Flash Photolysis Experiment of o‑Methyl Red as a Function of pH: A Low-Cost Experiment for the Undergraduate Physical Chemistry Lab

• Published in: Journal of chemical education Vol. 93; no. 12; pp. 2096 - 2100
• Main Authors: Larsen, Molly C, Perkins, Russell J
• Format: Journal Article
• Language: English
• Published: Easton American Chemical Society and Division of Chemical Education, Inc 13.12.2016; Division of Chemical Education, Inc and ACS Publications Division of the American Chemical Society; American Chemical Society
• Subjects: Bleaches; Chemistry; College Science; Flash photolysis; Isomerization; Laboratory Experiments; Laboratory Procedures; Low cost; Molecules; Organic chemistry; Photolysis; Physical chemistry; Physics; Rate constants; Science Experiments; Science Instruction; Science Laboratories; Spectroscopy; Spectrum analysis; Students; Thermodynamics; Time; Undergraduate Students; Colorado; Hands-On Learning/Manipulatives; Spectroscopy; UV−vis Spectroscopy; Upper-Division Undergraduate; Laboratory Instruction; Physical Chemistry; Lasers; pH; Kinetics; Photochemistry
• ISSN: 0021-9584; 1938-1328
• DOI: 10.1021/acs.jchemed.6b00403

A low-cost, time-resolved spectroscopy experiment appropriate for third year physical chemistry students is presented. Students excite o-methyl red in basic solutions with a laser pointer and use a modular spectrometer with a CCD array detector to monitor the transient spectra as the higher-energy cis conformer of the molecule converts back to the thermodynamically more stable trans form. The transient absorption dynamics are monitored as a function of time averaged between 340 and 360 nm, and the transient bleach dynamics are monitored as a function of time averaged between 440 and 460 nm. The bleach dynamics are monitored in basic solutions at several values of pH, and the observed rate constants are used to extract the rate constant for the isomerization of the protonated form and the deprotonated form of the molecule.