Mixing Up a Better Lab: Wavelength Choice Affects Accuracy and Consistency of the UV-Vis Spectroscopy Analysis

Overview

Objectives/Goals For this study, I moved to scientifically ascertain the source of the inconsistencies in the results of the students who performed the UV-Vis Spectroscopy Lab as as part of their Analytical Chemistry 251 lab at SDSU, and then to remedy it. Thus, I asked the question, #what causes unreliable results when using spectrophotometry to find the concentration of Iron (II) and how does one overcome such obstacles?# Methods/Materials I predicted that the wavelength used by the original lab procedure, 562nm, was not best for detecting the iron in the solution and was, thus, the source of the inconsistencies reported by the students. I thought that changing the wavelength to one that can detect a higher absorbance would negate such unfortunate effects by rendering any error insignificant. I first measured the absorbances of a single solution of Iron (II) marked with 1, 10 phenanthroline in a spectrophotometer at different wavelengths to find the one with the highest results. I then measured the absorbances of solutions with a constant amount of unknown and varying amounts of standard at the original wavelength and the new one to find the concentration of the unknown (absorbance is directly proportional to concentration). Results The wavelength with the maximum absorbance was found to be 512nm. After using the relative absorbances for the known concentrations of the standards to to derive the concentration of the unknown at both 512nm (experimental) and 562nm (control), I found that the error of the experimental wavelength was 3 times smaller than that of the control. Its results were also reproducible, falling consistently within an error of 10%. Conclusions/Discussion the experiment concluded that the wavelength that was used by the students before this study, 562nm, was too high and not as sensitive to the particular shade of green that was absorbed by the solution. Meanwhile, 512nm was an excellent wavelength that allowed for optimal measurements to be made under the circumstances and the skills being used.

Competition history

• CSEF 2016 Chemistry · Entry S0606

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