See the Light by Making a Cell Phone Spectrophotometer Questions

[deleted-330896]

Hello!
I am doing this project with various colored liquids. I was hoping I could find the concentration of dye in a certain drink with the Beer-Lambert equation (A = e*c*l). After inserting my pictures into the software, I receive the graph with the maximum absorbance and the corresponding wavelength. Would I be able to plug in the maximum absorbance into the equation to find the concentration? If not, is there a possible way to find the absorbance with the information I have (the graph)?

Thanks in advance!

[norman40]

Hello sophia2101,

I’m assuming that you’re working on the project described here:

https://www.sciencebuddies.org/science- ... p100.shtml

In this project you build a spectrophotometer and use it to observe the wavelength at which maximum absorbance occurs for several colored liquids.

You could use the spectrophotometer to find the concentration of a colored solution. As you pointed out, the Beer-Lambert equation relates absorbance to concentration (c in the equation). To calculate the concentration you need the absorbance, the extinction coefficient (e in the equation) and the sample path length (l in the equation).

Because you don’t know the extinction coefficient or the path length, a calibration experiment is needed. To do this, make up three or four solutions with different concentrations of the material of interest. In your case this would be the dye in the drink that you want to test. Measure the absorbance for each solution with your spectrophotometer. Use the same size cuvette and spectrophotometer set up for all tests.

Now make a graph with concentration on the x-axis and absorbance on the y-axis. Your data should form a straight line. The slope of the line is equal to the extinction coefficient multiplied by the path length. You can use the graph to find the concentration of an “unknown” sample (one that you didn’t use for the calibration). Just find the concentration on the x-axis that corresponds to the absorbance value for your “unknown” sample.

I hope this helps. Please post again if you have more questions.

A. Norman

[deleted-330896]

Thanks so much! That was a great help. When referring to the absorbance, should I be using the maximum absorbance?

[norman40]

Hi sophia2101,

If you are doing an experiment like the one I described in my previous post, you should find the wavelength that gives the maximum absorbance for the material you’re testing. Then you should use the absorbance measured at that wavelength for the calibration and “unknown” sample tests in the experiment.

Otherwise, the visible light wavelength at which maximum absorbance is found is an indication of the color of the solution. For example, wavelengths of maximum absorbance for several dyes are listed in the Table 1 of the project procedure.

I hope this helps. Please post again if you have more questions.

A. Norman

[deleted-330896]

Hi again,

You mentioned before that the slope of a line graphing concentration vs. absorbance is equal to the extinction coefficient multiplied by the path length. Why does this occur?

Thank you

[norman40]

Hi sophia2101,

The equation for a straight line with an intercept of zero (y = mx) has the same form as the Beer’s Law equation (A = e*l*c). Comparing the two equations, the Beer’s Law “A” is the “y” in the linear equation, “c” from Beer’s Law corresponds with “x”. The Beer’s Law “e*l” (path length multiplied by the extinction coefficient) is the slope (m) in the linear equation. So the proportionality between concentration and absorbance is the product of the path length and extinction coefficient.

The extinction coefficient is the amount of light at a given wavelength a mole of a chemical compound will absorb. Light absorption is a result of light interacting with the chemical compound. The amount of interaction increases when light passes through a larger number of molecules of the compound. Because of this, the light absorbance will vary with concentration and with the distance the light travels through a solution of the compound (the path length). The path length is normally kept constant in experiments aimed at finding an extinction coefficient.

I hope this helps. Please post again if you have more questions.

A. Norman