Mark Cannon (Brigham Young University – Hawaii, USA)
Red cabbage juice is commonly used in introductory chemistry laboratories as a visible indicator of acidity because its color varies with pH: red, violet, green, blue and then yellow color is observed with increasing pH as various anthocyanins from the cabbage become deprotonated. Previously reported lab procedures using red cabbage have mostly been qualitative explorations; safe and fun, with a high “gee whiz” factor. We have developed a lab procedure that gives General Chemistry-level students a more quantitative experience: by carefully measuring UV-Vis absorbance spectra of cabbage juice samples over a range of pH students can develop a process for accurately measuring the pH of an unknown solution, with a small added volume of cabbage juice, from its absorbance spectrum. In addition, careful analysis of the visible spectra gives students an appreciation for absorbance spectroscopy and the connection between absorbance peaks and observed color.
