Stability studies of pH reference materials under repeated use conditions by the primary measurement method

This work presents the results from stability studies for a series of pH-certified reference materials submitted to repeated use conditions (repeated sampling) to meet the requirements from the latest versions of ISO 17034 and ISO Guide 35. The study was applied to reference materials with nominal pH values of 1.68, 4.00, 6.86, 9.18, and 10.01, which were submitted to a repeated use simulation under controlled conditions to reproduce their routine use in analytical laboratories. The materials were analyzed once a month by the primary pH measurement method to check for deviations, trends, or significant variations in the results, using linear regression and normalized error statistical tests. The results indicated that the pH variations found for the materials with pH values of 1.68, 4.00, and 6.86 could be considered normal or statistically insignificant, not affecting their certified values for the time studied. For the materials with pH values of 9.18 and 10.01, the pH variations found were statistically significant and even higher than those from the stability monitoring (bottles not submitted to the repeated use simulation), making it possible to estimate a new associated uncertainty source to the certified value and increase the reliability in the use of these materials. Graphical abstract.