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Acid–base indicators are among the first complex molecules that many students encounter in their study of chemistry. The molecules featured in the August 2008 issue were three common indicators with an emphasis on structural changes occurring on going from the acid form to the base form (1).
The JCE Featured Molecules for this month are also indicators (Figure 1) and come from the paper by Donald W. Mundell on the phenomenon of Marangoni flows (2). The indicators are used to make the demonstration more graphic, or to use the term coined by Dick Ramette, more exocharmic (3).
 indigo carmine
 neutral red
 alizarin yellow
Figure 1. Three of the molecules added to the JCE Featured Molecules collection from the article by Mundell (2).
Marangoni flow (sometimes called Gibbs–Marangoni flow) remains a topic of interest in various fields where interfaces are important. Recent papers have reported such flow for silver nanoparticles in a water/alcohol solvent containing a surfactant (4) and in very high temperature melts (5).
Those interested in the history of chemistry might wish to explore the role played by Willard Gibbs in explaining the phenomenon, and those interested in interfacial processes and intermolecular interactions will find a wealth of information in the primary literature. This seems an appropriate topic for student presentations or papers in a physical chemistry or introductory chemistry course.
Students will also find that indicators have a life outside of titrimetry, although indicators are still used to respond to changes in hydrogen ion concentration. Several recent patent applications involve one or more indicators. An application from Xerox Corporation describes the use of indicators in what they are calling inkless printer paper. This application, particularly the various claims set forth, is an excellent way to introduce students to the patent literature and process (6). In a second application, alizarin yellow R is the key ingredient in a carbon dioxide sensor designed to measure CO2 emissions during the ripening process of fruits and in the detection of food spoilage (7).
Students with an interest in computation could explore the extent of nitrogen participation in delocalized bonding in neutral red, indigo carmine, and the methyl violet B cation.
Literature Cited
- Coleman, William F. J. Chem. Educ. 2008, 80, 1152.
- Mundell, Donald W. J. Chem. Educ. 2009, 81, 833–836.
- Ramette, Richard W. J. Chem. Educ. 1980, 57, 68
- Johnson, Donald D. Jr.; Kang, Barry; Vigorita, John L.; Amram, Alec; Spain, Eileen M. J. Phys. Chem. A 2008, 112 (39), 9318–9323.
- Hibiya, Taketoshi; Ozawa, Shumpei. Marangoni Flow and Surface Tension of High Temperature Melts. In High-Temperature Measurements of Materials, Advances in Materials Research, Volume 11; Fukuyama, H., Waseda, Y., Eds.; Springer: Berlin Heidelberg, 2009; Chapter 3.
- Inkless Printing Paper and Method–Patent (accessed May 2009).
- European Patent Office. Enter “WO2008079024” into search to find “Sensor Device” (accessed May 2009).
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