CCQM P37.1: preparation of Ag/AgCl electrodes for harned cell measurements.

A questionnaire was completed by 14 participants (INMETRO, NIM, PTB, DFM, LNE, NMIJ, SMU, NIST, CENAM, GUM, NPLI, IPQ, BIM and KRISS) to study the influence of several variables in the preparation of Ag/AgCl electrodes on the accuracy of Harned cell measurements of pH. The performance of each national metrology institute in the last decade has been assessed based on their results in 8 key comparisons involving the measurement of pH of phosphate, phthalate, carbonate, borate and tetroxalate buffer materials (in CCQM - K9 and K9.2, K17, K18 and K18.1, K19 and K19.1 and K20 respectively). The performance of each laboratory has been correlated to the results of the questionnaire to determine the critical parameters in the preparation of Ag/AgCl electrodes and their sensitivities with respect to the accuracy of pH measurement. This study reveals that the parameters most closely correlated to performance in comparisons are area of electrode wire exposed to the electrolyte, diameter and porosity of the Ag sphere prior to anodisation, amount of Ag converted to AgCl during anodisation, stability times employed for electrodes to reach equilibrium in solution prior to measurement, electrode rejection criteria employed and purity of reagents.