An autonomous freshwater pH sensor: development and applications

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DeGramdpre0337460Intellectual merit of the proposed activity. pH is a commonly measured parameter in freshwater systems. Continuous pH measurements are desired in many situations, yet present day technology is incapable of providing good quality, long-term, operator-free measurements. We propose to rigorously evaluate a novel spectrophotometric pH sensor that offers the potential for drift-free freshwater pH measurements. The design derives directly from the Submersible Autonomous Moored Sensor for CO2 (SAMI-CO2)(DeGrandpre et al. 1995, 1999). The SAMI-pH sensor utilizes the same design as SAMI-CO2. The availability of the SAMI technology through Sunburst Sensors (www.sunburstsensors.com) will greatly facilitate development, testing and future commercialization of SAMI-pH. Spectrophotometric pH methods are widely thought to be inadequate for analysis of weakly buffered waters such as freshwater because the indicator alters the pH of the sample. The SAMI-pH design utilizes a straightforward methodology to quantify the perturbation-free pH. The objectives of this grant are to 1) rigorously test the design in the laboratory using a series of weakly buffered samples, 2) quantify indicator equilibrium constants through a range of temperature and ionic strength, and 3) rigorously test the sensor performance in natural waters with a wide range of alkalinities, humic content, and particle loading. SAMI-pH data from the field studies will be compared to in situ pH electrode measurements and pH computed from pCO2 and alkalinity. Long-term (weeks to months) pH accuracy, precision and dynamic range will be assessed and design changes will be made based on these results. Field studies of mine-impacted streams will utilize two SAMI-pH sensors to quantify longitudinal gradients and pH rates of change (previously very difficult using potentiometric electrodes). The pH data will be used to model geochemical equilibria to predict metal loading. The combined models will enable metal variability and loading to be attributed to specific in-stream processes.Broader impact of the proposed activity . There are three broad impacts: 1) students trained in the area of chemical sensor development will subsequently become the next generation of leaders in this important research area; 2) an improved understanding of biogeochemical cycling in aquatic ecosystems will allow better prediction of anthropogenic impacts and hence management of our natural resources; and 3) advancements in autonomous chemical sensing technology will benefit the environment, industry and national security by making it possible to remotely and continuously monitor chemical species in the environment.