Traceability of pH Measurements by Glass Electrode Cells: Performance Characteristic of pH Electrodes by Multi-Point Calibration

摘要

Routine pH measurements are carried out using pH meter-glass electrode assemblies. In most cases the glass and reference electrodes are thereby fashioned into a single probe, the so-called 'combination electrode' or simply 'the pH electrode'. Using these electrodes, various effects, described below, producing uncertainties of unknown magnitude must be noted. Therefore the measurement of pH of a sample requires a suitable calibration by certified standard buffer solutions (CRMs) traceable to primary pH standards. The procedures in use are based on calibrations at one point, at two points bracketing the sample pH and at a series of points, the so-called multi-point calibration. The multi-point calibration (MPC) is recommended if minimum uncertainty and maximum consistency are required over a wide range of unknown pH values. Details of uncertainty computations for the two-point and MPC procedure are given. Furthermore the multi-point calibration is a useful tool to characterise the performance of pH electrodcs. This is demonstrated with different commercial pH electrodes.