Results of CCQM-K99 key comparison on unknown phosphate buffer pH ∼ 7.5 at 5 °C, 15 °C, 25 °C, 37 °C and 50 °C are reported. Good agreement is found between the majority of participants. 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).

The first practical comparison of the characteristics of Ag|AgCl electrodes prepared at different National Metrology Institutes (NMIs) has been carried out as a pilot study organised by the Consultative Committee for Amount of Substance (CCQM). Each NMI prepared at least three Ag|AgCl electrodes representative of those employed in their primary pH calibration facilities. The electrodes were sent to a coordinating laboratory where comparative measurements of their potential difference to a de facto reference, slope as a function of chloride ion concentration and electrochemical impedance were made. Electrodes from most NMIs were highly repeatable and consistent within the typical rejection criteria applied during production. An analysis of the electrode slopes in a phosphate buffer containing different concentrations of NaCl revealed the influence of variances in the Ag|AgCl electrodes between NMIs on the certified pH value of the buffer solution. The difference between NMIs is consistent with submitted values in CCQM-K9, although biases were smaller in the analysis here, suggesting the presence of other unknown contributions to the uncertainty such as cell design. The smaller biases observed in this work may also reflect improvements in the Ag|AgCl electrodes since CCQM-K9 was performed. Electrochemical impedance spectroscopy suggests a similar microstructure for electrodes prepared by most NMIs. (C) 2015 Elsevier Ltd. All rights reserved.

Detection of the end point in potentiometric titrations has wide application on experiments that demand very low measurement uncertainties mainly for certifying reference materials. Simulations of experimental coulometric titration data and consequential error analysis of the end-point values were conducted using a programming code. These simulations revealed that the Levenberg-Marquardt method is in general more accurate than the traditional second derivative technique used currently as end-point detection for potentiometric titrations. Performance of the methods will be compared and presented in this paper.

The COOMET. QM-K36 key comparison 'Electrolytic conductivity: 0.5 S/m' is a comparison in the field of electrolytic conductivity measurements conducted by COOMET and carried out in 2012. It used a solution of KCl in water and the results are connected to those of the CCQM key comparison CCQM-K36. a through common participation of VNIIFTRI (Russia), SMU (Slovakia) and Ukrmetrteststandart (Ukraine). The purpose of this key comparison was to establish the equivalence of measurements of electrolytic conductivity performed at the National Metrology Institutes of COOMET member states for the value of 0.5 S/m. The results can be used to support the CMCs claims over the range of 0.1 S/m to 1.3 S/m.

The COOMET.QM-K36 key comparison 'Electrolytic conductivity: 0.5 S/m' is a comparison in the field of electrolytic conductivity measurements conducted by COOMET and carried out in 2012. It used a solution of KCl in water and the results are connected to those of the CCQM key comparison CCQM-K36.a through common participation of VNIIFTRI (Russia), SMU (Slovakia) and Ukrmetrteststandart (Ukraine). The purpose of this key comparison was to establish the equivalence of measurements of electrolytic conductivity performed at the National Metrology Institutes of COOMET member states for the value of 0.5 S/m. The results can be used to support the CMCs claims over the range of 0.1 S/m to 1.3 S/m. 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).

Participants: Akiharu Hioki, Toshiaki Asakai, Igor Maksimov, Toshihiro Suzuki and Tsutomu Miura (NMIJ); Krairerk Obromsook, Nongluck Tangpaisarnkul and Patumporn Rodruangthum (NIMT); Siu-Kay Wong and WaiHing Lam (GLHK); Osman Zakaria and Khirul Anuar Mohd. Amin (NML-SIRIM); Ngo Huy Thanh (VMI); Michal Máriássy, Leos Vyskocil and Zuzana Hankova (SMU); Paola Fisicaro and Daniela Stoica (LNE); Nahar Singh and Daya Soni (NPLI); Galia Ticona Canaza (INDECOPI); Viatcheslav Kutovoy (VNIIFTRI); Fabiano Barbieri Gonzaga and Júlio Cesar Dias (INMETRO); Alena Vospelova (CMI); Nickolay Bakovets (BelGIM); Bibinur Zhanasbayeva (KazInMetr).

The key comparison (KC) studies of the Consultative Committee for Amount of Substance—Metrology in Chemistry help ensure the reliability of chemical and biochemical measurements relevant to international trade and environmental-, health-, and safety-related decision making. The traditional final evaluation of each measurement result reported by a KC participant is a "degree of equivalence" (DEq) that quantitatively specifies how consistent each individual result is relative to a reference value. Recognizing the impossibility of conducting separate KCs for all important analytes in all important sample matrices at all important analyte levels, emphasis is now shifting to documenting broadly applicable critical or "core" measurement competencies elicited through a series of studies. To better accomplish the necessary synthesis of results, data analysis and display tools must be developed for objectively and quantitatively combining individual DEqs. The information detailed in the 11 KCs of primary method pH measurements publically available as of 2013 provides an excellent "best case" prototype for such analysis. We here propose tools that enable documenting the expected primary pH measurement performance of individual participants between pH 1 and pH 11 and from 15 °C to 37 °C. These tools may prove useful for other areas where the uncertainty of measurement is a predictable function of the measured quantity, such as the stable gases. That results for relatively simple measurement processes can be combined using relatively simple analysis and display methods does not ensure that similarly meaningful summaries can be devised for less well understood and controlled systems, but it provides the incentive to attempt to do so.

The aim of the key comparison CCQM-K105 was to demonstrate the measurement capabilities of the participating institutes with respect to the conductivity of multi-component aqueous salt solutions. Practical salinity results are currently not traceable to metrological references consistent with the International System of Units (SI). Nevertheless, salinity is one of the most important input quantities for oceanographic models, whose measurement data must be accurate on very long time scales. Thus, in order to determine the practical salinity value, there is a strong interest on the part of oceanographic researchers in establishing the traceability to the SI of conductivity measurements. To this end, the conductivity of a standard seawater sample, provided by the support laboratory (PTB), was measured in a way traceable to the SI. The nominal conductivity values of the solution were 5.3 S.m(-1) at 25 degrees C and 4.3 S.m(-1) at 15 degrees C. Thirteen institutes taking part in the comparison had to measure the conductivity values of the sample at both the temperatures. The median was chosen for both values as an estimator for the KCRV, evaluated on the basis of the Monte Carlo method. An institute requested to be excluded from the determination of the KCRV because of contact problems of its cell. At 25 degrees C the KCRV is 5.3024 S.m(-1) with an interval of confidence (at the 95.45 % level of significance) from 5.3005 S.m(-1) to 5.3044 S.m(-1). At 15 degrees C the KCRV is 4.2892 S.m(-1) with an interval of confidence (at the 95.45 % level of significance) from 4.2877 S.m(-1) to 4.2907 S.m(-1). For the "How far the light shines" statement the CMCs can cover the range 1 - 15 S.m(-1) both for the values 5.3 S.m(-1) at 25 degrees C and 4.3 S.m(-1) at 15 degrees C. This comparison is a follow-up of the CCQM Pilot Study P111.

Results of the CCQM-K91 key comparison on pH of an unknown phthalate buffer with a nominal pH value of pH similar to 4.01 at 25 degrees C are reported. Measurements are performed at 15 degrees C, 25 degrees C, and 37 degrees C and optional also at 5 degrees C and 50 degrees C.

The aim of the key comparison CCQM-K92 was to demonstrate the capabilities of the participating NMIs to measure electrolytic conductivity of an unknown sample.Two samples with nominal electrolytic conductivity values of 0.05 S.m(-1) and 20 S.m(-1) have been prepared for comparison. For the first time conductivity value larger than those given in the IUPAC document [1] was measured in CCQM comparison. Thus no calibration standards with similar conductivity value were available. The comparison was an activity of the Electrochemical Working Group (EAWG) of the CCQM and was coordinated by SMU.In the comparison NMIs from fifteen countries took part. The higher conductivity (20 S.m(-1)) was measured by ten participants. Good agreement of the results was observed for the majority of participants.

This work presents a new differential potentiometric cell for the standardization of pH buffer solutions and its evaluation, by means of a bilateral interlaboratory comparison, in relation to a traditional Baucke cell. The results obtained with the two cells were exactly the same for three of the pH buffer solutions analyzed (1.68, 4.01, and 6.86) and similar to each other for the remaining buffer solution (9.18). The new cell showed an average measurement time of only 21minutes, in comparison with one to three hours for other cells described in the literature (including the Baucke cell).

The subsequent key comparison CCQM-K19.1 has been carried out in 2010 as a follow-up comparison to CCQM-K19. Three laboratories (NMIJ, PTB and SMU) took part in both comparisons. Their results are consistent. The assigned uncertainties are reliable and in the same order. These results provide the link to CCQM-K19.As well as the original comparison the subsequent key comparison was organised to demonstrate the capability of the interested National Metrology Institutes to measure the pH value of an unknown borate buffer by a primary method at 15 degrees C, 25 degrees C and 37 degrees C. Eight laboratories took part in CCQM-K19.1. The participants either were not able to participate in the original comparison or participate only in the Pilot Study CCQM-P82 running in parallel to CCQM-K19. In one case the laboratory expressed doubts on the integrity of the sample provided in CCQM-K19.The result in CCQM-K19.1 is the acidity function at zero chloride molality, AF(0) of the unknown borate buffer solution.Most participants in the key comparison CCQM-K19.1 demonstrated improved capabilities. This especially applies for the laboratories original taken part in the pilot study CCQM-P82. In some cases previous results could be confirmed. The hidden reasons for that have still to be clarified.

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.

The subsequent key comparison CCQM-K18.1 was started in order to evaluate the equivalence of metrology institutes as a follow-up to the previous key comparison CCQM-K18 for institutes that could not take part in the comparison at that time or did not consider their results representative of their capabilities. A carbonate buffer of a slightly different composition to that in CCQM-K18 was used.There were seven institutes plus the coordinating laboratory participating in this subsequent comparison. Most of the participating institutes showed significant improvement; in some cases the new results confirmed the previous ones. The reasons for this have still to be elucidated.

The key comparison CCQM-K18 was started in order to evaluate the equivalence of metrology institutes as a follow-up of the previous study CCQM-P52. As a sample a carbonate buffer solution was used with a composition slightly different to the usual one. There were 13 institutes participating in the comparison using a primary method for pH measurement. The fair agreement between the results reflects increased difficulty in measurement, where the buffer composition may slightly change during the measurement and extrapolation to the start time of measurement is usually necessary. 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 (MRA).

This paper surveys the state of the art for primary methods for the evaluation of electrolytic conductivity in aqueous solutions as they are currently carried out in several national metrological institutes (NMIs). The theoretical and practical basic knowledge of this measurement is described. Analysis of and comments on the different approaches are offered to give emphasis to technical difficulties and possible solutions. Further development is foreseeable, ensuring a common effort for the sharing of expertise that has been undertaken at the NMIs. In particular, improvements are expected towards traceable measurements of solutions with conductivity values lower than those actually standardized, down to the level of ultrapure water.

The second key comparison for the quantity pH, CCQM-K17, was carried out to assess the degree of equivalence of the national primary measurement procedures used to determine the pH of primary standard buffer solutions.

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.