Introduction pKa values are quite useful when determining qualitative information of acids. The pKa helps determine the strength of an acid and describes its basic functionalities and ionization state1-2. The pKa of drugs influence factors such as solubility, protein binding, and diffusion rate across a membrane3. pKa values can also be utilized to determine the effects of active pharmaceutical ingredients in biological systems2. pKa, otherwise known as the acid dissociation constant, can be found through the pH of a the solution and the concentrations of the acid and conjugate base4. Given the importance of pKa, this study demonstrates how UV/Vis spectroscopy is used to determine pKa. There are several methods to find pKa, but the method of UV/Vis spectroscopy offers precision and efficiency compared to others, such as potentiometric titrations4 or calorimetry5. In this paper, the method for determining pKa bromothymol blue using UV/Vis spectroscopy is presented. Several solutions of bromothymol blue with varying pH were created and the analyses were performed under a UV/Vis spectrophotometer. Once absorbance and pH were determined, pKa was solved for. Experimental Materials and Methods. All chemical components were provided by the Boston University Analytical Chemistry stockroom. Solids were massed by top loading balances and volumes were measured using calibrated volumetric equipment. All pH measurements were made using a Vernier pH sensor that was calibrated with buffers at a pH of four, seven and ten. All absorbances were measured using a Cary 60 UV/Vis spectrophotometer. Preparation of Standard Solution. A 6.0 mL aliquot of bromothymol blue indicator, 0.37 g of KH2PO4, and 1.13 g of Na2HPO4?7H2O were added to a reaction flask. After the flask was filled to the 250 mL mark, the solution was mixed and then transferred to containers in 50 mL portions. The solutions were then labeled “A”-“E”. The pH of solution A was taken. The pH of solutions “B” and “C” were then adjusted by adding 1.0 M NaOH drop wise. The pH of solution “B” was raised by approximately 0.3 units, while the pH of solution “C” was raised by approximately 0.6 units. Solutions “D” and “E” had 1 M HCl added to decrease the pH. The pH of solution “D” was lowered by approximately 0.3 units, while the pH of solution “E” was lowered by 0.6 units. Recording of t