In this experiment absorption spectroscopy is conducted to detect Cr(VI)in aqueous solutions and with it the concentration of Cr(VI).

Question

How would each of the following affect the Cr(VI) concentration determined in this experiment? Explain the answer.

1. A student adds more standard solution than intended in the initial dilution step when performing serial dilutions.

Would this increase the concentration since there would be a more detection of the Cr(VI)?

2. A student only fills a cuvette 1/4 full when taking the absorption spectrum of the simulated lake water sample.

Would this decrease the concentration of Cr(VI) because there is less absorption because there is a less quantity in the cuvette?

3. A student mistakenly uses the cuvette containing the most dilute standard solution, rather than the cuvette containing deionized water, as the blank solution.

Would this decrease? I don't know why though.

4. A student does not wipe off dirty fingertips on the cuvette containing the unknown solution before measuring its absorbance.
Would the concentration decrease because there is less absorption of light of the solution because of the smudge marks blocking the light?

Thank you for helping!

Answer 1

I agree with 1. The standard curve is too high so readings from it will be too high.

2. It depends upon where the beam is. If the liquid in the cuvette is enough to encounter all of the light beam there will be no effect. If the beam is higher than the top of the liquid, there will be less absorption; therefore, less Cr.

3. So the blank reads too high, Abs sample-Abs blank is too low and that read from the standard curve will be too low so Cr is too low.

4. I don't agree with your answer here. Dirty finger smudges will absorb (or reflect) more light so the abs will be too high leading to high results. Read your last sentence. If the smudge marks BLOCK the light wouldn't that mean the absorbance was too high (i.e., transmittance was too low).