Estimation of measurement uncertainty of routine pH measurement in selected chemical laboratories

Abstract

A procedure for estimation of measurement uncertainty of routine pH measurement using pH meter with pH range -2 to 16, three-point calibration, easy recall of calibration point and slope data, temperature compensation and combination glass electrode based on the ISO method is presented. The uncertainty of pH strongly depends on the pH meter itself. Buffers of different ions (NH3/NH4Cl, CH3COOH/CH3COO-) at specified pH-values are prepared gravimetrically according to the Henderson Hasselbach equation. Both calibrations and measurements were performed at 25±1.0 °C on the same day. The pHvalues of the buffer solutions were close to the expected values but notable uncertainties were also found. Basic statistical calculations and the law of propagation of uncertainty in a spreadsheet model will be used for analysis of uncertainty. The uncertainty of pH measurements will be compared to manufacturers’ specifications. In essence, interval and confidence level are needed, in order to quantify uncertainty. This investigation was performed to provide adequate confidence that pH meters performed satisfactorily and corresponded with laboratory requirements. This approach is used to solve quality related problems in the industry as well ensuring product quality in the industry and to improve knowledge about quality assurance. The study is proposed to be used by assessing organizations, Quality Assurance specialists/ managers/ officers and analytical staff, both in industry and the academic world. It provides principles from which assessing organizations such as accreditation (the ISO/IEC standard 17025) or certification bodies could specify assessment criteria.