Posted in Education, Physical Chemistry, Spectroscopy, UV-VIS-NIR

Quantitative Color

I have always been bothered by subjective references to color, especially when used in acceptance specifications or quality evaluations.  One specification spelled out that the product must be burnished gold.  Great.  What exactly is burnished gold. 

This sent me down the path toward colorimetry, and since this was lacking in my B.S. and Ph.D. chemistry curricula, I decided to include it in the Physical Chemistry course at Sam Houston State University.  Here are the abstracts for two articles I wrote for the Journal of Chemical Education

Williams, D. L., Flaherty, T. J., Jupe, C. L., Coleman, S. A., Marquez K. A., Stanton J. J., Beyond Lambda-Max: Transforming Visible Spectra into 24-bit Color Values, Journal of Chemical Education, 84(11), 1873-1877 (2007).

This article introduces the standardized definition of color, and provides a spreadsheet tool in the Supplemental Material that allows the assignment of standard 24-bit color values to solutions based upon their visible spectra. A digital photograph of several colored solutions was taken. The visible spectra of these solutions in the range of 400–700 nm were obtained with a spectral bandpass of 5 nm. The spectra were exported to a spreadsheet where they were transformed into tristimulus values, chromaticity coordinates, and standard 24-bit RGB colors. Using the spreadsheet drawing tools, a color swatch for each solution was created to display the corresponding RGB color. Finally, the digital image of the solutions was imported and compared to the color swatches to explore the differences and similarities of photographic and spectrophotometric colorimetry. The spreadsheet data analysis is emphasized in this article, since the lab procedures for obtaining visible spectra are well-established in most undergraduate curricula.

Williams, D. L.; Flaherty, T. J.; Alnasleh B. K., Beyond Lambda-Max Part 2: Predicting Molecular Color, Journal of Chemical Education, 86(3), 333-339 (2009).

A concise roadmap for using computational chemistry programs (i.e., Gaussian 03W) to predict the color of a molecular species is presented. A color-predicting spreadsheet is available with the online material that uses transition wavelengths and peak-shape parameters to predict the visible absorbance spectrum, transmittance spectrum, chromaticity diagram, and the standard 24-bit color values, sRGB, at several theoretical concentrations. The technique is demonstrated on azulene derivatives and phenolphthalein. The use of this color-prediction spreadsheet in our introductory chemistry lectures and the physical chemistry laboratory is described along with the applicable topic areas. The theory behind color prediction is explained, but the tool is well-suited for novice users. It has been successful in increasing student engagement with the concepts of spectroscopic absorbance and transmittance while introducing the unfamiliar concepts of standard color spaces.

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Author:

Dr. Darren L. Williams (pchem4u) is an Associate Professor of Physical Chemistry at Sam Houston State University. When he is not blending solvents, he is tinkering with contact angle measurements, FTIR microspectroscopy, etc. http://www.shsu.edu/~chm_dlw/ . He welcomes your comments and questions by phone (936-294-1529) and by email (williams “at” shsu.edu).

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