Question

1) The equation A=Ebc directly relates the concentration of a sample, its molar absorptivity (E), and the absorbance of a sample. Instead of bothering with a standard curve, why not make two measurements: first, measure the absorbance of a sample of known concentration in order to compute the molar absorptivity. Second, measure the absorbance of the solution with unknown concentration and use the molar absorptivity from the first to calculate the concentration. Explain why this would, or would not, be a better method than creating a standard curve and measuring the absorbance of an unknown.

2) Define interpolation and extrapolation. Which is the more reliable/acceptable pracitce? Why do we avoid extrapolation when we do an experiment with Beer's Law? Why do we avoid extrapolation when we use linear regression? Explain.

3) Neither o-phenanthroline nor Fe2+(aq) are colored in solution. Explain the appearance of color when these two are mixed.

4) What are the important factors to consider when choosing a colorimetric reagent to use for a particular analysis?

Answer 1

ANs1: measure the absorbance of a sample of known concentration in order to compute the molar absorptivity. Since you are going to have just one reading, it is not sufficient to justify the value of E i.e. epsilon. Yoa have to take number of reading i.e. may be at leat 7-8 to confirm the value of E. If you have consistent E from all the reading then you can measure the absorbance of the solution with unknown concentration and use the molar absorptivity from the first to calculate the concentration.

Ans 2:  interpolation is estimation of values on a graph between any two known points (already obayined through any reading) and extrapolation is estimation of values by plotiing a graph beyond the last value obtained. In Labert beer law, if you will extrapolate any value, you can be misled with data since nothing can be predicted beyond what you have obtained, i.e. you can not predict any value on graph. but on the other hand interpolate means you will skectch the graph between two points alredyy obatined. So, it will not lead to any scientific error.

Ans 3: You are wrong with the question that Fe(II) is not coloured. Fe(II) is pale green coloured solution but when you mix it with o-phenanthroline, it gives a complex, known as ferrion, which is red in colour due to charge tranfer complex formation. The charge transfer is from ligand to metal which gives it intense colour.

Ans 4: important factors to consider when choosing a colorimetric reagent to use for a particular analysis:

the colorimetric solution should be stable to enviornmental variation.

the life of that agent should be of months at 25oC to 40oC

it should be cheap and easily available

the ease for measurement should be good and

the results produced should be reliable.