In this experiment absorption spectroscopy is conducted to detect Cr(VI)in aqueous solutions and with it the concentration of Cr(VI).
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!
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).
1. If a student adds more standard solution than intended in the initial dilution step, it would actually decrease the concentration of Cr(VI) determined in the experiment. Serial dilutions are done to accurately measure concentrations by diluting a standard solution multiple times. If the student adds more standard solution than intended, the dilution factor will be smaller than expected, resulting in a higher concentration of Cr(VI) in the final solution. This means that the measured concentration of Cr(VI) will be higher than the actual concentration.
2. When only filling a cuvette 1/4 full when taking the absorption spectrum of the lake water sample, it won't directly affect the concentration of Cr(VI) in the solution. However, it will affect the accuracy of the measurement. Absorption spectroscopy measures how much light is absorbed by a solution, which is directly related to the concentration of the absorbing species. If the cuvette is not filled to the appropriate level, the path length of the light through the solution will be shorter than expected, affecting the recorded absorbance values. This can result in an inaccurate determination of the Cr(VI) concentration.
3. If a student mistakenly uses the cuvette containing the most dilute standard solution as the blank solution instead of using deionized water, it could lead to an overestimation of the Cr(VI) concentration. The blank solution is used as a reference to account for any interference or background absorbance. By using a cuvette containing a standard solution instead of water, the absorbance measured for the blank solution will include the absorption of Cr(VI) present in the solution. This additional absorbance will be incorrectly attributed to the unknown solution, leading to a higher recorded concentration of Cr(VI).
4. When a student does not wipe off dirty fingertips on the cuvette containing the unknown solution before measuring its absorbance, it can potentially affect the accuracy of the concentration determination. Fingerprints or smudge marks on the cuvette can absorb or scatter light, interfering with the accurate measurement of absorbance. This can lead to incorrect absorbance values and subsequently an inaccurate determination of the Cr(VI) concentration. It is important to ensure the cuvette is clean and free from any contaminants that may interfere with the absorption spectroscopy measurement.