When measuring absorbance of solutions, why are the measurements taken in order of increasing concentration?

Question

One of the experiments I have to do is 'The Beer Lambert law' - Spectroscopy'. Where I have to measure the absorbance of standard solutions of Ferroin complex.
I would also like to know:
1) how can I calculate the concentration of iron from the absorbance?
2) Ater plotting a graph of absorbance vs. concentration (for the samples of known concentration), how can I then calculate the molar absorptivity of ferroin at 470nm?
3)How can I then use the graph to establish the total mass of iron in the the tablet toether with estimated errors?
4)What colour of light does 470nm correspond to and why was this wavelength chosen for this experiment?

[p.s. Sorry for asking so much questions - It's just that I cannot find any suitable answers and my lecturers won't help me!] Thanks to anyone that can help me out.

Answer 1

Well you know the law:

A=E*d*C

You are using the same cuvette for all measurements, therefore when you change the solution that you will measure there is the possibility that some quantity of the previous solution will remain in the cuvette. If the previous solution is more concentrated then the error will be big, since it will change a lot the actual concentration of the new solution in the cuvette. If it is less concentrated the error will be small.

To answer (1) you need to do (2) first.
Since you want E470, you should be measuring the absorbance of the standards at 470 nm. You do the plot. Then you do least squares method to find the equation that describes the standard curve (linear). The slope that you find is E*d. d depends on your cuvette and usually it is 1cm so slope=E expressed in the respective units.

Once you have that equation you use it to calculate the concentration of ferroin of an unknown sample based on the absorbance. From then on you have to find the relationship between iron and ferroin (is it 1:1 stoichiometry?) so that you can find the concentration of iron

For the mass, I don't know the exact experimental procedure, but once you have the concentration of iron you just need to do some calculations (depending on the experimental set up) and calculate it.

As for 4, usually you pick wavelengths where absorbance is maximum (peak in the spectrum). I don't know if there is another reason as well (e.g. you have other substances in your sample whose spectra overlap with that of ferroin at some wavelenghts but not significantly at 470 nm; even in this case ferroin should have a peak at or near 470 nm) The color that corresponds to 470nm you can definitely find on the web

Answer 2

Molar Absorptivity Of Ferroin