How would each of the following affect the Cr(VI) concentration determined in this experiment? Justify the answers with an explanation.
In the experiment, absorption spectroscopy was used to obtain the concentration of Cr(VI) ions.
1) A student adds more standard solution than intended in the initial dilution step when performing serial dilutions.
This would cause an increase because the other series of dilutions would have more concentration, right?
2) As student only fills a cuvette 1/4 full when taking the absorption spectrum of the simulated lake water sample.
There would be a decrease because the scanner probably did not scan the liquid obtaining little or no concentration, right?
3) A student mistakenly uses the cuvette containing the most dilute standard solution, rather than the cuvette containing deionized water, as the blank solution.
I don't understand why there would be lower concentration of Cr(VI) ion...
4) A student does not wipe off dirty fingerprints on the cuvette containing the unknown solution before measuring its absorbance.
There would be more absorbance since the oil from the fingers would absorb some amount of light, leading to a higher concentration of Cr(VI) in the unknown solution, right?
Thank you for helping!
I don't agree with #1 but I do the others. I think the results will be low if I've interpreted the question correctly and this is not a standard additions method.
I can't draw it on this board but here is what you do.
On a sheet of graph paper, mark the Y axis as A (for absorbance) and the x axis for C (for concentration). Now we will make a standard curve. The first one right and second one wrong.
A.....C
--------
0.1...0.1 mg/L
0.2...0.2 mg/L
0.3...0.3 mg/L
Now, on the same sheet of graph paper and with the same coordinates and concns suppose we put in more of the standard before making the serial dilutions. So we might have something like this.
A.....C in mg/L
---------
0.l...0.2 (it should have read 0.1 if we did it right.
0.2...0.3
0.3...0.4
If you draw those lines they will be straight lines one above the other. NOW, suppose we measure a sample and it actually is A = 0.2. So you read it on the upper curve and you obtain 0.1 mg/L. Reading it on the lower curve you obtain 0.2. So you should be reading 0.2 mg/L but you read (on the wrong curve) 0.1 mg/L so the results are low.
1) When the student adds more standard solution than intended in the initial dilution step, it would affect the Cr(VI) concentration determined in the experiment. This is because serial dilutions involve diluting a solution multiple times, and each dilution decreases the concentration. By adding more solution than intended in the initial dilution step, the subsequent dilutions will have a higher starting concentration. As a result, the concentration of Cr(VI) ions determined through absorption spectroscopy will be higher than expected due to the initial higher concentration.
2) If the student only fills a cuvette 1/4 full when taking the absorption spectrum of the simulated lake water sample, it would indeed affect the Cr(VI) concentration determined. Absorption spectroscopy measures the absorption of light by a sample to determine its concentration. If the cuvette is only partially filled, there is less sample volume for the light to pass through and interact with. This results in a reduced absorption reading, leading to an underestimated concentration of Cr(VI) ions in the sample.
3) When a student mistakenly uses the cuvette containing the most dilute standard solution as the blank solution instead of deionized water, it would lead to an underestimated concentration of Cr(VI) ions. The blank solution is used as a reference to account for any background absorbance or interference from other substances present in the sample. The most dilute standard solution should ideally have no Cr(VI) ions, allowing it to act as a blank with minimal absorbance. However, if this solution is used as the blank, it will contribute some absorbance due to the presence of Cr(VI) ions. As a result, the absorbance of the other solutions will appear lower, leading to an underestimated concentration of Cr(VI) ions.
4) If a student does not wipe off dirty fingerprints on the cuvette containing the unknown solution before measuring its absorbance, it would affect the Cr(VI) concentration determined in the experiment. The presence of fingerprints or any other contaminants on the cuvette surface can interfere with the transmission of light through the sample. These contaminants can absorb or scatter light, resulting in a higher measured absorbance. This would lead to the perception of a higher concentration of Cr(VI) ions in the unknown solution than the actual concentration. Thus, ensuring a clean cuvette without any fingerprints or contaminants is essential for accurate absorbance measurements and concentration determination.