A simple, inexpensive, and environmentally friendly undergraduate laboratory experiment is described in which students use visible spectroscopy to determine a numerical value for an equilibrium constant, K[subscript c]. The experiment correlates well with the lecture topic of equilibrium even though the subject of the study is an acid-base indicator, bromothymol blue. The experiment gives excellent results and offers an inexpensive, zero-waste alternative to traditional equilibrium experiments, such as the Fe(SCN)[superscript 2+] equilibrium. (Contains 1 table, 1 figure, 1 scheme, and 2 notes.)