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!
1) If a student adds more standard solution than intended in the initial dilution step when performing serial dilutions, it would actually result in a decrease in the determined concentration of Cr(VI) ions. This is because the additional volume of standard solution would result in a higher overall dilution factor. As a result, the concentration of Cr(VI) ions would be more diluted in the final solution, leading to a lower concentration determination.
To calculate the effect, you would need to know the intended volume of the standard solution added and the volume of the diluent (e.g., water) used in the dilution. By comparing the intended dilution ratio to the actual dilution ratio, you can determine the impact on the concentration of Cr(VI) ions.
2) If a student only fills a cuvette 1/4 full when taking the absorption spectrum of the simulated lake water sample, it would likely result in an underestimate of the concentration of Cr(VI) ions. This is because the cuvette is not filled to its standard volume, which may affect the absorbance readings obtained during the spectroscopy. The readings might be lower than expected due to the reduced sample volume, leading to a lower determined concentration.
To account for this, it is important to ensure that the cuvette is filled to the appropriate volume specified in the experimental procedure. This ensures accurate and consistent absorbance measurements.
3) If a student mistakenly uses the cuvette containing the most dilute standard solution, rather than the cuvette containing deionized water, as the blank solution, it would result in an overestimate of the concentration of Cr(VI) ions.
In absorption spectroscopy, a blank solution is used to account for any absorbance contributed by the solvent or matrix. By using the cuvette containing the most dilute standard solution as the blank, the student would be using a solution that contains Cr(VI) ions, albeit at a low concentration. This would introduce an additional absorbance reading that should have been contributed solely by the solvent or matrix, leading to an incorrect determination of the concentration of Cr(VI) ions.
To mitigate this error, it is vital to use the correct blank solution, which should not contain any analyte (Cr(VI) ions in this case). This ensures that the absorbance reading used for determining the concentration is solely due to the Cr(VI) ions in the sample solution.
4) If a student does not wipe off dirty fingerprints on the cuvette containing the unknown solution before measuring its absorbance, it would likely result in an overestimate of the concentration of Cr(VI) ions. This is because the finger oil or any other contaminants on the cuvette surface can absorb or scatter light, leading to an increased absorbance reading.
To avoid this, it is essential to clean the cuvette thoroughly and ensure that it is free from any fingerprints, smudges, or other contaminants before measuring the absorbance. This ensures accurate absorbance readings and reliable determination of the concentration of Cr(VI) ions in the unknown solution.