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Water in its three ambient phases plays the central thermodynamic role in the terrestrial climate system. Clouds control Earth's radiation balance, atmospheric water vapour is the strongest "greenhouse" gas, and non-equilibrium relative humidity at the air-sea interface drives evaporation and latent heat export from the ocean. On climatic time scales, melting ice caps and regional deviations of the hydrological cycle result in changes of seawater salinity, which in turn may modify the global circulation of the oceans and their ability to store heat and to buffer anthropogenically produced carbon dioxide. In this paper, together with three companion articles, we examine the climatologically relevant quantities ocean salinity, seawater pH and atmospheric relative humidity, noting fundamental deficiencies in the definitions of those key observables, and their lack of secure foundation on the International System of Units, the SI. The metrological histories of those three quantities are reviewed, problems with their current definitions and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10. It is concluded that the International Bureau of Weights and Measures, BIPM, in cooperation with the International Association for the Properties of Water and Steam, IAPWS, along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for these long standing metrological problems in climatology.

Water dissolves many substances with which it comes into contact, leading to a variety of aqueous solutions ranging from simple and dilute to complex and highly concentrated. Of the multiple chemical species present in these solutions, the hydrogen ion, H+, stands out in importance due to its relevance to a variety of chemical reactions and equilibria that take place in aquatic systems. This importance, and the fact that its presence can be assessed by reliable and inexpensive procedures, are the reasons why pH is perhaps the most measured chemical parameter. In this paper, while examining climatologically relevant ocean pH, we note fundamental problems in the definition of this key observable, and its lack of secure foundation on the International System of Units, the SI. The metrological history of seawater pH is reviewed, difficulties arising from its current definition and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent TEOS-10 seawater standard. It is concluded that the International Bureau of Weights and Measures (BIPM), in cooperation with the International Association for the Properties of Water and Steam (IAPWS), along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for these long standing metrological problems.

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.

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 aim of 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 timescales. 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 that was traceable to the SI. The nominal conductivity values of the solution were 5.3 S m-1 at 25 ?C and 4.3 S m-1 at 15 ?C. Thirteen institutes taking part in the comparison had to measure the conductivity values of the sample at both 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 ?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 ?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 S m-1 to 15 S m-1 for the values 5.3 S m-1 at 25 ?C and 4.3 S m-1 at 15 ?C. This comparison is a follow-up of the CCQM Pilot Study P111. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

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 a conductivity value larger than those given in the IUPAC document [1] was measured in a 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. 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).

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

CCQM-K9.2 was performed supplementary to the key comparison CCQM-K9 on the pH determination of a phosphate buffer with nominal pH ~ 6.9 (at 25 ?C). The sample composition was very similar in both comparisons. Only the source of the starting material used for sample preparation was different. The comparison was restricted to the use of the primary method for pH (Harned cell measurement) as defined in the IUPAC Recommendations [2]. The measurement temperatures were 15 ?C, 25 ?C, 37 ?C. CCQM-K9.2, CCQM-K.9 and the first supplementary comparison CCQM-K9.1 [5] are activities of the Electrochemical Working Group (EAWG) of the CCQM. All three comparisons were coordinated by the PTB, Germany. The Danish Primary Laboratory (DPL) successfully took part in the CCQM-K9. Meanwhile the primary set-up for pH in Denmark moved from DPL affiliated to Radiometer Medical to DFM, Denmark. The subsequent comparison allows assessing the degree of equivalence for the measurement of pH at DFM after the move. Due to the interest of other laboratories in demonstrating their progress in pH measurements on the primary level the CCQM-K9.2 supplementary comparison was extended to other participants than DFM, namely NMIJ, VNIIFTRI, INMETRO and CMI. The reported quantity for CCQM-K9.2 was not the pH of the sample but the acidity function at zero chloride molality (see chapter 12). To calculate the pH value from the acidity function it is necessary to know the ionic strength of the sample buffer solution, which was undisclosed by the coordinator. With the exception of the Czech Metrology Institute, CMI, good agreement in the determined acidity function is found between the participants. The results reported by DFM and by PTB agree within their measurement uncertainty at all measurement temperatures. 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).

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).

A comparison of the primary methods for pH measurement of Russia and Germany was carried out. Three buffer solutions: potassium tetraoxalate, 0.05 mol/kg, potassium hydrogen phthalate, 0.05 mol/kg and disodium tetraborate decahydrate, 0.01 mol/kg, prepared from materials of the All-Russian Research Institute for Physical, Technical and Radio-Technical Measurements (VNIIFTRI) and Merck KGaA, Germany, were used. The pH values obtained at 25 °C in this comparison agree within an expanded uncertainty of 0.004 for the borate and oxalate buffers and 0.005 for the phthalate buffer (k = 2).

The key comparison CCQM-K19 was carried out to demonstrate the capability of the interested national metrology institutes to measure the pH value of an unknown borate buffer by a primary method. The buffer of nominal pH ~ 9.17 was measured at 15 ?C, 25 ?C and 37 ?C. The pilot study CCQM-P82 was performed in parallel in order to allow interested laboratories, mainly newcomers in this field, to compare their capabilities to the more advanced NMI using the same batch of sample. The comparison was an activity of the Electrochemical Working Group (EAWG) of the CCQM and was coordinated by the Physikalisch-Technische Bundesanstalt, Germany. All participants applied the primary method for pH [1]. The result for the unknown buffer solution is the acidity function at zero chloride molality, pa0. Good agreement of the results was observed for the majorities of participants. The Key Comparison Reference Value (KCRV) was agreed upon during the EAWG meeting in Berlin in October 2005 as the median of the reported results. Accordingly the Statements of Equivalence were calculated. 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).

The EUROMET project 696, a trilateral comparison between PTB, Germany, LNE, France and VNIIFTRI, Russia was performed in order to demonstrate and document the capability of the participants to measure the pH of a phthalate buffer by the primary measurement procedure for pH. Good agreement of the reported results was observed. The sample was very similar to the one used in the comparison CCQM-K17. PTB acts as pilot laboratory in CCQM-K17 and in EUROMET 696. This comparison allows one to link the results obtained by LNE to the CCQM-K17 key comparison through the degree of equivalence of PTB. On the other hand, the discrepancy between measured pH values at the VNIIFTRI and PTB for the same type of buffer solution decreased, as compared with a bilateral comparison in 1997. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the Mutual Recognition Arrangement (MRA).

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 CCQM-K17 comparison allows estimations of the capability of ten national metrology institutes (NMIs) to determine the pH of a phthalate buffer of unknown concentration at different temperatures. The key comparison was co-ordinated by the CCQM Working Group on Electrochemical Analysis and piloted by the Physikalisch-Technische Bundesanstalt (PTB) with assistance from the National Institute of Standards and Technology (NIST) and the Slovak Institute of Metrology (SMU). The measurement results of the NMIs agreed not at the same level as for the first key comparison on pH CCQM-K9. The results obtained by seven of the eleven participants agree very well within the uncertainty stated by the participants. Four participants obtained results somewhat lower. The two groups of results are most obviously evident at a measurement temperature of 37 °C. 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 Mutual Recognition Arrangement (MRA).