Spectrophotometry of chromate-dichromate equilibrium

[24forChromium]

The goal is to determine the relative abundance of the ions using the spectrophotometer. As far as I understand, to determine the relative (or absolute) concentration of a coloured ion in solution, one must have the absorbance/transmittance of the solution at multiple wavelengths.

Once the absorbance/transmittance of a solution over various wavelengths of light is acquired, one should do something with the maximum point of the absorbance/transmittance over frequency graph, which I do not know.

Please explain the following steps, point out any mistakes in my procedure and suggest a wavelength range to examine the chromate-dichromate mixture in.

 

[Borek]

As far as I understand, to determine the relative (or absolute) concentration of a coloured ion in solution, one must have the absorbance/transmittance of the solution at multiple wavelengths.

No, you select one wavelength for that. Typically we will select the one where absorbance is the highest, as it gives you the highest sensitivity (you can measure lower concentrations).

Google Beer-Lambert law.

 

[24forChromium]

No, you select one wavelength for that. Typically we will select the one where absorbance is the highest, as it gives you the highest sensitivity (you can measure lower concentrations).

Google Beer-Lambert law.

Thanks for the reply, so what happens is that, across a certain length of the liquid, the solution will absorb an amount of light that is related to its concentration. Is there a way to find out how much of each ion there is if there is a mixture of two liquids?

Here is what I think: first decide on a certain wavelength, find the total absorbance of the mixture (A_total)

A_total = A_1 + A_2 where A_1 and A_2 represent the absorbance of each chemical at this wavelength
A_total = a_1 * L * C_1 + a_2 * L * C_2 where a represent the absorptivity of each chemical, L is the length of the examined liquid, C represent the concentration of each solute.

Is that right?

 

[Borek]

OK so far. Now, if you select another wavelength you will have another measurement result. That will yield second equation in the same two unknowns (c1 and c2).

You just have to select such pair of wavelengths that a1/a2 ratios are as different as possible (that will help minimalize errors).