Key comparison of liquid density standards

At the SIM meeting in Buenos Aires, from 30 May to 1 June 2012, it was decided a RMO key comparison on pH measurement coordinated by INMETRO would be performed, named SIM.QM-K91, in order to investigate the degrees of equivalence of measurement procedures for the pH determination of a phthalate buffer solution (nominal pH around 4.01 at 25 °C). Phthalate buffer is widely used to calibrate pH electrodes in the acid range. A buffer solution of 0.05 mol kg-1 potassium hydrogen phthalate, KHC8H4O4, is one of the primary pH reference buffer solutions recommended by IUPAC. It was only allowed to participate in this comparison by using a differential cell or a glass electrode, instead of primary cells, if the highest metrological standard in the institute or if the CMCs are based on the type of cell to be used. The results obtained by INMETRO and NIST (who also participated in CCQM-K91 comparison) were used to link the results from the other institutes to the key comparison reference value of CCQM-K91. In this comparison, pH measurements were performed at 25 °C, and optionally also at 15 °C and 37 °C. Nine institutes took part in the comparison and the results are presented in this report. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).