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For primary pH measurements, a platinum hydrogen electrode and a silver chloride electrode are immersed in the same solution in a cell without transference (Harned cell). The platinum electrode is covered with dispersed platinum (platinum black) to increase the surface area. To determine the influence of specific deposition conditions (current density, duration, and composition of the electrolyte) on the properties of platinum black, the surface area, and the electrode performance, platinized platinum electrodes were investigated systematically using scanning electron microscopy and optical microscopy. Confocal laser scanning microscopy was applied to obtain quantitative information about roughness parameters. After assessment of the surface structures, improved conditions for the fabrication of platinized platinum electrodes were derived. It was found that all investigated electrochemically coated platinum electrodes obtained comparable values of standard potential of Ag/AgCl-electrodes within the measurement uncertainty. The platinum layer obtained by sputtering was significantly less distinct and less homogenous than the layer of platinum obtained by electrochemical deposition. The examinatio of shiny (uncoated) platinum electrodes evidenced a significant difference between multiple-used and freshly prepared platinum electrodes. Thus, uncoated platinum electrodes are not suitable for the Harned cell measurement either. Thus, to obtain reliable and reproducible results in pH measurements using the primary method, an electrochemical coating of the platinum electrodes under defined conditions is required.

pH measurements are used to assess both the free acidity of aqueous solutions and the concentrations, mi, of chemical species i related to hydrogen ion, H+, through chemical equilibrium, provided that measurements in reference solutions with well-known compositions are also carried out. Seawater is a highly complex electrolytic matrix of variable composition, rich in NaCl, where the determination of pH = − lg aH + (aH + = mH+ γH+ is the activity of the specified ion, H+ with the concentration mH+ and activity coefficient γH+ ≠ 1) is an issue of controversy. Electroneutrality prevents measurement of individual ion activity coefficients, which are affected and depend on the other ions in presence, namely the counter ions. Nevertheless, mean activity coefficients, γ ± = γ + γ − , in this case γ ± = γ H + γ Cl − , can be experimentally assessed from the application of the Nernst equation to Harned cell (Pt | H2 vs. Ag | AgCl electrodes, without transference) in solutions of known mH+ and mCl−. In this work measurements of electrical potential, E/V, have first been taken in strategically planned solutions, ranging from 0.01 mol kg− 1 HCl + (NaCl) to 0.01 mol kg− 1 HCl + (NaCl + KCl + CaCl2 + MgCl2), of ionic strength, I ≈ 0.67 mol kg− 1 at 15 °C, 25 °C and 35 °C with experimental pressure corrected to reference pressure of 1 atm. Mean activity coefficients were experimentally assessed with associated uncertainty. Further addition of sodium sulfate, Na2SO4, targeting artificial seawater (ASW), originates acid–base equilibria that affect the free hydrogen ion concentration, mH+. The changes introduced by the addition of sulfate in the measured Harned cell potential have been used to assess the free proton concentration and, based on the obtained results, to propose for the first time a procedure aiming to calculate values of bisulfate association constants with the corresponding uncertainty at 25 °C for both studied media. Further, the effective ionic strength of artificial seawater (ASW) was calculated and compared with the value obtained using the salinity based equation largely conveyed in the literature.

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.

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.

The free acidity of an aqueous solution is expressed in terms of its pH, a measure of the hydrogen ions activity. Being the most often measured chemical parameter, pH is used in the assessment of other chemical species and in comparative exercises. The validity of conclusions requires that the values of pH measurements are traceable to common references and ultimately to the SI. Measurements are recommended in pH meters submitted to prior calibration with reference pH buffers whose pH values have been established against conventionally assigned pH values of primary pH buffer standards of low ionic strength, I≤ 0.1 mol kg-1. This chain of operations is supported on the quality associated with, namely, buffers, electrodes, procedures, calculations and model assumptions. An overview is given of recent fundamental and applied studies addressing these various issues, for the particular case of highly saline waters approaching the complex composition of seawater.

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.

Metrological comparability as well as reliability of ion activity results measured with ion-selective electrodes (ISE) was investigated within the framework of an interlaboratory comparison between eight partners from national metrology institutes and expert laboratories. Two electrolyte solutions containing the clinically most relevant ions sodium, potassium, magnesium, calcium and chloride having ion activities near the physiological range served as samples. The calibration of the measurement set-ups of the participants was carried out using gravimetrically prepared aqueous electrolyte solutions. The ion activities of these calibration standards were calculated by means of the semi-empirical Pitzer model. The measurement uncertainty of the measurement results was calculated according to the guide to the expression of uncertainty in measurement, GUM. Based on a new scale for ion activities traceable to the SI system of units, comparability and reliability of ISE measurement results of clinically relevant ions is realised.

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.

Consistency of observed oceanographic salinity data is discussed with respect to contemporary metrological concepts. The claimed small uncertainty of salinity measurement results traceable to the conductivity ratio of a certified IAPSO Standard Seawater reference is not metrologically justified if results are compared on climatic time scales. This applies in particular to Practical Salinity SP, Reference Salinity SR, and the latest estimates of Absolute Salinity using the TEOS-10 formalism. On climate time scales an additional contribution to the uncertainty that is related to unknown property changes of the reference material must be accounted for. Moreover, when any of these measured or calculated quantity values is used to estimate Absolute Salinity of a seawater sample under investigation, another uncertainty contribution is required to quantify the accuracy of the equations relating the actually measured quantity to the Absolute Salinity. Without accounting for these additional uncertainties, such results cannot be used to estimate Absolute Salinity with respect to the International System of Units (SI), i.e. to the unit chosen for the mass fraction of dissolved material in the sample, which is "g kg−1". From a metrological point of view, such deficiencies in the calculations involving other quantities will produce SI-incompatible results. We outline how these problems can be overcome by linking salinity to primary SI measurement standards.

Traceable measurement of the quantity pH in media outside the limits of the recent IUPAC recommendation requires an alternative to the Bates–Guggenheim convention. The Pitzer’s approach to ion interactions is widely appraised and offers a suitable approach to the estimation of single-ion activity coefficients. For this purpose, the underlying electrolyte data need to be consistent and the uncertainty in existing Pitzer’s data needs to be assessed. Such an assessment is attempted in this study for pH in potassium acetate buffer medium. The re-evaluation of Pitzer’s data from potassium nitrate data revealed inconsistencies and wide uncertainty margins. The uncertainty contribution from binary Pitzer’s parameters to an assessment of pH in seawater resulted in an uncertainty in pH of about 0.03 (95% confidence interval). This contribution has to be taken into account in the overall uncertainty budget. Lack of appropriate data for other pH buffer systems (e.g., phosphate buffers) is noted.

The present paper focuses on the currently available reliability of the results of electrolytic conductivity measurements in the low conductivity range, emphasizing its weaknesses with respect to SI traceability. Electrochemical and physical effects at the electrode–solution interface influencing the measured impedance spectra are outlined, giving an overview of models that can be employed to investigate the frequency response of electrolytic conductivity cells to an applied AC voltage. Particular attention is devoted to the determination of the bulk resistance of the solution, which is necessary to derive the electrolytic conductivity of aqueous solutions traceable to the SI in the sub mS m −1 range.

The extent to which technical barriers to international trade caused by non-acceptance of chemical measurement results can be removed, largely depends on the success in building up national and international traceability structures also in metrology in chemistry. Achieving metrological comparability of measurement results requires the definition of metrological calibration hierarchies providing metrological traceability chains which enable the establishment of metrological traceability of measured quantity values to a common metrological reference [1]. For pH, over the last decade, work has been carried out in a number of countries to build up meteorological calibration hierarchies within a national framework. An overview on the metrological traceability chain for pH is given with focus on the primary measurement procedure of pH.

This paper discusses the basis and historical development of the traceability chain for pH. The quantity pH, first introduced in 1909, is among the most frequently measured analytical quantities. The practical measurement of the pH value of a sample is inexpensive, easy to perform, and yields a rapid result. However, the problems posed by the traceability of pH are not easy to solve. Most pH measurements are performed by potentiometry, using a glass electrode as the pH sensor. Such pH electrodes must be calibrated at regular intervals. Confidence in the reliability of pH measurements requires establishment of a metrological hierarchy including an uncertainty budget for calibration that links the pH measured in the sample to an internationally agreed and stated reference. For pH, this reference is the primary measurement of pH. A traceability chain can be established that links field measurements of pH to primary buffer solutions that are certified using this primary method. This allows the user in the field to estimate the measurement uncertainty of the measured pH data. As the realization of the primary measurement is sophisticated and time-consuming, primary standards are generally realized at national metrology institutes. A number of potentiometric methods are suitable for the determination of the pH of reference buffer solutions by comparison with the primary standard buffers. The choice between the methods should be made according to the uncertainty required for the application. For reference buffer solutions that have the same nominal composition as the primary standard, the differential potentiometric cell, often called the Baucke cell, is recommended.

The brackish water of the Baltic Sea is a mixture of ocean water from the Atlantic/North Sea with fresh water from various rivers draining a large area of lowlands and mountain ranges. The evaporation-precipitation balance results in an additional but minor excess of fresh water. The rivers carry different loads of salts washed out of the ground, in particular calcium carbonate, which cause a composition anomaly of the salt dissolved in the Baltic Sea in comparison to Standard Seawater. Directly measured seawater density shows a related anomaly when compared to the density computed from the equation of state as a function of Practical Salinity, temperature and pressure. Samples collected from different regions of the Baltic Sea during 2006–2009 were analysed for their density anomaly. The results obtained for the river load deviate significantly from similar measurements carried out forty years ago; the reasons for this decadal variability are not yet fully understood. An empirical formula is derived which estimates Absolute from Practical Salinity of Baltic Sea water, to be used in conjunction with the new Thermodynamic Equation of Seawater 2010 (TEOS-10), endorsed by IOC/UNESCO in June 2009 as the substitute for the 1980 International Equation of State, EOS-80. Our routine measurements of the samples were accompanied by studies of additional selected properties which are reported here: conductivity, density, chloride, bromide and sulphate content, total CO2 and alkalinity.

The first part of the study assessed the equivalence of practical salinity measurement results of a slightly diluted seawater sample from the North Atlantic, which were traceable to the SI. The study shows that the practical salinity reference value S RV (here S RV = 34.967) can be determined with a relative standard uncertainty of 3×10−4. This quantifies the uncertainty range, in which long-term comparability of practical salinity measurement results can be guaranteed currently. In the second part of the study, eight laboratories determined the mass fractions of five seawater components to quantify the equivalence of these measurement results. The results were: Na+ (10.726 ± 0.134) g/kg, Mg2+ (1.288 ± 0.018) g/kg, Sr2+ (0.00755 ± 0.00011) g/kg, Cl− (19.360 ± 0.047) g/kg, SO4 2− (2.650 ± 0.025) g/kg. The investigation confirmed the reference composition mass fractions of sodium, magnesium and chloride ions, but it showed a disagreement for strontium and sulphate ions.

To guarantee the quality of bioethanol and avoid damage to the fuel system in a car, the determination of acidity, water content, as well as chloride, sulfate, alkali metal and alkaline earth metal content are important. Electrochemical quantities like pH and electrolytic conductivity are frequently used in biofuel analysis as sum parameters mainly to indicate the risk of corrosion. Many standards and measurement methods used today in the analysis of biofuel are adopted from specifications and test methods originally developed for fossil-based fuels and commercial alcohol. This results from the rapid expansion of the biofuel production. There are efforts to solve remaining metrological and regulatory issues on a regional and international basis on a short term. In the following an overview on the fundamentals for the measurement of pH in bioethanol blends is given. It will be discussed if the electrolytic conductivity of the bioethanol could be a reliable measure of corrosion causing ionic contaminations in biofuel. State of the art, limitations and future tasks for metrology will be discussed.

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.

Measurements of practical salinity are based on conductivity measurements, which are traceable to the conductivity of a defined KCl reference solution. The conductivity of this reference solution must be traceable to the SI in order to state a reliable uncertainty for practical salinity measurements and to guarantee their comparability. Currently, the conductivity ratios from various old standard seawater batches are compared instead. However this method is inadequate on a long time-scale, which is important for oceanography studies, because it lacks a time-invariant reference. Therefore the equivalence of conductivity measurements traceable to primary conductivity standards has been investigated in Euromet project 918. Several European metrology institutes measured the conductivity of a 5 S/m KCl solution and the practical salinity of IAPSO standard seawater. The study estimates relative standard uncertainties to be of the order of 10−4 to 10−3 for such measurements.

International efforts concentrate on the traceability of electrolytic conductivity at the field level having small associated measurement uncertainties. Although the measurement of conductivity at ...

The carbonate solution is one of seven primary standard buffers set recently by the International Union of Pure and Applied Chemistry (IUPAC) recommendations (Buck et al., Pure Appl Chem 74:2169–2200, 2002). The peculiarity of the pH measurement of this buffer in a traditional Harned cell (classic cell) is due to the loss of carbon dioxide with hydrogen flow and the ensuing change of its chemical composition. This results in a continuous increase of the recorded electromotive force (E) of the cell. Therefore, almost all National Metrology Institutes (NMIs) are now using the extrapolation of measured E data to zero time point in order to find the initial pH. The National Metrology Institute of Japan (NMIJ) has developed a different design of Harned cell (NMIJ cell) with compensation of the lost carbon dioxide from the intermediary saturation vessel. The results of the study of the NMIJ cell are presented, together with those of the trilateral NMIJ–PTB–ZMK (NMI of Japan with the NMI of Germany [Physikalisch-Technische Bundesanstalt, PTB] and the German Centre for Measurements and Calibrations [Zentrum fur Messen und Kalibrieren GmbH, ZMK]) comparison of pH values for the carbonate solution in different designs of Harned cell.

A major issue with the electrolytic conductivity measurement for pure water is the lack of standard or reference methods. A primary method traceable to SI and suitable for pure-water conductivity measurement was developed at the Physikalisch-Technische Bundesanstalt (PTB), Germany, as the base for the calibration method for the conductivity measuring devices at the low conductivity level. This paper provides a novel method to calculate the bulk resistance of pure water using impedance measured at a single frequency, which is one of the key procedures for the primary methods.

The measured cell potentials for suspension potentiometric cells have been interpreted and explained by a detailed analysis of the schemes for these cells ["Guidelines for potentiometric measurements in suspensions. Part A. The suspension effect (IUPAC Technical Report", Pure Appl. Chem.79, 67 (2007)]. Some former disagreements amongst investigations have been clarified. A new unambiguous operational definition of the suspension effect (SE) is presented. It is defined as the difference in cell potential for two suspension potentiometric cells, one with both electrodes in the separated equilibrium solution (eqs) and the other with both electrodes in the sediment or suspension. This potential difference is the sum of the change in the indicator electrode (IE) potential and the change in the liquid junction potential of the reference electrode (RE), when the electrodes are used for measurement, once in the sediment of the suspension and then in its eqs.

An explanation of the origin and interpretation of the suspension effect (SE) is presented in accordance with "pH Measurement: IUPAC Recommendations 2002" [Pure Appl. Chem.74, 2169 (2002)]. It is based on an analysis of detailed schemes of suspension potentiometric cells and confirmed with experimental results. Historically, the term "suspension effect" evolved during attempts to determine electrochemically the thermodynamically defined activity of H+ (aq) in suspensions. The experimental SE arises also in determining other pIon values, analogous to pH values.The SE relates to the observation that for the potential generated when a pair of electrodes (e.g., reference electrode, RE, and glass electrode) is placed in a suspension, the measured cell voltage is different from that measured when they are both placed in the separate equilibrium solution (eqs). The SE is defined here as the sum of: (1) the difference between the mixed potential of the indicator electrode (IE) in a suspension and the IE potential placed in the separated eqs; and (2) the anomalous liquid junction potential of the RE placed in the suspension. It is not the consequence of a boundary potential between the sediment and its eqs in the suspension potentiometric cells as is stated in the current definition of the SE.

There is currently a major issue with the calibration of conductivity meters used for high purity water: the lack of availability of a reference material or reference methods for low conductivity ranges (conductivity below 1 μS cm−1 at 25.0 °C, resistivity >1 MΩ cm at 25.0 °C). This paper describes the current status of conductivity measurements in high purity water. A new and improved approach, currently being investigated, should allow us to make the calibration of conductivity meters used for low conductivity ranges traceable to the SI.

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.

Routine pH measurements are carried out with pH meter-glass electrode assemblies. In most cases the glass and reference electrodes are thereby fashioned into a single probe, the so-called 'combination electrode' or simply 'the pH electrode'. The use of these electrodes is subject to various effects, described below, producing uncertainties of unknown magnitude. Therefore, the measurement of pH of a sample requires a suitable calibration by certified standard buffer solutions (CRMs) traceable to primary pH standards. The procedures in use are based on calibrations at one point, at two points bracketing the sample pH and at a series of points, the so-called multi-point calibration. The multi-point calibration (MPC) is recommended if minimum uncertainty and maximum consistency are required over a wide range of unknown pH values. Details of uncertainty computations for the two-point and MPC procedure are given. Furthermore, the multi-point calibration is a useful tool to characterise the performance of pH electrodes. This is demonstrated with different commercial pH electrodes. ELECTRONIC SUPPLEMENTARY MATERIAL is available if you access this article at http://dx.doi.org/10.1007/s00216-002-1506-5. On that page (frame on the left side), a link takes you directly to the supplementary material.

Measurements of pH are performed on a large scale at laboratory level, and in industry. To meet the quality-control requirements and other technical specifications there is a need for traceability in measurement results. The prerequisite for the international acceptance of analytical data is reliability. To measure means to compare. Comparability entails use of recognised references to which the standard buffer solutions used for calibration of pH meter-electrode assemblies can be traced. The new recommendation on the measurement of pH recently published as a provisional document by the International Union on Pure and Applied Chemistry (IUPAC) enables traceability for measured pH values to a conventional reference frame which is recognised world-wide. The primary method for pH will be described. If analytical data are to be accepted internationally it is necessary to demonstrate the equivalence of the national traceability structures, including national measurement standards. For the first time key comparisons for pH have been performed by the Consultative Committee for Amount of Substance (CCQM, set up by the International Bureau of Weights and Measures, BIPM) to assess the equivalence of the national measurement procedures used to determine the pH of primary standard buffer solutions. The results of the first key comparison on pH CCQM-K9, and other international initiatives to improve the consistency of the results of measurement for pH, are reported.

The definition of a “primary method of measurement ” has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harned cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Système International d’ Unités). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [Pure Appl. Chem. 57, 531 (1985)]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 °C. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances.

A statistical analysis of multi-point calibration procedure for practical measurement of pH under laboratory conditions is given. The procedure is shown to provide the necessary elements for simple assignment of an uncertainty to a pH measurement – an essential element of quality control. Performance of ordinary least squares regression for prediction of confidence limits is compared to orthogonal regression, inverse regression and Monte Carlo simulations. In case of pH measurement by multi-point calibration procedure, methods considering uncertainties in both axes are found to be statistically more satisfactory than ordinary least squares regression. By analysis of 50 pH calibration data, randomly selected from 250 5-point glass electrode calibrations, it is found that the practical differences, however, are of minor importance. The significance of uncertainty in pH is demonstrated for an example from metal ion speciation in aqueous solution.

Reliable analysis results are indispensable in clinical chemistry for unambiguous diagnosis. Hence today traceability of the patient measurement to the SI system is established within the scope of a traceability system consisting of a national metrology institute and routine- and reference laboratories.

The standard hydrogen electrode (SHE) acts as a primary reference in electrochemistry. The standard potentials of all other reference electrodes are linked to that of the SHE at the same temperature. The SHE contribution to the cell potential is by convention zero at all temperatures (see Chap. 1).

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).

An explanation of the origin and interpretation of the suspension effect (SE) is presented in accordance with "pH Measurement: IUPAC Recommendations 2002" [Pure Appl. Chem. 74, 2169 (2002)]. It is based on an analysis of detailed schemes of suspension potentiometric cells and confirmed with experimental results. Historically, the term "suspension effect" evolved during attempts to determine electrochemically the thermodynamically defined activity of H + (aq) in suspensions. The experimental SE arises also in determining other plon values, analogous to pH values. The SE relates to the observation that for the potential generated when a pair of electrodes (e.g., reference electrode, RE, and glass electrode) is placed in a suspension, the measured cell voltage is different from that measured when they are both placed in the separate equilibrium solution (eqs). The SE is defined here as the sum of: (1) the difference between the mixed potential of the indicator electrode (IE) in a suspension and the IE potential placed in the separated eqs; and (2) the anomalous liquid junction potential of the RE placed in the suspension. It is not the consequence of a boundary potential between the sediment and its eqs in the suspension potentiometric cells as is stated in the current definition of the SE.

We present results of the European Metrology Research Project on the SI traceability of electrolytic conductivity measurements in bioethanol. As a first step to this aim secondary conductivity measurements have been performed to characterize reproducibility, stability, measurement uncertainty and the significance of the measurement results. The relative standard measurement uncertainty is of the order 0.3%, while inter-laboratory reproducibility is around 6.9%. The measured conductivities of two samples from different sources show a relative difference of around 30%. These results show that conductivity is an appropriate quality indicator for bioethanol. However, it also demonstrates that inter-laboratory reproducibility has to be improved, in particular with respect to SI traceability.

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).