A review of the state of the art of electrochemical methods at the highest metrology level in national metrology institutes (NMIs) is given, with emphasis on standardization work (primary methods) in the fields of pH and electrolytic conductivity, as well as use of coulometry. Attention is also given to certain technical issues in the implementation of these methods.
The possible approaches to realising a link to the SI system and the status of primary direct methods in the traceability chain of chemical measurements are discussed. Some results obtained with the new coulometric standard system are presented.
In the 1990s the CCQM (Comité consultatif pour la quantité de la matière) defined the term 'primary method'. It also identified some methods that are considered to be primary. One of these methods is titrimetry. In this paper, the principle of titrimetry and two practical examples performed at the highest metrological level in the frame of key comparison CCQM-K8 are discussed. It is evident that titrimetry is a very powerful method leading to highly accurate results when performed correctly, and does not depend on whether the classical metrological approach or a new experimental design using modern laboratory technology is applied.
Key Comparison CCQM-K36.2016 was a follow-up comparison for K36 and provided updated support for the corresponding calibration and measurement capability (CMC) entries in the BIPM CMC database. It aimed to demonstrate the capabilities of the participating NMIs to measure electrolytic conductivity of aqueous electrolyte solutions in the conductivity range 0.15 S m−1 to 1.5 S m−1 and in the conductivity range 1.5 mS m−1 to 15 mS m−1. To this end electrolytic conductivity of a potassium chloride solution (nominal conductivity 0.5 S m−1) and of a HCl solution (nominal conductivity 5 mS m−1) had to be measured. 17 NMIs participated in the comparison. The key comparison reference value (KCRV) of the KCl solution was (0.50999 +/-0.00032) S m−1 and the KCRV of the HCl solution was (4.9877 +/-0.012) mS m−1. Both values were estimated from the medians of the results considered eligible for KCRV calculation. They were given with their expanded uncertainties (95% coverage). The majority of the 0.5 S m-1 results were consistent with the KCRV. Two institutes showed a small inconsistency, one outlier was observed. The conductivity of the HCl solution showed a small, but steady linear drift of 0.00006843 mS m−1 per day during the measurement period and was corrected for KCRV calculation. Some institutes reported unstable measurement conditions for this solution. The results of seven participants have been inconsistent with the KCRV.
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 the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
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