Technical Note: Stability of tris pH buffer in artificial seawater stored in bags

Abstract. Equimolar tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer in artificial seawater is a well characterized and commonly used standard for oceanographic pH measurements. We evaluated the stability of tris pH when stored in flexible, gas impermeable bags across a variety of experimental conditions, including bag type, tris batch, and storage in air vs. seawater over 300 days. Bench-top spectrophotometric pH analysis revealed that the pH of tris stored in bags drifted at a rate of −0.0058 ± 0.0008 yr−1 (mean slope ± 95 % confidence interval of slope). Analyses of total dissolved inorganic carbon confirmed that a combination of CO2 infiltration and/or microbial respiration led to the observed decrease in pH. Eliminating bagged tris pH drift remains a goal, yet the pH drift rate of 0.006 yr−1 is lower than many processes of interest and demonstrates the value of bagged tris to sensor calibration and validation of autonomous in situ pH measurements.