A review of pH measurement at high temperatures

Measurement of pH in aqueous solutions at up to 300 degrees and 150-300 bar is reviewed. Potentiometric membrane electrodes are identified as the sensors giving the most immediate hope of being practical. Zirconia membranes work well above 200 degrees and in alkaline solution, whereas glass membranes are best up to 150 degrees and in acidic solutions. Both membranes are largely free from interferences. Metal-metal oxide electrodes offer poor prospects, deviating from the ideal Nernstian response at all temperatures and being susceptible to interference from many redox and complexing agents, but systems based on iridium oxide have some promise. The hydrogen electrode remains the standard for pH measurement, but its analytical application is limited by the need to know the hydrogen partial pressure. A practical solution to this problem has yet to be found, except in restricted and artificial circumstances. Palladium hydride electrodes may be useful up to about 200 degrees , but in hydrogen-saturated waters revert to being hydrogen electrodes in any case. Non-potentiometric pH measurements with semiconducting oxides have been shown to be possible, but there are many unanswered questions about possible interferences. Considerable extra instrumentation is required, compared with potentiometry. Fibre-optic sensors based on indicator dyes have been investigated at room temperature, and have the great merit of not requiring a reference electrode. They seem, however, prone to many interferences and have an inherently limited working range of approximately 2 pH. No measurements at high temperature have been reported. Improved reference electrodes for potentiometric systems are still needed, although there have been advances in the design of external pressure-compensated electrodes working at room temperature. The silver-silver chloride system is still the one most favoured. There has been little rigorous work on standard buffer solutions at above 100 degrees and none at above 200 degrees . Neutral and alkaline buffers are especially needed. The establishment of proper pH standards for high-temperature work would make the testing of sensors both speedier and more reliable. Doubtless because of the experimental difficulties involved, few measurements have actually been made at high temperature, and those in a rather restricted range of conditions. In particular, measurements in dilute, poorly buffered, solutions, which provide the most rigorous test of a system's capability, are completely lacking.     

Numerical Simulation of pH Measurement in Dilute Solutions of the System H<Subscript>2</Subscript>O–KCl–HCl

Potentiometric pH measurements on cells with liquid junctions are known to be biased with respect to the notional pH in dilute acid solutions, but detailed evaluation of the problem is obstructed by experimental difficulties. In this work, pH measurements are simulated numerically on a kind of the Harned cell with a free-diffusion junction between the saturated solution of KCl and dilute solutions of HCl + KCl with ionic strength and acid concentration varying from 0.0001 to 0.1 in terms of molarity. The pH is standardized against the solution 0.0001 M HCl + 0.05 M KCl, and the simulations are based on known solution properties (transport numbers, activity coefficients and diffusion coefficients). The bias is found to range from ?0.012 to 0.056 in the composition range studied. The cell response is nearly linear in the notional pH in solutions with varying acid concentration, but no such relation is found in solutions with varying ionic strength at fixed acid concentration. It is shown that the Henderson equation underestimates the residual liquid-junction effect in very dilute solutions, largely due to failure to account for activity coefficients varying along the junction.     

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.     

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.     

Methods for Reliable pH Measurements of Precipitation Samples

Abstract

Significant errors in precipitation acidity determinations can result from improper use of pH electrodes. Conventional electrodes measure free hydrogen ion activity instead of hydrogen ion concentration or free acidity. Correction from activity to concentration is a function of ionic strength and can be large for the low ionic strengths typical of precipitation samples. Also, differences between sample and standard calibration buffer solution ionic strengths can result in liquid-junction potentials that affect electrode readings. Streaming potentials due to the stirring of precipitation samples can cause the single, largest error in pH. Certain procedures can be employed to reduce individual types of errors. These and methods to assess pH electrode performance are discussed.     

Determination of the performance of glass electrodes in aqueous solutions in the physiological ph range and at the physiological sodium ion concentration

A method for testing glass electrodes in the physiological pH range (6.4–7.6) and at the physiological sodium ion concentration (0.15 mol l^-1), based on indirect comparison of potentials with the hydrogen gas electrode, is developed according to a scheme described earlier. The hydrogen ion sensitivity and the sodium ion error of a glass electrode can be determined with three different aqueous solutions of amine buffers and their hydrochlorides; two of these have different pH values and one also contains a sodium salt (at the higher pH value). A cell without a liquid/liquid junction, containing silver/silver chloride reference electrodes, is used at 37° C. The accuracy of both determinations is ±0.2 mV (±0.003 pH). The results for some commercial glass electrodes tested with this method are presented.     

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.     

Systematic error arising from ‘sequential’ standard addition calibrations. 2. Determination of analyte mass fraction in blank solutions

The use of a sequential standard addition calibration (S-SAC) can introduce systematic errors into measurements results. Whilst this error for the determination of blank-corrected solutions has previously been described, no similar treatment has been available for the quantification of analyte mass fraction in blank solutions - a crucial first step in any analytical procedure. This paper presents the theory describing the measurement of blank solutions using S-SAC, derives the correction that needs to be applied following analysis, and demonstrates the systematic error that occurs if this correction is not applied. The relative magnitudes of this bias and the precision of extrapolated measurements values are also considered.     

Polynomial mass bias functions for the internal standardization of isotope ratio measurements by multi-collector inductively coupled plasma mass spectrometry

The development and application of a calibration strategy for routine isotope ratio analysis by multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is described and assessed. Internal standardization was used to account for the mass dependant determinate error (mass bias). The general solution for polynomial isotope ratio mass bias functions for use with internal standardization and isotope ratio measurements by multi-collector inductively coupled plasma mass spectrometry was derived. The resulting linear isotope ratio mass bias function was demonstrated to be mathematically consistent and experimentally realistic for the analysis of acidified aqueous solutions of analyte and internal standard elements (clean solutions) by multi-collector inductively coupled plasma mass spectrometry.     

Determination of pH values over the temperature range –60°C for some operational reference standard solutins and values of the conventional residual liquid-junction potentials

Direct measurements of the pH of six new operational standard solutions for the British Standard pH scale are reported from measurements with hydrogen gas electrodes in cells with liquid junctions reproducibly formed in vertical capillary tubes (1 mm internal diameter). Comparison with assigned pH values from cells without liquid junction enables values of the conventional residual liquid-junction potential to be calculated.     

Performance of reference electrodes with free-diffusion junctions : The Effect of Ionic Strength and Bore Size on Junction with Simple Cylindrical Geometry

The performance of free-diffusion liquid junctions formed in a capillary between saturated potassium chloride solution and a range of solutions with ionic strengths varying from 10^?5 to 0.5 mol kg^?1 is described. Precision, response time and noise associated with convection were adversely affected at ionic strengths less than 10^?3 mol kg^?1. Increasing the bore of the capillary from 0.5 mm to 3 mm also had a detrimental effect. Capillaries with 0.5-mm bore performed optimally with errors <0.2 mV for solutions with ionic strength > 10^?4 mol kg^?1, but deteriorating to 1 mV for 10^?5 mol kg^?1 solutions. Equivalent errors if free-diffusion junctions were used in pH measurements would be 0.003 and 0.017 pH, indicating that, given well-controlled experimental conditions, it is possible to achieve precise measurements in very dilute solutions.     

Nafion® Coated Electropolymerised Flavanone-based pH Sensor

This work summarizes the electrochemical response of flavanone carbon composite electrodes in comparison with Nafion®-coated flavanone carbon composite electrodes, for use as voltammetric pH sensors in both buffered and low-buffered media. Square wave voltammetric measurements suggest the peak potential achieved from the electrochemical polymerization after the electron-proton oxidation responds with accuracy to buffered pH solutions for both coated and non-coated electrodes, with a potential shift of 55.1 mV and 54.6 mV per pH unit respectively. However, a considerable improvement in stability, accuracy and sensitivity is provided by the proton-transfer Nafion® layer in CO₂ bubbled sea water. Furthermore, Nafion®-coated flavanone carbon composite electrodes predicted a pH of 8.04 for the commercial seawater, which is in excellent agreement with the measured pH 8.05 value.     

pH measurements in non-aqueous and aqueous–organic solvents – definition of standard procedures

pH standardisation procedures in non-aqueous and aqueous-organic solvents are discussed in the light of the newly prepared IUPAC recommendation on measurement of pH in dilute aqueous solutions. Both scientific and metrological aspects are considered, as required by the definitions of primary and secondary methods of measurements recently endorsed by BIPM (Bureau International de Poids et de Mesures, France).     

Potentiometric determination of silver(I), palladium(II) and gold(III) in mixtures by use of an iodide-selective electrode

Two procedures are proposed for the potentiometric determination of Ag(I), Pd(II) and Au(III) in binary mixtures, by titration with potassium iodide solution, and use of a commercial iodide electrode as sensor. In the first procedure, two aliquots of the mixture are titrated, at pH 2.0 ± 0.2 and 9.0 ± 0.2, adjusted with dilute sulphuric acid and ammonia solution. At pH 2.0, the titrant reacts with both metals, whereas at pH 9.0, Ag(I) is the only reactant. The second procedure utilizes titration of two aliquots of the mixture in the presence and absence of a selective masking agent. The methods have been applied to the determination of these metals in some jewellery alloys.     

Standardization of chemical shifts of TMS and solvent signals in NMR solvents

The standard for chemical shift is dilute tetramethylsilane (TMS) in CDCl3, but many measurements are made relative to TMS in other solvents, the proton resonance of the solvent peak or relative to the lock frequency. Here, the chemical shifts of TMS and the proton and deuterium chemical shifts of the solvent signals of several solvents are measured over a wide temperature range. This allows for the use of TMS or the solvent and lock signal as a secondary reference for other NMR signals, as compared with dilute TMS in CDCl3 at a chosen temperature; 25 degrees C is chosen here. An accuracy of 0.02 ppm is achievable for dilute solutions, provided that the interaction with the solvent is not very strong. The proton chemical shift of residual water is also reported where appropriate.     

Gold and Silver Micro‐Wire Electrodes for Trace Analysis of Metals

Micro‐wire electrodes were made from gold and silver wires (diameter: 25 μm; length: 3–21 mm) and sealed in a polyethylene holder; micro‐disk electrodes were made from the same wires and polished. The gold electrodes were electrochemically coated with mercury before use; the silver wires were used without coating. Comparative measurements demonstrated that the micro‐wire electrodes had much higher sensitivity, and a much (10–100×) lower limit of detection, than micro‐disk electrodes, and the sensitivity increased linearly with the area and length of the electrodes. Using a gold micro‐wire electrode of 21 mm and a deposition time of 300 s the limit of detection was 0.07 nM Pb in seawater of natural pH, compared to a limit of detection of 10 nM Pb (more than 100×greater) using a gold micro‐disk electrode of the same diameter. Using the silver micro‐wire electrode the limit of detection of lead was improved by a factor of 10 to 0.2 nM in acidified seawater. It is expected that the improved sensitivity of micro‐wire electrodes will lead to successful in situ detection of metals in natural waters.     

The response of the sulfide-selective electrode to sulfide,iodide and cyanide

The responses of solid-state sulfide-selective electrodes in the presence of sulfide, iodide and cyanide are described. Linear near-Nemstian responses were obtained down to ca. I × 10^-9 M solutions for these anions. Appropriate procedures of sample treatment limiting the interfering redox responses, adjusting pH and total ionic strength and preserving the ion of interest without contamination of the sample are described. These procedures improve the useful activity range as well as the reliability of the measurements.     

Applications of flow injection analysis in a power plant determination of pH,ammonia and hydrazine in an AVT-conditioned water steam cycle

The quality of information concerning management of systems and subsystems of industrial processes is greatly affected by the analytical techniques uses. Flow-injection methods are described for monitoring pH (in the range 4.5–95), ammonia (0.25–2 mg l^?1 N) and hydrazine (50?300 μg l^?1 N) in a water/steam cycle, based on the use of potentiometric sensors. The procedures are discussed in terms of both applicability to very dilute solutions (with simple management of the sytem) and measurability. The parameters considered were precision, analysis frequency and application range. The application range and analysis frequency were good for hydrazine but only the application range was good fro pH and ammonia. Data evaluation by means of a “measurability” model gave acceptable results for ammonia and hydrazine, but not for pH measurements. The frequency of analysis is the main factor affecting the quality of the results.     

A method for field measurements using alpha-spectrometry

Alpha-ray spectrometry in combination with sample preparation methods are commonly performed within a laboratory. These procedures are both technically demanding and time consuming. The current work describes what information alpha spectrometry will give if it is performed in the field with a limited amount of equipment. The aim was to find a mobile method usable in the field or mounted inside a vehicle. Experiments were made on four electrodeposited samples with different nuclide mixes, measured during normal and reduced air pressure conditions. A prototype of an instrument made for mobile use is also briefly described.     

Determination of Hydrogen Single Ion Activity Coefficients in Aqueous HCl Solutions at 25°C

The single ion activity coefficients of hydrogen and chloride ions in aqueous HCl solutions have been estimated at 25°C at concentrations up to 1 mol-kg^–1, using potentiometric measurements with ion-selective electrodes and appropriate calibration procedures. Two methods are described for an internal calibration of the electrodes in the extended Debye–Hückel concentration range. The results are compared to the conventional pH calibration with external buffer solutions. Since the latter calibration method does not account for the liquid junction potential E _J which arises at the reference electrode, the resulting activity coefficients are quite different in HCl solutions of higher concentration. These differences between internal and external calibration decrease significantly, when a correction for E _J is introduced into the conventional pH calibration. Hence, in solutions of higher ionic strength the accuracy of the conventional pH electrode calibration using buffer solutions is very limited, when exact H⁺ activities are required. The consistency of the results indicates that the liquid junction potentials in the examined systems calculated by the Henderson/Bates approximation are of reasonable precision.