Stability of Tris pH buffer in artificial seawater stored in bags

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.