Residual Liquid Junction Potential Research Articles

Toward Unified pH of Saline Solutions

Fluctuations of pH in coastal systems are generally surveyed through potentiometric pH measurements. A new concept of a unified pH scale was introduced with the great advantage of enabling comparability of absolute values, pHabs, pertaining to any medium. Using water as an anchor solvent, yielding pHabsH2O, enables referencing the pHabs values to the conventional aqueous pH scale. The current work aims at contributing to implement pHabsH2O to saline solutions. To this purpose, differential potentiometric measurements, with a salt bridge of ionic liquid [N2225][NTf2], were carried out aiming at overcoming problems related to residual liquid junction potentials that affect the quality of such measurements. The ability to measure pHabsH2O with acceptable uncertainty was evaluated using Tris-Tris·HCl standard buffer solutions prepared in a background matrix close to the characteristics of estuarine systems (salinity of 20) as well as with NaCl solutions with ionic strength between 0.005 and 0.8 mol kg−1. The present study shows that for high ionic strength solutions, such as seawater, challenges remain when addressing the assessment and quantification of ocean acidification in relation to climate change. Improvements are envisaged from the eventual selection of a more adequate ionic liquid.

Standards for pH Measurements of Bis-[(2-Hydroxyethyl)amino]acetic Acid (BICINE) from (278.15 to 328.15) K

The second dissociation constant, pK2, and related thermodynamic data for BICINE have been previously de- termined and reported from temperatures (278.15 to 328.15) K. For the present study, three buffer solutions without NaCl, and five with NaCl yielding an ionic strength (I = 0.16 mol·kg -1 ) similar to that of blood plasma were prepared. These buffer solutions have been evaluated in the temperature range of (278.15 to 328.15) K using the extended form of the De- bye-Huckel equation. The Bates-Guggenheim convention is only valid when I< 0.1 mol·kg -1 . Values of the residual liquid junction potential (δEj) between the BICINE solutions and the saturated KCl calomel electrode have been estimated at (298.15 and 310.15) K. Two BICINE buffer solutions are recommended as primary standard reference solutions for pH measurements of biological fluids.

Buffers for the Physiological pH Range: Studies of 4-(N-Morpholino)butanesulfonic Acid (MOBS) from 5 to 55 °C

In this study, we report the pH values of two buffer solutions without chloride ion and eight buffer solutions with NaCl with an ionic strength I=0.16 mol⋅kg−1. Electromotive force (emf) techniques have been used to get the cell potentials at 12 temperatures from 5 to 55 °C, including 37 °C. An extended form of the Bates-Guggenheim convention is used in the entire ionic strength range, 0.04 to 0.16 mol⋅kg−1. The residual liquid junction potentials (δEj) of the buffer solutions of MOBS have been estimated from previous measurements with a flowing junction cell. These values of δEj have been used for correction in order to ascertain the operational pH values of four buffer solutions of MOBS at 25 and 37 °C. These solutions are recommended as pH standards for physiological application in the pH range 7.4 to 7.7.

Buffer standards for the physiological pH of N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine (TRICINE) from T = (278.15 to 328.15) K

The pH values of two buffer solutions without NaCl and seven buffer solutions with added NaCl, having ionic strengths (I=0.16mol·kg−1) similar to those of physiological fluids, have been evaluated at 12 temperatures from T=(278.15 to 328.15)K by way of the extended form of the Debye–Hückel equation of the Bates–Guggenheim convention. The residual liquid junction potentials (δEj) between the buffer solutions of TRICINE and saturated KCl solution of the calomel electrode at T=(298.15 and 310.15)K have been estimated by measurement with a flowing junction cell. For the buffer solutions with the molality of TRICINE(m1)=0.06mol·kg−1, NaTRICINE(m2)=0.02 mol·kg−1, and NaCl(m3)=0.14mol·kg−1, the pH values at T=310.15K obtained from the extended Debye–Hückel equation and the inclusion of the liquid junction correction are 7.342 and 7.342, respectively. These are in excellent agreement. The zwitterionic buffer TRICINE is recommended as a secondary pH standard in the region for clinical application.

Buffer Standards for Physiological pH of the Buffer N-(2-Acetamido)-2-aminoethanesulfonic Acid from 5&amp;#176;C to 55&amp;#176;C

Electromotive force (emf) measurements of the Cell Pt(s), H2(g)|ACES(m1) + NaACES(m2) + NaCl (m3)| AgCl, Ag(s) have been carried out from 5°C to 55°C . The agreement of pH values between two calculated (extended Debye-Huckel and liquid junction correction) is very good. Two buffer solutions without the chloride ion and seven buffer solutions with NaCl, at an ionic strength (I = 0.16 mol.kg-1) similar to that of physiological fluids, have been studied. The pH values for these buffer solutions have been evaluated in the temperature range of 5°C to 55°C using the extended Debye-Huckel equation of the Bates-Guggenheim convention. Values of the residual liquid junction potential (δEj) between the ACES solutions and the saturated KCl calomel electrode solution have been estimated at 25°C and 37°C from the previously determined Ej values using the flowing junction cell to determine the operational pH values at 25°C and 37°C. These ACES buffer solutions are recommended as secondary standard reference solutions for pH measurements in the range of physiological applicati on at I = 0.16 mol.kg-1.

Evaluation of the residual liquid junction potential contribution to the uncertainty in pH measurement: A case study on low ionic strength natural waters

The residual liquid junction potential (RLJP) needs to be accounted for in pH uncertainty estimation. Attempts to do this and the currently available methods for evaluating the RLJP are critically discussed and their weak sides are pointed out. In this work an empirical approach to the problem is proposed. It is based on the use of the RLJP bias estimated on a variety of measurement conditions for a specific class of analytical objects essentially differing in ionic strength from the pH calibration buffers. The data from five independent studies, including interlaboratory comparisons, on pH measurement in low ionic strength waters were used to find the overall bias observed in the 10 −4 mol dm −3 strong acid solution. The procedure includes quantifying the uncertainty of bias values from separate studies by combination of the relevant uncertainty components and testing the consistency of the data. The weighted mean bias in pH was found to be 0.043 ± 0.007 ( k = 2). With this estimate, the pH measurement uncertainties calculated according to the previously suggested procedure (I. Leito, L. Strauss, E. Koort, V. Pihl, Accredit. Qual. Assur. 7 (2002) 242–249.) can be enlarged to take the uncorrected bias into account. The resulting uncertainties on the level of 0.10–0.14 ( k = 2) are obtained in this way for pH measurement in water and poorly buffered aqueous solutions in the range of pH 7.5–3.5.

Buffer Standards for the Physiological pH of the Zwitterionic Compound, ACES from 5 to 55°C.

The values of the second dissociation constant pK2 and related thermodynamic quantities of [N-(2-acetamido)-2-aminoethanesulfonic acid] (ACES) have already been reported over the temperature range 5 to 55°C including 37°C. This paper reports the paH values of four chloride ion free buffer solutions and eight buffer solutions with I = 0.16 mol·kg -1, matching closely to that of the physiological sample. Conventional paH values for all twelve buffer solutions from 5 to 55°C, are reported. The residual liquid junction potential correction for two widely used temperatures, 25 and 37°C, has been made. The flowing-junction calomel cell method has been utilized to measure Ej , the liquid junction potential. The operational pH values for four buffer solutions at 25 and 37°C are calculated using the physiological phosphate buffer standard based on NBS/NIST convention. These solutions are recommended as pH standards in the pH range of 6.8 to 7.2 for physiological fluids.

Acidity constants in different media (I=0 and I=0.1 M KCl) from the uncertainty perspective

Procedures for estimating the measurement uncertainty for the acidity constant Ka (or the pKa value) in different media (I=0 and I=0.1 mol L(-1) KCl), as determined by potentiometric titration, are presented. The uncertainty budgets (the relative contributions of the different input quantities to the uncertainty in the result) of the pKa (I=0) and pKa (I=0.1 mol L(-1) KCl) values are compared. Unlike the values themselves, the uncertainties and uncertainty budgets of the values are comparable. The uncertainty estimation procedures are based on mathematical models of pKa measurement and involve the identification and quantification of individual uncertainty sources according to the ISO GUM approach. The mathematical model involves 52 and 48 input parameters for pKa (I=0) and pKa (I=0.1 mol L(-1) KCl), respectively. The relative importance of each source of uncertainty is discussed. In both cases, the main contributors to the uncertainty budget are the uncertainty components due to the hydrogen ion concentration/activity measurement, which provide 63.7% (for pKa (I=0)) and 89.3% (for pKa (I=0.1 mol L(-1) KCl)) of the uncertainty. The remaining uncertainty contributions arise mostly from the limited purity of the acid. From this work, it is clear that the uncertainties of the pKa (I=0.1 mol L(-1) KCl) values tend to be lower than those of the pKa (I=0) values. The main reasons for this are that: (1) the uncertainty due to the residual liquid junction potential is nominally absent in the case of pKa (I=0.1 mol L(-1) KCl) due to the similarly high concentrations of background electrolyte in the calibration solutions and measured solution; (2) the electrode system is more stable in solutions containing the 0.1 mol L(-1) KCl background electrolyte and so the readings obtained in these solutions are more stable.

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.

High-precision pH measurement by means of electrochemical cells with transference

pH Measurements were carried out by means of cells containing Pt/hydrogen and glass electrodes, both equipped with the same type of ground glass sleeve junction carefully selected with respect to the leak rate. The cells were calibrated with the same set of standard buffers using multiple point calibration with linear regression. The slopes of the regression lines were compared with the theoretical (Nernst) slope of the E versus pH function and yielded the electromotive efficiency of the two cells. The accuracy of the measurements was obtained from the residual liquid junction potential errors.

Ionic binding, net charge, and Donnan effect of human serum albumin as a function of pH

The ionic activities and total molalities of sodium, potassium, calcium, lithium, and chloride in a solution of human serum albumin were measured at different values of pH between 4 and 9. The same quantities were measured simultaneously in a protein-free electrolyte solution in membrane equilibrium with the albumin solution. Taking the residual liquid-junction potential and bias from unselectivity of the electrodes into account, we determined the own, bound, and net charges of albumin. Chloride was amply bound at low pH, and calcium at high pH. The varying charge of ions bound to albumin opposed the effect of acid or base on the net charge. All ions were distributed across the membrane according to the same electric potential difference, which equalled the Donnan potential. The high concordance between observation and theory favors the Donnan theory and furthermore implies that the electrodes are as accurate in a solution with albumin as in a protein-free solution.

Calibration solutions for the simultaneous potentiometric measurement of sodium, potassium and calcium in blood plasma: examination of the electrochemical factors affecting precision and accuracy in direct potentiometric clinical analysers

A series of well defined calibration solutions is proposed to eliminate some of the discrepancies in the measurement of sodium, potassium and calcium in blood plasma by ion-selective electrode potentiometry. The calibration matrix has been chosen to resemble the electrolyte composition of blood plasma in terms of ionic composition and ionic strength. This reduces liquid-junction and activity coefficient bias. A flow-through test rig was constructed from sodium, potassium and calcium ion-selective electrode modules from commercial instruments and a single ceramic plug KCl reference electrode. These modules were calibrated with the proposed solutions and used in blood plasma. Preliminary experiments were performed to evaluate residual liquid-junction potentials and the effects of heparin and carbon dioxide.

Sodium activity measurements in protein solutions with sodium amalgam electrodes

A method using sodium amalgam electrodes has been developed to study the effect of protein on activity measurements and residual liquid junction potential of sodium in protein-containing solutions. It was tested for reliability by application to aqueous sodium chloride solutions before it was applied to protein-containing solutions. The experimentally determined activity coefficients are in good agreement with the activity coefficients for aqueous sodium chloride solutions reported in literature. Therefore, we used this method to measure the activity of NaCl solutions containing varying concentrations of bovine serum albumin (BSA). The electrode slope of the electrode responses in a protein-containing medium does not differ from that of pure aqueous solutions. A functional relationship has been established between the concentration of protein and the percent bias in the measurements. Measurements of electromotive force made in protein-containing solutions do not show any evidence of protein effect on liquid junction potential.

Conventional residual liquid junction potentials in dilute solutions

Conventional residual liquid junction potentials were measured between NBS 1:1 phosphate buffer and various dilute solutions with ionic strengths of 1-100 mmol kg −1, using 3.5 mol kg −1 potassium chloride as the bridge electrolyte. All junctions were of the free-diffusion type, formed within a 1-mm capillary. The conventional residual junction potential was indistinguishable from zero (± 0.5 mV), for dilute solutions of primary reference standards, for which calculations based on the Henderson equation predict values of 0.8–1.3 mV. Solutions of Tris-HCl (20–100 mmol kg −1) and dilute HCl (1–50 mmol kg −1) had appreciable conventional residual liquid junction potentials (1.1–3.3 mV). The experimental values for dilute HCl solutions compare favourably with those calculated with the Henderson equation. These results suggest that the pH of hard waters, measured using the NBS pH scale, will be a good approximation (within 0.01 pH) to p a H, so that pH can be interpreted in terms of the activity of the hydrogen ions. Further measurements are required to ascertain whether this approximation is true for soft and acidic waters.

An isotonic potassium chloride liquid junction minimises the effects of ionic strength, protein and haematocrit on ionised calcium measurement.

When the reference electrode liquid junction of a Nova 2 analyser was changed to isotonic potassium chloride, increasing the ionic strength of aqueous solutions containing a constant total calcium concentration had a negligible effect on measured ionised calcium. In contrast, measurements using hypertonic potassium chloride, hypertonic sodium formate and isotonic sodium chloride liquid junctions showed significant sample ionic strength effects. Interferences by sample protein concentration and haematocrit were marked with hypertonic, but negligible with isotonic junctions. Ionised calcium values in samples containing 25 mmol/L acetate, bicarbonate, beta-hydroxybutyrate, lactate or pyruvate were all lower by 6-7% with an isotonic than a hypertonic potassium chloride junction. Thus, anions that replace bicarbonate during metabolic acidosis have a similar effect on residual liquid junction potential. The clinical usefulness of an isotonic potassium chloride liquid junction needs to be evaluated.

The pH measurements in seawater on the NBS scale

A new standardization method for glass electrodes is proposed for pH measurements on the NBS scale. An expression is obtained to evaluate the apparent activity coefficient of hydrogen ions as a function of salinity and temperature, from determinations obtained by Mehrbach et al., 1973 (Limnol. Oceanogr., 18: 897–907). This expression must be used to estimate systematic errors produced by variations of residual liquid-junction potential by estimating the apparent activity coefficient of hydrogen ions. In this way, values of pH obtained in oceanographical cruises can be standardized in order to determine and compare different parameters of the CO 2 system. Some experiments to study the variation of pH with temperature are carried out to compare the different series of acid constants used at present. A simple and precise polynomial equation is also obtained to determine the variation of pH with temperature.

Theory and practice in the electrometric determination of pH in precipitation

Basic theory and laboratory investigations have been applied to the electrometric determination of pH in precipitation samples in an effort to improve the reliability of the results obtained from these low ionic strength samples. The theoretical problems inherent in the measurement of pH in rain have been examined using natural precipitation samples with varying ionic strengths and pH values. The importance of electrode design and construction has been stressed. The proper choice of electrode can minimize or eliminate problems arising from residual liquid junction potentials, streaming potentials and temperature differences. Reliable pH measurements can be made in precipitation samples using commercially available calibration buffers providing low ionic strength quality control solutions are routinely used to verify electrode and meter performance.

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.

The Achilles' Heel of Potentiometric Measurements, the Liquid Junction Potential

The mechanical design of the liquid junction and materials used for the construction of the potentiometric measuring cell are discussed. Single ion conductances at 37 °C for common ions in calibration solutions are presented and a calibrating solution without residual liquid junction potential when compared with blood plasma is proposed. A new salt bridge solution containing sodium formate, which eliminates the suspension effect during whole blood measurements, is also presented, as are results from a comparison on whole blood between the new salt bridge solution and the traditional KCI salt bridge solution.

An Interlaboratory Test of pH Measurements in Rainwater.

An interlaboratory test of pH measurements in rainwater has been conducted. Various types of electrodes and junction materials were used in this test. The results of this exercise verify that there are significant differences in the pH values of low ionic strength solutions reported by various laboratories. Other work suggests that these differences are due to residual liquid junction potentials. Furthermore, this test confirms the efficacy of using dilute solutions of a strong acid as working standards for pH measurements in acid deposition studies.