Stability of Tris pH buffer in artificial seawater stored in bags

Wiley Wolfe1, Kenisha Shipley2, Yuichiro Takeshita3, Philip Joseph Bresnahan Jr2, Taylor Wirth4 and Todd R Martz4, (1)University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (2)Scripps Institution of Oceanography, La Jolla, CA, United States, (3)Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States, (4)Scripps Institution of Oceanography, La Jolla, United States
Tris buffer in artificial seawater (Tris-ASW) is commonly used by oceanographers to calibrate seawater pH measurements. If Tris-ASW could be measured in situ by autonomous sensors, this would provide the ability to calibrate and assess sensor drift during a deployment. However, the pH stability of Tris-ASW when stored in bags has not been assessed. Here, we present results from several experiments where Tris-ASW was stored in bags for up to one year while pH was tracked using spectrophotometry. Throughout the experiment, the pH of seawater CO2 certified reference materials (CRMs) was used to independently verify spectrophotometer performance. In every case, a linear negative pH drift was observed for Tris-ASW stored in bags, with a mean slope of -0.006 ± 0.003 per year. We will discuss the implications of the magnitude of the observed drift in terms of potential applications for Tris-ASW stored in bags and outline the limitations that would result from such instability. Finally, we will provide recommendations for additional experiments aimed at determining a source for the drift and, possibly, ways to reduce it to a negligible level.