We tested the stability of a well-characterized seawater pH buffer, tris, during long-term storage in gas-impermeable bags. Tris is used to validate pH measurements; it is critical that we understand how its chemistry changes over time. Correspondingly, we prepared multiple batches of tris buffer in artificial seawater, stored the buffer in multiple types of gas impermeable bags, and analyzed its pH over the course of 300 d, discovering an average change of −0.006 yr⁻¹.