Effect of Silver Annealing Conditions on the Performance of Electrolytic Silver/Silver Chloride Electrodes used in Harned Cell Measurements of pH

We have studied the long and short term stability of electrolytic Ag/AgCl electrodes fabricated from Ag wire that has been subjected to a range of different annealing conditions. At elevated temperatures, the presence of oxygen during the annealing process has been shown to be detrimental to the performance of electrodes produced. This phenomenon has been attributed to the dissolution of oxygen in the Ag lattice leading to structural changes in the Ag/AgCl electrode material. Electrodes prepared from Ag wire annealed in the absence of oxygen have shown no appreciable change in performance throughout the temperature range employed. This work has resulted in an improved understanding of the optimum annealing conditions required for Ag used in the preparation of electrolytic Ag/AgCl reference electrodes. This work has positive implications for the accuracy of Harned cell measurements of pH

Sensitivities of Key Parameters in the Preparation of Silver/Silver Chloride Electrodes Used in Harned Cell Measurements of pH

A questionnaire was completed by fourteen world leading national metrology institutes 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 institute in the last decade has been assessed based on their results in eight key comparisons, organized by the Bureau International des Poids et Measures Consultative Committee for Amount of Substance, involving the measurement of pH of phosphate, phthalate, carbonate, borate and tetroxalate buffer solutions. 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

Key comparison CCQM-K73 amount content of H+ in hydrochloric acid (0.1 mol·kg-1)

This key comparison (KC), CCQM-K73, was performed to demonstrate the capability of the participating National Metrology Institutes (NMIs) to measure the amount content of H+ , νH + , in an HCl solution with a nominal νH + of 0.1 mol·kg-1 . A parallel Pilot Study, CCQM-P19.2, was performed for NMIs that did not desire to participate in the KC. The comparison was a joint activity of the Electrochemical Working Group (EAWG) and Inorganic Analysis Working Group (IAWG) of the CCQM and was coordinated by NIST (USA) and CENAM (México). The method of determination of νH + was left to the individual participant. All participants used either coulometry or titrimetry with potentiometric determination of the endpoint. The agreement of the results was not commensurate with the claimed uncertainties of the subset of participants that claimed small uncertainties for this determination. A workshop on technical issues relating to the CCQM-K73 measurements was conducted at the joint IAWGEAWG meeting at the Bureau International des Poids et Mesures (BIPM), Paris (Sèvres) in April 2010. Several possible sources of bias were investigated, but none could explain the observed dispersion among the participants’ results. In the absence of a specific cause for the dispersion, the IAWG and EAWG decided to assign a Key Comparison Reference Value, KCRV, and standard uncertainty of the KCRV, uKCRV, based on the DerSimonian-Laird statistical estimator. The uKCRV is dominated by the between-laboratory scatter of results in CCQM-K73. The uncertainty estimates from the participants with the lowest reported uncertainties remain unsupported by this KC.Fil: Pratt, Kenneth W. National Institute of Standards and Technology (NIST); Estados UnidosFil: Ortiz-Aparicio, Jose Luis. Centro Nacional de Metrología (CENAM); MéxicoFil: Matehuala-Sanchez, Francisco Javier. Centro Nacional de Metrología (CENAM); MéxicoFil: Jakobsen, Pia Tønnes. Dansk Fundamental Metrology (DFM); DinamarcaFil: Pawlina, Monika. Główny Urząd Miar (GUM); PoloniaFil: Kozłowski, Władysław. Główny Urząd Miar (GUM); PoloniaFil: Borges, Paulo P. Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMetro); BrasilFil: da Silva Junior, Wiler B. Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMetro); BrasilFil: Borinsky, Mónica B. Instituto Nacional de Tecnología Industrial (INTI); ArgentinaFil: Hernandez, Ana. Instituto Nacional de Tecnología Industrial (INTI); ArgentinaFil: Puelles, Mabel. Instituto Nacional de Tecnología Industrial (INTI); ArgentinaFil: Hatamleh, Nadia. Instituto Nacional de Tecnología Industrial (INTI); ArgentinaFil: Acosta, Osvaldo. Instituto Nacional de Tecnología Industrial (INTI); ArgentinaFil: Nunes, João. Instituto Português da Qualidade (IPQ); PortugalFil: Guiomar Lito, M. J. Instituto Português da Qualidade (IPQ); PortugalFil: Camões, M. Filomena. Instituto Português da Qualidade (IPQ); PortugalFil: Filipe, Eduarda. Instituto Português da Qualidade (IPQ); PortugalFil: Hwang, Euijin. Korea Research Institute of Standards and Science (KRISS); Corea del SurFil: Lim, Youngran. Korea Research Institute of Standards and Science (KRISS); Corea del SurFil: Bing, Wu. National Institute of Metrology (NIM); ChinaFil: Qian, Wang. National Institute of Metrology (NIM); ChinaFil: Chao, Wei. National Institute of Metrology (NIM); ChinaFil: Hioki, Akiharu. National Metrology Institute of Japan (NMIJ); JapónFil: Asakai, Toshiaki. National Metrology Institute of Japan (NMIJ); JapónFil: Máriássy, Michal. Slovenský Metrologický Ústav (SMU); EslovaquiaFil: Hanková, Zuzana. Slovenský Metrologický Ústav (SMU); EslovaquiaFil: Nagibin, Sergey. Ukrainian State Research and Production Center of Standardization Metrology, Certification, and Consumers’ Rights Protection (UMTS); UcraniaFil: Manska, Olexandra. Ukrainian State Research and Production Center of Standardization Metrology, Certification, and Consumers’ Rights Protection (UMTS); UcraniaFil: Gavrilkin, Vladimir. Ukrainian State Research and Production Center of Standardization Metrology, Certification, and Consumers’ Rights Protection (UMTS); UcraniaFil: Kutovoy, Viatcheslav. All-Russian Scientific Institute for Physical-Technical and Radiological Measurements (VNIIFTRI); Rusi

Meeting the Requirements of the Silver/Silver Chloride Reference Electrode

Potentiometric cells from which the pH values of standard buffer solutions are computed rely on the silver-silver chloride reference electrode which has proved to be convenient, reproducible and reliable. Of the various methods of preparation of silver-silver chloride electrodes only one concerns us here: the thermal-electrolytic type that has been used more extensively than any other form. Once prepared, the electrodes need to be equilibrated before use and between experiments. The equilibration technique must ensure voltage stability and inter-electrode, or bias, potential below 0.1 mV. In potentiometry the stability of a reference electrode is of utmost importance since an offset of 1 mV is equivalent to a deviation of about 0.02 in the pH value. A thorough study has been conducted, involving a critical assessment of the various protocols which have been developed and adopted by different research schools and have been used in the course of inter-laboratory exercises. Factors that affect the electrode performance, such as size, mass, chloridization, storage and conditioning were systematically investigated, through measurements of bias potential, leading to a set of recommended procedures that meet the requirements for high quality results. In this work, the adequacy of the optimized methodology is confirmed by means of primary pH measurements

Effect of condensation phenomena on potentiometric measurements

Results of potentiometric analysis, namely those of pH measurements, depend on temperature control of the experimental setup, as it is expressed in the analytical law, the Nernst equation, starting from the primary level, where reference values are conventionally assigned to standard solutions, through the whole traceability chain, down to the service laboratory. Fundamental studies of pH standards, based on the measurement of the potential of an electrochemical cell without transference, known as Harned cell, containing a platinum-hydrogen electrode and a silver-silver chloride reference electrode, refer condensation phenomena on the portions of the cell walls which are not immersed in the thermostatic bath, as one of the major sources of erTor in the assessment of both the silver-silver chloride electrode standard potential and on pH values. In this work such effect, which is bound to happen due to significant temperature differences between the ambient air temperature and the water bath, has been quantified, presenting an original contribution to the improvement of the quality of potentiometric analysis results. This was possible due to the availability of a climatic cabin "WALKIN" with a temperature control of +/- 0.01 degrees C, which permitted that temperature gradients were built between the thermostat water bath (controlled to 0.005 degrees C where cells filled to about 2/3 full were immersed up to 90% of their height, and the surrounding environment. (c) 2006 Elsevier B.V. All rights reserved

PORTUGUESE pH INTERLABORATORY COMPARISON

Abstract − The first interlaboratory comparison at national level for pH measurement of a primary standard phosphate buffer solution was carried out. The main aim of the exercise was to verify the way laboratories are planning and performing pH measurements and evaluates the performance of each laboratory. Seven laboratories have participated in this study

Improving the quality of potentiometric pH measurements

Like all experimentally determined physical and chemical properties, pH measurements are affected by the limited precision and accuracy of the measurement procedures. Fundamental studies of pH standards, based on measurement of the potential of an electrochemical cell without transference, known as the Harned cell, containing a platinum-hydrogen electrode and a silver-silver chloride reference electrode, indicate that vapour condensation phenomena on potentiometric cell walls not immersed in the thermostatic bath are a major source of error in assessment of pH values. In this work a study was conducted on phthalate buffer, 0.05 mol kg(-1)supercript stop KHPhth, and results are reported for the effect of this phenomenon on the assignment of pH values and on their corresponding uncertainties. Identification and quantification of this effect constitute an original contribution to improvement of the primary method of pH measurement and, therefore, more rigorous pH (PS) values

Uncertainty introduced by extrapolation in the conventional assignement of reference ph values

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