Project: Development of Spectrophotometric pH Measurement Capabilities in Estuaries

NSF Award Abstract:

Extremely accurate pH measurements are required to fully evaluate changes of the inorganic carbon chemistry in seawater as it equilibrates with rising atmospheric CO2. After decades of careful chemical characterization of a series of organic molecules that change color with changing pH, called pH indicator dyes, this accuracy is possible for full strength seawater. In dynamic coastal systems where riverine freshwater and seawater meet, large variations in the physical chemistry and pH have made extremely accurate measurements of pH difficult and not routinely available. The researcher on this project, having pioneered the development of oceanic pH indicator dyes, would extend their chemical characterization over the much broader range of conditions (e.g. temperature, salinity) required to measure pH in coastal mixed systems. Results from this research would allow oceanographers to quantify variability in carbon chemistry at the interface between rivers and seawater with accuracy not currently possible, relieving pH measurement constraints and allowing greatly increased confidence in evaluating seawater CO2 dynamics in coastal systems. This project will involve graduate and undergraduate students in field studies, giving them hands-on experience in oceanographic research, with results from this research used by a University of South Florida minority student in a dissertation. The PI also plans to continue his history of mentoring summer interns from Eckerd College and local high schools.

Current characterizations for purified sulfonephthalein indicator dyes are highly specific to measure pH values in 'full strength' open ocean seawater. They are not currently adequate for making extremely accurate pH measurements in systems with salinities lower than 20. This creates uncertainties in all of the CO2 system variables (dissolved inorganic carbon, total alkalinity, CO2 fugacity, etc.) that are connected to pH in equilibrium calculations, thus hampering the ability of oceanographers to quantify estuarine CO2 dynamics with the high degree of confidence required to examine changing coastal carbon cycles. Meticulous characterization of the sulfonephthalein indicator dye series across estuarine conditions, combined with previous open ocean work, will allow for researchers to extend highly precise and accurate pH measurements over salinities from 0 to 40. The calibrations will be performed using procedures to characterize the hydrogen ion exchange parameters and the spectral absorbance behavior of sulfonephthaleins, as well as the chromatographic procedures to obtain purified indicators. The new analytical capability for pH measurements generated by this project will enable researchers and coastal management professionals to generate trusted pH measurements in lakes, rivers, and mixed salinity systems, a major advance in the accurate physical chemical characterization of natural waters.