The standardization of pH measurements

The determination of pH values suffers from two sources of error. One is the method of calibration of glass electrode cells. Multiple point calibration with linear regression is highly recommended instead of the usual bracketing procedure in cases where a precision of pH<±0.02 is required. Advantages of this method are discussed, particularly regarding the contribution of the liquid junction potentials to the cell emf. The second uncertainty is caused by batch to batch variations of Standard Reference Materials and their respective standard pH(S), actual values of which are part of the data set of microprocessor-controlled pH meters. Differential potentiometry is proposed as the method of choice to avoid these differences. It results in pH(S) independent of a particular material. Experimental data of the restandardization of pH reference materials are also presented.     

Traceable measurements of pH

 The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to these primary pH standards. To assess the degree of equivalence, comparisons of primary measurement procedures for pH were organized in co-operation with EUROMET. Typical results will be presented. In 1998, the Consultative Committee for Amount of Substance (CCQM) decided to include the field of pH in its working programme. The first key comparison for this quantity was recently carried out on two phosphate buffer solutions. Received: 14 June 2000 / Accepted: 22 August 2000     

Thermodynamic examination of pH measurements

The electrometric determination of pH is essentially an evaluation of the thermodynamic quantity -loga(H(3)O)(+) from a measurement of an electrical potential difference between two electrodes. It is accordingly necessary to examine the electrometric method in light of thermodynamic concepts in order to disclose the meaning of experimental pH values and their limitations. In this paper it is shown by thermodynamic considerations that the evaluation of -loga(H(3)O)(+) is actually a special case of the evaluation of the thermodynamic quantity The operational definition of pH prescribes a general method to evaluate this quantity, but not -loga(H(3)O(+)) as commonly believed.     

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.     

Spectrophotometric pH measurements of freshwater

The use of cresol red (CR) indicator for determination of freshwater pH is evaluated. Ionic strength effects and indicator pH perturbation are discussed and quantified using theoretical and empirical approaches. Spectrophotometric and potentiometric methods are directly compared by repeated analyses of a low ionic strength pH buffer. The mean and standard deviation of the two methods were 7.618±0.008 (spectrophotometric) and 7.484±0.040 (potentiometric) (N=18) with systematic errors of 0.003 and 0.137 pH units relative to the true pH (7.621). Field data from an alkaline river (pH∼7.8-8.8) show that measurement reproducibility is better than 0.01 pH units, making it possible to resolve very small spatial and temporal changes in riverine pH. Uncertainty in the indicator apparent dissociation constant limits the accuracy of the pH measurement to ∼0.05 pH units. An alternative method for estimating the dissociation constant, based on calculation of pH from two other carbonate parameters, is proposed.     

Improved traceability of pH measurements

Traceability is a prerequisite for the comparability and uniformity of measurements. Although pH-measurements are carried out on a large scale in laboratory and industry, the problems involved in the traceability of pH values have not adequately been solved in the past. The comparability of pH measurements is limited, among other parameters, by the accuracy of the pH values of the standard buffer solutions used to calibrate the pH meter-electrode assemblies. The measured pH(X) value must be traceable to primary standard pH(PS) values through an unbroken chain of comparisons, all values having stated uncertainties. A new primary standard measurement device for pH is used to certify primary pH reference materials from which these secondary reference materials can be derived.     

The effect of platinum electrode surfaces on precise primary pH measurements

Journal of Solid State Electrochemistry - For primary pH measurements, a platinum hydrogen electrode and a silver chloride electrode are immersed in the same solution in a cell without transference...     

Raman based pH measurements with polyaniline

The reversible acid–base equilibrium between the emeraldine salt (ES) and the emeraldine base (EB) form of thin electropolymerized films of polyaniline (PANI) has been studied between pH 2 and 9 with in situ Raman spectroscopy using the 514, 633 and 780 nm laser excitation wavelengths. The electropolymerization of PANI was done with cyclic voltammetry in a flow cell from a solution consisting of 50 mM aniline and 1.0 M HCl. It is shown that the Raman signal of the CN stretching vibration at 1439 cm⁻¹, originating from the quinoid units of the EB form, can be used for pH measurements between pH 3 and 6 with the 633 nm laser. This vibration is strongly resonance enhanced by the 633 nm laser and its intensity grows therefore considerably in the pH interval where the ES–EB transition mainly takes place (pH 3–6). The influence of the film conditioning pH was also studied as well as the reproducibility of the Raman spectra when pH was changed several times from pH 6 to 3.     

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 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.     

The pH response of the reticulated vitreous carbon electrode

Reticulated vitreous carbon (RVC) electrodes show variable mV responses to changes in pH. The mV—pH slopes are Nernstian for the pH range 1–7 between 39 and 49°C. Treatment with permanganate solution greatly enhances the response. The RVC material was analyzed for carbon, hydrogen, and nitrogen and trace metallic impurities. Reasons for the response are discussed.     

The history and development of a rigorous metrological basis for pH measurements

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.     

pH measurements based on a palladium electrode

The feasibility of a metal palladium electrode as the pH sensor in air-saturated aqueous solutions was evaluated. The measurement was based on an “on-site” precleaning step and a potential decay process. The experimental results showed a linear pH response with a sensitivity of around 60 mV/pH and good reproducibiliry. The temperature effect on the measurement was also evaluated.     

Two-photon nano-PEBBLE sensors: subcellular pH measurements

Intracellular pH mapping is of great importance as it plays a critical role in many cellular events. Also, in tissue, pH mapping can be an indicator for the onset of cancer. Here we describe a biocompatible, targeted, ratiometric, fluorescent, pH sensing nano-PEBBLE (Photonic Explorer for Biomedical use with Biologically Localized Embedding) that is based on two-photon excitation. Two-photon excitation minimizes the photobleaching and cell autofluorescence drastically, leading to an increase in the signal-to-noise ratio. PEBBLE nanosensors provide a novel approach for introducing membrane impermeant dyes, like HPTS, into cells. We use both non-targeted and F3 peptide targeted PEBBLE nanosensors for intracellular pH measurement of 9L cells. The intracellular measurements suggest that the non-targeted nanosensors are mostly trapped in endosomes, whereas the F3 peptide targeting enables them to escape/avoid these acidic compartments. Combining the advantages of pH sensitive PEBBLE nanoparticles, including their specific targeting, with the advantages of two-photon microscopy provides an attractive and promising prospect for non-invasive real-time monitoring of pH inside cancer cells and tissues.     

Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements

The oceans are a major sink for anthropogenic atmospheric carbon dioxide, and the uptake causes changes to the marine carbonate system and has wide ranging effects on flora and fauna. It is crucial to develop analytical systems that allow us to follow the increase in oceanic pCO₂ and corresponding reduction in pH. Miniaturised sensor systems using immobilised fluorescence indicator spots are attractive for this purpose because of their simple design and low power requirements. The technology is increasingly used for oceanic dissolved oxygen measurements.     

Potentiometric pH sensors for measurements in fluoride-containing solutions

The results of tests in fluoride-containing solutions were presented for an antimony electrode, solidstate polyvinyl chloride-graphite electrodes modified by a quinone-hydroquinone series system, and electrodes made of silicate glasses, in particular, of commercial formulations. A technique and criteria were proposed for comparison of the performance characteristics of different glass electrodes in fluoride-containing solutions, and samples most resistant to hydrofluoric acid were identified.     

The interactions of macrocyclic ethers by 13C dipole-dipole relaxation time measurements

¹³C dipole–dipole relaxation time measurement studies of free and cation complexed cyclic polyethers in solution are reviewed briefly. The ¹³C dipole–dipole relaxation time of the macrocyclic ether backbone has been recognized to be a function of a motion of the molecular segment acting as a macrocyclic ligand site. This has been particularly investigated by NMR under extreme narrowing conditions.     

Inexpensive device for measurements with ion-selective and pH electrodes

The design and construction of a simple device for measuring ionic concentrations (or pH) with ion-selective electrodes are described. The automated system includes a special electronic circuit with an operational amplifier, a signal conditioner and a personalcomputer. A digital multimeter can be used if automation is not required. The results obtained in tests with iodide-, chloride- and nitrate-selective electrodes and glass electrodes show very good agreement with those obtained with sophisticated commercial apparatus.     

Planar micro sensors for in vivo myocardial pH measurements

Potentiometric responses of microelectronically fabricated planar pH sensors based on both TDDA or ETH 5294 proton neutral carriers, in aminated PVC matrix, were evaluated with respect to main analytical parameters (linear ranges, slopes, reproducibility of potential measurements, potential drift and membrane resistance). In order to increase the electrode life time, increased amounts of membrane material were applied on the Ag/AgCl-poly-HEMA active spots of the polyimide substrate. The electrodes were implanted into an in situ porcine beating heart preparation at a midmyocardial depth in order to monitor H(+) concentration changes during the course of coronary artery occlusion.     

Standardization of pH measurements based on the ion interaction approach

The Pitzer method was used to calculate the pH values on the conventional and “true” scales for the TRIS—TRIS·HCl−NaCl−H₂O buffer system in the 0–40 °C temperature region and 0–4 NaCl molality interval. This buffer can be used as a standard for pH measurements in a wide range of ionic strengths. The conventional scale is used in cells without a salt bridge. The “true” scale is recommended for pH measurements using cells with a salt bridge. At the same concentrations of the buffer solution, the “true” scale is essentially transformed into the scale of the National Bureau of Standards (NBS) of the USA. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 676–680, April, 2000.