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<p>Equimolal 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 purportedly gas-impermeable bags across a variety of experimental conditions, including bag type and storage in air vs. seawater over 300 <span class="inline-formula">d</span>. Bench-top spectrophotometric pH analysis revealed that the pH of tris stored in bags decreased at a rate of <span class="inline-formula">0.0058±0.0011</span> <span class="inline-formula">yr⁻¹</span> (mean slope <span class="inline-formula">±95</span> <span class="inline-formula">%</span> confidence interval of slope). The upper and lower bounds of expected pH change at <span class="inline-formula"><i>t</i>=365</span> <span class="inline-formula">d</span>, calculated using the averages and confidence intervals of slope and intercept of measured pH change vs. time data, were <span class="inline-formula">−</span>0.0042 and <span class="inline-formula">−</span>0.0076 from initial pH. Analyses of total dissolved inorganic carbon confirmed that a combination of <span class="inline-formula">CO₂</span> infiltration and/or microbial respiration led to the observed decrease in pH. Eliminating the change in pH of bagged tris remains a goal, yet the rate of pH change is lower than many processes of interest and demonstrates the potential of bagged tris for sensor calibration and validation of autonomous in situ pH measurements.</p>