Longitudinal meta-analysis of NIST pH Standard Reference Materials(®): a complement to pH key comparisons

Area of Science:

• Metrology
• Analytical Chemistry
• Physical Chemistry

Background:

• Accurate pH measurements are critical in various scientific fields.
• Standard Reference Materials® (SRMs) are essential for ensuring measurement traceability and comparability.
• Long-term stability data for pH SRMs is crucial for metrological assessments.

Purpose of the Study:

• To assess the long-term (up to 70 years) within-laboratory variation of NIST pH SRMs.
• To quantify the impact of buffer composition deviations and measurement methodology on pH stability.
• To provide a long-term dataset complementing international pH key comparisons.

Main Methods:

• Meta-analysis of historical pH measurement data for tetroxalate, phthalate, phosphate, borate, and carbonate buffers.
• Graphical analysis of ΔpH(S) (difference between certified pH and mean pH) as a function of SRM issue and temperature.
• Calculation of mean ΔpH(S) to characterize constant shifts due to buffer deviations.
• Calculation of standard deviation of ΔpH(S) to characterize time-dependent variations.

Main Results:

• In most cases, the absolute difference |ΔpH(S)| was less than 0.004.
• Deviations in the base:acid mole ratio resulted in constant, temperature-independent shifts in ΔpH(S), with impurity mole fractions generally below 0.3%.
• Measurement variations (equipment, personnel, materials, methodology) resulted in temperature-dependent changes, with standard deviations of ΔpH(S) typically ≤ 0.0015.

Conclusions:

• NIST pH SRMs exhibit high long-term stability within a single laboratory.
• The study quantifies the contributions of buffer composition and measurement factors to pH variability.
• The findings offer valuable long-term metrological data for pH standardization.