Question

Q1. A compound was characterized using UV-visible absorption spectroscopy. The absorbance spectrum of a 1.00 x 10–4 mol/L solution in a 1.00 cm path length cell peaked at the wavelength of 345 nm to a value of 1.32.

(1) What was used as a reference? Specify the cell and its contents.

(2) What is the transmittance at that wavelength?

(3) What is the molar absorptivity at that wavelength?

Q2. Answer the following questions regarding quantitative analysis using UV-visible spectroscopy. Assume that the spectrometer gives you the raw intensity spectrum of the transmitted light as a function of the wavelength only, and the light source is stable.

(1) You are given a water-soluble compound. Explain how you can obtain the spectrum of specific absorption coefficient ε(λ) = Abs/(bc), where Abs is the absorbance spectrum, b is the path length, c is the mass concentration (wt% or g/mL).

(2) Let us assume that you have obtained specific absorption coefficient εa(λ) and εb(λ) for two water-soluble compounds a and b having different colors. You are given a mixture that consists of the two compounds and a third transparent compound. How can you find the concentrations of and b (ca and cb, respectively) when the total concentration of the mixture in water is c (c > ca + cb). Explain the procedure.

Answer 1

1.

(1) The solvent used for making the solution of compound was used as reference.

(2)Here

concentration, c =  1.00 x 10^–4 mol/L

path length, l = 1 cm

Absorbance, A = 1.32

Now.

Transmittance, T = 10^-A = 10^-1.32 = 0.0479

(3) According to Beer-lambert law;

A = c l ; where = molar absorptivity

= A/cl

= 1.32 / [ (1.00 x 10^–4 mol L^-1) (1 cm)

= 1.32 x 10⁴ mol^-1 L cm^-1

2.

(1) Here

A = -log T

Where A= absorbance and T= tansmittance

So,

(λ) = -log T / (bc).

(2) Absorbances are additive properties.

Thus, in a solution containing n independently absorbing species, the total absorbance is given by

A = A1 + A2 + ......An

= 1c1 + 2c2 + ......ncn

where c1, c2, ......cn are the concentrations of the species and 1, 2, .....n are their respective absorption coefficients.

By solving these equations we can find the concentrations.