Quantitative Use of Red Cabbage To Measure pH through Spectrophotometry: A Laboratory Experience for General Chemistry Students

摘要

Red cabbage juice is a natural pH indicator: red, purple, blue, green and then yellow color is observed with increasing pH as the conjugated ring structures in the cabbage anthocyanins are rearranged. Classroom demonstrations and home kitchen lab procedures commonly employ cabbage juice extract to qualitatively display acid/base chemistry in a visual manner. We have developed a lab procedure that gives general chemistry-level students a more quantitative experience: by carefully measuring 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 from its absorbance spectrum in the presence of a small amount of the cabbage extract. Using curve-fitting software, a mathematical function is developed from the absorbance data that can be fit to an equation connecting absorbance and pH.