Long-term stability monitoring of pH reference materials using primary pH method.

This work presents the results from a series of stability studies for some batches of different aqueous pH reference materials, packed in high-density polyethylene (HDPE) bottles, taking primary pH measurements over long periods (time lengths from 18.4 to 21.0 months). The results obtained over time for acid and neutral buffer solutions (nominal pH values of 1.68, 4.00, and 6.86 at 25 °C), considering their uncertainties, were statistically similar, demonstrating the high stability of these materials. On the other hand, for the alkaline buffer solutions (nominal pH values of 9.18 and 10.01 at 25 °C), there was a clear decrease in the results over time, with pH variation rates around -8.5 × 10(-4) per month. The results showed that reference materials of the acid and neutral buffer solutions can be easily provided with small uncertainty values and long shelf lives in simple HDPE bottles closed under air atmosphere, whereas reference materials of the alkaline buffer solutions must have more limited shelf lives and higher uncertainty values (taking into account the pH decrease over time) or must be provided in special packaging (such as ampoules) to prevent carbon dioxide interference.