Question

3) What two factors are held constant when constructing a calibration curve? 4) Briefly explain the purpose of using a "blank" before measuring your calibration solutions. 5) For a solution that appears blue in color, which wavelength of light in nm is expected to be absorbed most strongly? Give the approximate value or range of values.

Answer 1

3) Beer’s law is given as

A = ε*c*l

where A is the absorbance of a solution of a sample having concentration c and l is the path length of the solution (length of the spectrophotometer cuvette). ε is the molar absorptivity of the solution. ε is a characteristic of the sample and depends only on the sample of interest. The value of ε remains constants for a particular sample or analyte.

In experiments related to Beer’s law, the concentrations of a series of solutions are varied and the absorbances are measured. The constant parameters are the molar absorptivity (ε) and the path length of the solution. A cuvette of a particular length is used in the spectrophotometer.

4) In experiments of spectrophotometry, the absorbances of a series of solutions having different concentrations are measured and a calibration plot is constructed. The solutions are prepared by dissolving different amounts (weights or moles) of a solute in varying volumes of a solvent.

The solvent is used as a “blank” in the experiment and the absorbance of the blank is measured prior to measuring the absorbance of the solution. This is because the solvent (blank) may absorb light at the wavelength of the measurement and the absorbance of the solvent will add to the absorbance of the solution. In order to nullify the effect from any possible absorbance of the blank, a blank solvent is run before any measurement is taken so that the absorbance due to the solvent is nullified in the experiment.

5) The solution appears blue in color. Hence, light of the complimentary color is absorbed. The complimentary color of blue is orange, having a wavelength of 440-490 nm. Since the solution shows blue color, hence, the solution absorbs in the region 440-490 nm.