New IUPAC (International Union of Pure and Applied Chemistry) recommendations on the measurement of pH - background and essentials

The IUPAC Recommendations on pH (1985) have serious metrological deficiencies (recommendation of two pH scales and of several pH definitions and procedures to measure pH). Background and essential features of new recommendations, which replace the 1985 document, are reported in this paper. The new document is strictly based on metrological principles. pH is defined (notionally) by the negative logarithm of the hydrogen ion activity according to S?rensen and Linderstr?m-Lang (1924), that is pH=-lg a(H). Because pH is a single ion quantity it is immeasurable and is therefore experimentally verified, with stated uncertainties, by pH(PS) values of primary standard buffer solutions. The assignment of pH(PS) is carried out in a Harned cell (without transference), which is defined as a primary method of measurement, and involves the Bates-Guggenheim convention. pH(PS) is thus a conventional quantity. Consideration of the uncertainty of the Bates-Guggenhein convention, however, permits its incorporation into the internationally accepted SI system of measurement. Comparison of the pH of secondary buffer solutions with pH(PS) values in recommended cells with transference yields secondary standards, whose pH(SS) can be traced back to pH(PS) and consequently to the definition of pH. The traceability chain is continued "downwards" by practical cells with transference containing glass electrodes for the measurement of pH(X) values of unknown solutions, for which three calibration procedures are recommended. The measurement of pH is thus represented by the traceability chain pH(X)-->pH(SS)-->pH(PS)-->pH as defined, each step having stated uncertainties. This hierarchical system of measurement excludes any pH 'scale'. Tabulated pH(PS) values are given as examples, and it is recommended that actual pH(PS) and pH(SS) be taken from certificates, which are to accompany each lot of certified reference material (CRM). Target uncertainties and examples of their calculation, a sign convention for pH cells and conventions for presenting cell schemes are given in the new document.     

The Current Usage of Selectivity Coefficients for the Characterization of Ion‐Selective Electrodes. A Critical Survey of the 2000/2001 Literature

The articles describing new ion‐selective electrodes published during the years 2000/2001 in eight leading analytical chemistry journals have been surveyed. Compliance with the IUPAC recommendations of the procedures used for characterizing the selectivity of the new electrodes has been checked. It was found that the information accompanying the selectivity data obtained according to the procedures recommended by IUPAC or their variations was frequently insufficient for interpreting of the electrode behavior and comparing with other sets of data, thus reducing the significance of the values provided. Some criteria for improving the communication between researchers in the field are suggested.     

Electrodes for Potential Measurements in Aqueous Systems at High Temperatures and Pressures

The measurement of pH, redox potentials, and corrosion potentials at high temperatures and pressures is often desirable for the control and monitoring of industrial processes; e.g., for controlling water chemistry in conventional and nuclear power generators, for process control in the chemical industry, and for monitoring performance in petroleum extraction and refining. Such measurements are also needed for research on the thermodynamics of high-temperature aqueous solutions including those associated with geothermal and hydrothermal reactions. Zirconia electrodes for these purposes are described, and their application is illustrated with examples. While not yet routinely available on a commercial basis, useful versions can be readily fabricated in the laboratory. Some of the pitfalls encountered in making such measurements at elevated temperatures are also discussed because great care is often required to assure that valid and useful data are being obtained.     

Liquid membrane electrodes for the selective determination of nicotine in cigarette smoke

Five liquid membrane electrode systems responsive to the nicotinium cation are described. These electrodes are based on the use of the ion-association complexes of the nicotinium cation with tetraphenylborate, 5-nitrobarbiturate, flavianate, reineckate and picrolonate counter anions in nitrobenzene solvent as ion-exchange sites. The performance characteristics of these electrodes, evaluated according to IUPAC recommendations, reveal fast, stable and near-Nernstian responses for 10(-2)-10(-5) M nicotine over the pH range 3.5-7. Many inorganic and organic cations do not interfere. The direct potentiometric determination of 3 micrograms ml-1-1.6 mg ml-1 of nicotine in aqueous solutions showed an an average recovery of 99.5% and a mean standard deviation of 1.2%. The electrodes were also used for monitoring the titration of nicotine with sodium tetraphenylborate, measuring the pK of nicotine and determining nicotine in the smoke from different cigarettes. The results compare favourably with those obtained by the standard gas chromatographic method.     

The terms “tochnost’” and “pravil’nost’” as applied to the results of chemical analysis

In this paper, attention is given to the inadequacy of the Russian translation of the term “accuracy” as “pravil’nost’” in publications of the IUPAC recommendations for the presentation of the results of chemical analysis and vocabularies of analytical terms. The essence of the concepts tochnost’ and pravil’nost’ in Russian measurement terminology is considered in historical context, and the evolution of the concepts accuracy in the terminological standards on analytical chemistry (IUPAC Recommendations), metrology, and statistics is traced. It is demonstrated that the one-to-one correspondence between the terms “accuracy” and “tochnost’” and “trueness” and “pravil’nost’,” respectively, which occurs in the present-day standardized terminologies, should also be followed in analytical chemistry     

Redefinition of basic chromatography terms

Summary The most important chromatography terms we are concerned with frequently are efficiency and selectivity. Through appropriate redefinitions the relation between these terms and the resolution becomes more obvious. To evaluate a particular separation the measurement of the behaviour of an unretained compound is not required; with common measurements on the actual chromatogram it is easily possible to calculate simple rations, the efficiency and the selectivity, so that different systems can be compared.Symbol used with the same meaning in the IUPAC recommendations [3].Symbol not in the IUPAC recommendations but compatible with it.     

Extended traceability of pH: an evaluation of the role of Pitzer's equations

The 2002 IUPAC recommendation on pH (provisional) has taken its own philosophy to provide a basis for comparable and traceable assignment of a value, from a measurement, to the quantity pH. Whereas the substituted 1983 IUPAC recommendation relied heavily on precisely prescribed experimental techniques and procedures, the current recommendation defines a hierarchical relationship between references for comparison (primary and secondary standards) and objective criteria on the comparison of measurements with these standards. The recommendation aims at a traceability chain from the national metrological institution (NMI) level down to field and laboratory measurements. Currently, however, the traceability chain is developed to the level of certified reference materials (CRM), namely the above mentioned primary and secondary standards. To complete the traceability chain, several theoretical and practical aspects have to be pondered. In part, the methods for comparative assessment of different options have yet to be developed. As an illustrating example of the complexity of issues to be considered in a further extension of the traceability chain is estimation of the doubt associated with Pitzer coefficients. The Pitzer equations for activity coefficient modelling are explicitly mentioned in the 2002 IUPAC recommendation on pH (provisional) as enabling possible improvement in the ionic strength extrapolations to zero ionic strength. An assessment of uncertainty of ternary Pitzer coefficients is given for the first time.     

Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008)

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the (1)H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating (13)C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice.     

Response time of ion-selective electrodes: Current usage versus IUPAC recommendations

The current usage in evaluating the response speed of ion-selective electrodes is critically surveyed. Examination of the articles introducing new ISEs published in eight leading analytical and electroanalytical chemistry journals in the period 2000-2001 and of sparse articles appeared afterwards shows that the current IUPAC definition of response time, based on the differential quotient ΔE/Δt, is only used by a minority of authors. In its place, former IUPAC definitions, to be presently considered obsolete, or different expressions of the response speed are commonly preferred. In most instances, the reported data do not describe adequately the electrode behaviour. Stricter adherence to the most recent IUPAC recommendation, which is more rational and nearly related with the practical reading time, is to be stimulated. Wider use of faithful emf versus time plots also deserves to be encouraged.     

Traceability of pH measurements by glass electrode cells: performance characteristic of pH electrodes by multi-point calibration

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.     

Potentiometric sensors enabling fast screening of the benign prostatic hyperplasia drug alfuzosin in pharmaceuticals, urine and serum

The construction and characterization of potentiometric membrane electrodes are described for the quantification of alfuzosin, a drug used in a mono- and combined therapy of benign prostatic hyperplasia (BPH). The membranes of these electrodes consist of alfuzosin hydrochloride-tetraphenyl borate, (Az-TPB), chlorophenyl borate (Az-ClPB), and phosphotungstate (Az(3)-PT) ion associations as molecular recognition reagent dispersed in PVC matrix with dioctylpthalate as plasticizer. The performance characteristics of these electrodes, which were evaluated according to IUPAC recommendations, revealed a fast, stable and liner response for alfuzosin over the concentration ranges of 8.3 x 10(-6) to 1.0 x 10(-2) M, 3.8 x 10(-6) to 1.0 x 10(-2) M, 7.5 x 10(-7) to 1.0 x 10(-2) M AzCl with cationic slopes of 57.0, 56.0 and 58.5 mV/decade, respectively. The solubility product of the ion-pair and the formation constant of the precipitation reaction leading to the ion-pair formation were determined conductometrically. The electrodes, fully characterized in terms of composition, life span and usable pH range, were applied to the potentiometric determination of alfuzosin hydrochloride ion in different pharmaceutical preparations and biological fluids without any interference from excipients or diluents commonly used in drug formulations. The potentiometric method was also used in the determination of alfuzosin hydrochloride in pharmaceutical preparations in four batches with different expiration dates. Validation of the method showed suitability of the proposed electrodes for use in the quality control assessment of alfuzosin hydrochloride. This potentiometric method offers the advantages of high-throughput determination, simplicity, accuracy, automation feasibility, and applicability to turbid and colored sample solutions.     

Thermodynamic Foundation for High Temperature Electrochemistry

Thermodynamic concepts required for the thermodynamic calculation of the potentials of electrodes for high temperature applications are briefly reviewed. A thermodynamic approach to the calculation of half cell potentials and the standard chemical potential of an electron at high temperatures which are related to the Standard Hydrogen Electrode(SHE) is discussed. As examples, an external Ag/AgCl reference electrode and a YSZ(Ag|O2) pH sensor for high temperature applications are analyzed by using the thermodynamic ap-proach to derive a high temperature pH measurement equation. The two electrodes are employed to measure high temperature pH and the measured pH was compared with the calculated pH by using a solution chem-istry method. Concepts and principles for electrode kinetics are also briefly introduced and a modification to the Tafel equations is suggested.     

Improved reliability of pH measurements

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.     

Basic equations and uncertainties in isotope-dilution mass spectrometry for traceability to SI of values obtained by this primary method

A current interest in chemistry concerns traceability of analytical measurements to the International System of Units (SI) and the estimation of their uncertainties in accordance with principles of metrology, that is, measurement science. “Primary methods of measurement” achieve traceability to SI directly without intermediate reference standards or materials and without significant empirical correction factors. Isotope-dilution mass spectrometry should be regarded as such a method. It has the potential of smallest presently achievable uncertainties for analytical measurements directly or for the certification of reference materials including those with abnormal isotopic composition. A simple explanation of the method including its basic equations is given. Full uncertainty estimation is emphasized in terms of these equations. The wider use of concepts of metrology in chemistry is discussed.     

Analytical chemistry in nuclear technology

Summary The objectives of analytical chemistry in nuclear technology are discussed. The analytical techniques and methods commonly used in the various phases of the nuclear fuel cycle, mining and fuel fabrication, reprocessing and nuclear waste management are described and their advantages and disadvantages are demonstrated. The question of applying in-line analytical instruments in this area is addressed and some techniques which show a proven potential for such an employment are defined.Presented at the 33rd IUPAC Congress, Budapest, August 17–22, 1991.     

An imaging pH optode for cell studies based on covalent attachment of 8-hydroxypyrene-1,3,6-trisulfonate to amino cellulose acetate films

The objective of this study was to create a thin film optode for fast pH measurements that meets the requirements for imaging pH-responses from cells as well as for a future hybrid design for detection of multiple analytes simultaneously. The sensor is based on the covalent attachment of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) to a film forming cellulose acetate material through a sulfonamide linkage. The synthesis routes of the cellulose material and regio-specific covalent attachment of the dye are described in detail. The sensor was sterilized in two different ways and showed excellent biocompatibility with Chinese hamster ovary cells. Imprints from cells and μm thin cell extensions were visualized when altering pH of the surrounding solution. The sensor was tested together with time-dependent sigmoidal calibrations giving pH determinations with an exceptional precision and accuracy during measurement within pH 6-8. The precision of the optode, calculated as pooled S.D. (n = 8) according to IUPAC recommendations between pH 6.641 and 7.742 was 0.0029. The accuracy was significantly better than the electrode used as reference during the measurements. The response time (0-95%) was 100 s between pH 6.641 and 7.742 and the reverse response (95-0%) was 80 s. The sensor shows great potential for extra-cellular pH determination over time during cell growth and pharmacological exposure.     

Use of uncertainty estimates as reported by participants in proficiency testing for the evaluation of their results: pros and cons

The aim of this paper is to study the measurement uncertainties reported in proficiency tests (PTs) using examples from PTs in the environmental sector and to compare the obtained measurement uncertainty estimates using different approaches. In addition, the paper focusses on the differences between the z-score and the zeta score. Since the year 2000, the Finnish Environment Institute has asked participants to report analytical methods as well as measurement uncertainties in connection with PT results. The measurement uncertainties of the assigned value have also been evaluated. On the basis of the results, the measurement uncertainties reported by the participants varied greatly. Participants often reported underestimated measurement uncertainties, but overestimated uncertainties were also reported. At the moment, it seems as if performance assessment should be based on the z-score because of a number of significant over- and underestimated measurement uncertainties. The zeta score should be used for information and educational purposes mainly.     

Secondary pH standards and their uncertainty in the context of the problem of two pH scales

Establishing a traceability route with all measurement and uncertainty relationships determined is an important aspect of traceability, and seems to be particularly striking in pH measurement. In this paper the issue of evaluation of secondary pH standards measured with reference to a primary standard in a differential cell with free diffusion type liquid junctions is considered. Relatively high uncertainty, U=0.015, has been assigned to such standards in the recent IUPAC Recommendations on pH (2001), because of a specific residual liquid-junction potential treated statistically as a contribution to the combined uncertainty. Close inspection of the problem leads to the conclusion that a correction for the residual liquid-junction potential should be applied to the measured value of a secondary pH standard. This can be considered as a correction for a known systematic effect on the traceability route. With available experimental data it is demonstrated that such a correction can reasonably be made for well-studied standard buffer systems. In this way the uncertainty associated with secondary pH standards is kept to a low level, and, what is more, the problem of two pH scales, a multi-standard scale and a single-standard scale, gains a proper solution. The need for different treatment of residual liquid-junction potentials at different levels in the measurement hierarchy is noted. Much attention is also given to rational categorization of pH standards in the hierarchy.     

Novel Green Potentiometric Method for the Determination of Lidocaine Hydrochloride and its Metabolite 2, 6‐Dimethylaniline; Application to Pharmaceutical Dosage Form and Milk

Sustainable chemistry has attracted the attention of scientists during the last decades owing to the great advantages encountered by its application. These include: waste reduction, energy conservation and substitution of hazardous substances with more eco‐friendly ones. Following this approach, a new sensitive and selective membrane sensor was developed and investigated for the determination of lidocaine hydrochloride (LD) and its carcinogenic metabolite 2,6‐dimethylaniline (DMA). Several polyvinyl chloride (PVC) based sensors were tried using different plasticizers as well as different cation exchangers. The best response was achieved upon using dioctylphthalate (DOP) as solvent mediator and phosphotungstate (PT) as cation exchanger. LD was selectively determined at pH 6 without interference from its carcinogenic metabolite, while DMA that had been reported to be the toxic inactive metabolite of LD secreted in the milk was determined in the milk at pH 2. Fast, stable Nernstian responses were achieved by the proposed sensors over a concentration range of 2.66×10⁻⁵ M to 1×10⁻² M for both LD and DMA. The method was validated according to the IUPAC recommendations and was successfully applied for the determination of LD in pure form and pharmaceutical dosage form, whereas DMA was successfully determined in pure form and spiked milk samples.