In both general chemistry and analytical chemistry courses, students are introduced to the concept of predominant species in solution when discussing acid/base chemistry. Speciation diagrams are often used to illustrate the concept and predict the relative abundance of species in solution. Herein, we describe a laboratory experiment for an undergraduate analytical chemistry course, in which students directly measure the speciation diagram and compare it to the diagram calculated using equations they derive in lectures. In this laboratory experiment, students prepare solutions of fixed pH in the pH range 0-14 and use them to measure the speciation diagram of thymol blue with UV-vis spectrophotometry. After collecting the absorbance spectra of all the solutions, students perform a series of data processing steps to derive the speciation diagram of thymol blue from the experimental data and compare it to the calculated diagram. Finally, students use the speciation diagram to corroborate the expected pH ranges, in which the three forms of thymol blue are predominant species. This experiment provides students with a hands-on experience with buffer preparation, enables a straightforward measurement of the speciation of different forms of a diprotic acid, and helps them visualize the concept of predominant species in solution.