PURIFICATION AND CHARACTERIZATION OF BROMOCRESOL PURPLE FOR SPECTROPHOTOMETRIC SEAWATER ALKALINITY TITRATIONS

The topic of my research is the purification of the sulfonephthalein indicator bromocresol purple (BCP). BCP is used as an acid-base indicator in seawater alkalinity determinations. Impurities in sulfonephthalein indicator salts often result in significant errors in pH values3. High-precision inorganic carbon (i.e. CO2) measurements are necessary to document the changes in ocean chemistry and pH due to anthropogenic CO2­ uptake by the earth’s oceans3. In order to reduce the error in seawater pH determinations and the general understanding of the CO2 cycle, BCP must be purified and characterized. Previous work has described the use of flash chromatography to efficiently produce large batches of purified indicator, so this method was used for BCP purification3. Since BCP is purified at a level useful for seawater pH calculations, the chemical can be characterized by more accurately calculating the constants associated with it1. The spectrophotometric pH values obtained by measuring absorbance ratios are directly related to indicator molecular properties, such as molar absorptivity ratios and protonation characteristics2. Once the accuracy of these values is improved, future and archived spectrophotometric pH data can be calculated using the improved indicator equilibrium and molar absorptivity values2. The available purification method of BCP will advance the understanding of ocean acidification in systems where the chemical is used