A Fresh Look at the Crystal Violet Lab with Handheld Camera Colorimetry Theodore R. Knutson, §,† Cassandra M. Knutson, §,‡ Abbie R. Mozzetti, † Antonio R. Campos, † Christy L. Haynes, † and R. Lee Penn* ,† † Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡ White Bear Lake High School, White Bear Lake, Minnesota 55110, United States * S Supporting Information ABSTRACT: Chemical kinetic experiments to determine rate laws are common in high school and college chemistry courses. For reactions involving a color change, rate laws can be determined experimentally using spectrophotometric or colori- metric equipment though this equipment can be cost prohibitive. Previous work demonstrated that inexpensive handheld camera devices can be used to quantify the concentration of a colored analyte in solution. This paper extends this approach to the kinetic study of the color fading of crystal violet upon reaction with sodium hydroxide. The results demonstrate accurate determination of the reaction order, with respect to crystal violet, using a method accessible in many high school and college laboratories. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Hands-On Learning/Manipulatives, Kinetics, Laboratory Equipment/Apparatus, Analytical Chemistry, Physical Chemistry, Dyes/Pigments M ost high school and college students have some practical knowledge about speeds of reactions before taking a chemistry course. For example, students understand that foods cook faster at higher temperature. This knowledge is supported and extended by the study of chemical kinetics. While studying kinetics, students learn that rate laws for chemical reactions can only be determined experimentally. Experiments suitable for exploration of kinetic concepts are essential for building connections to the curriculum. Rate laws can be determined by measuring initial rates or monitoring concentration over time. Monitoring changes in concentration of a colored analyte in solution can be accomplished through spectrophotometry or colorimetry. Traditional equipment used for these measure- ments can cost hundreds to thousands of dollars per instrument; many high schools do not have the financial means to purchase such instrumentation. Recently, Kehoe and Penn published a method for performing quantitative color- imetry using handheld camera devices. 1 Their work demon- strated suitable precision and accuracy; thus, quantitative colorimetry can be performed even in the absence of traditional equipment. Crystal violet, an intensely violet-colored triphenylmethane dye, reacts with hydroxide ions in aqueous solution to form a colorless compound (Scheme 1). For years, this reaction has been successfully used as a lab exercise for the experimental determination of a rate law. 2,3 A large excess of sodium hydroxide relative to crystal violet is used, which means that the reaction’s rate depends only on the concentration of crystal violet. Analytical spectrophotometry or colorimetry is per- formed at specified time intervals. Students monitor the concentration of crystal violet, which fades over time, by Scheme 1. Reaction Scheme between Crystal Violet and Hydroxide Ions a a Structures (a) and (b) are two resonance structures of crystal violet before the reaction, and structure (c) is the colorless product of the reaction. Laboratory Experiment pubs.acs.org/jchemeduc © XXXX American Chemical Society and Division of Chemical Education, Inc. A DOI: 10.1021/ed500876y J. Chem. Educ. XXXX, XXX, XXX-XXX