Question

1. Write out Beer’s Law for a mixture of 2 analytes and define the variables

2. Why are 2 wavelength reading necessary?

3. Why is HCl added to the solutions?

4. Calculate the concentration when 10.0 mL of 0.10 M HCl is diluted to 100 mL.

5. Calculate the volume required to make your working standards.

Answer 1

1) Let us assume that we have two analytes at concentrations C₁ and C₂ having molar extinction co-efficients ε₁ and ε₂. Let l be the length of the solution through which light passes.

We will assume that both the analytes absorb light at wavelengths λ. Let A be the absorbances at λ₁ and λ₂. Consequently,

A = ε₁C₁l + ε₂C₂l (ans).

2) In case of a mixture of two analytes, we assume that both the analytes are active to light, i.e, they absorb light. However, in order to determine their concentrations, we must make an assumption. We must select two wavelengths of light, λ₁ and λ₂. We will assume that one of the analytes absorb strongly at λ₁ while the absorbance of the second analyte at this wavelength is low (not negligible). Hence, the major portion of the light is absorbed by the first analyte at wavelength λ₁.

Similarly, we determine a second wavelength λ₂ where the second analyte absorbs strongly with low absorbance of the first analyte.

The two wavelengths are selected to ease the calculations. We approximate the absorbance at λ₁ to be mainly due to the first analyte and find its concentration. Similarly, we approximate the absorbance at λ₂ to be mainly due to the second analyte and find out the concentration.

Need to know the experiment for the remaining questions.