Question

Questions regarding the lab: Spectrophotometric Determination of Caffeine in a Soft Drink

1)How are the absorption spectra of atoms and molecules the same, and how are they different?

2)Beer’s law, A=e b c forms the basis of a calibration curve. It clearly shows that there is a theoretically linear relationship between absorbance and concentration under constant conditions of analyte absorptivity (e) and path length (b). However, it is important for an analyst to be aware that there are limitations analyte concentration range under which there truly is a linear relationship between absorbance and concentration. Under what conditions of concentration does the linear relationship break down?

3)Caffeine is not fluorescent. However, just for argument’s sake imagine that it was fluorescent. Would the wavelength of maximum fluorescence be shorter, longer, or equal to the wavelength of maximum absorbance? Why?

4)Why was it necessary to remove CO2 before the analysis?

Answer 1

1) Both atoms and molecules tend to absorb light of certain frequency matching with their valence shell or the bonds it has and emit the remaining light. when the amount of light absorbed is studies against the concentration of atoms or moelcules present at that time, we get absorption spectra. Atoms on the other hand tend to absorb light get excited and while coming back to the ground state emit light of certain pockets called photons corresponding to the type of transition it went through. Molecules on the other hand absorb light corresponding to it bond type, gets excited to higher level and then relax.

2) According to Beer's law the amount of light absorbed linearly depends upon the concentration of the solution. If however the concentration is very high, the absobance has reached its saturation and all the light will be absorbed. the absorption will be greater then 1 and this then does not follow Beer's law of absorbance. At very low concentrations the noise is very high and thus no significant absorbance values are obtained from the Beer's law.

3) For caffeine the wavelength of maximum absorbance would be longer then the maximum absorbance. This is a conjugated system, with conjugation the energy required for excitation of bonds gets lower (more single bond character) and thus the wavelength would be higher.

4) CO2 itlsef has a very strong absroption band in the UV which would intefere in the wavelength measurement of our sample. It is thus advised to remove all of CO2 present in before analysis