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.     

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 (??E _j ) of the buffer solutions of MOBS have been estimated from previous measurements with a flowing junction cell. These values of ??E _j 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.     

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.     

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

The values of the thermodynamic second dissociation constant, pK ₂, and related thermodynamic quantities of N-(2-hydroxyethyl)piperazine-N′-2-hydroxypropanesulfonic acid (HEPPSO) have already been reported from 5 to 55?°C, including 37?°C, by the emf method. This paper reports the results for the pH of one chloride-free buffer solution containing the composition: (a) HEPPSO (0.08 mol?kg^?1)+NaHEPPSO (0.04 mol?kg^?1). The remaining seventeen buffer solutions contain a saline medium of ionic strength I=0.16 mol?kg^?1, matching closely that of physiological fluids. Conventional pH values, denoted as pa _H, for all eighteen buffer solutions from 5 to 55?°C have been calculated. The operational pH values, designated as pH, with residual liquid-junction corrections for five buffer solutions, one without NaCl, and four with buffer solutions in saline media of I=0.16 mol?kg^?1 are recommended as pH standards in the range of physiological application. These are based on the NBS/NIST standard scale for pH measurements.     

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.     

Standard potential of the (Ag+AgCl) electrode in (acetonitrile+water)

The standard potentials of $\text{AgCl +}\text{1}\text{2}\text{H}{}_2\text{= Ag+HCl}$ in water +10, +20, and +30 mass per cent of acetonitrile were obtained in a cell without liquid junction, using as reference electrode the standard hydrogen electrode. They were determined at eight temperatures ranging from 298.15 to 263.15 K at intervals of 5 K. Hydrochloric acid solutions from 0.01 to 0.10 mol·kg^?1 were used. The Debye-Hückel theory and the standard potentials were used to calculate the mean activity coefficients for HCl in the solvents. The results were used to obtain the standard thermodynamic functions for the acidic dissociation of HCl in the solvents. High values of HCl activity coefficients were obtained, suggesting that the acid is completely dissociated in the mixed solvents.     

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 = 0.16 mol · 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 (δE_j) 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) = 0.06 mol · kg⁻¹, NaTRICINE(m₂) = 0.02 mol · kg⁻¹, and NaCl(m₃) = 0.14 mol · kg⁻¹, the pH values at T = 310.15 K 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.     

Surface Changes at Platinized Platinum Based Hydrogen Gas Electrodes Following Use in Highly Saline Aqueous Solutions

Platinized platinum based hydrogen gas electrodes, Pt(Pt)|H₂(g)|H⁺(aq), and silver‐silver chloride electrodes, Ag|AgCl|Cl^? (aq), make up the Harned cell, without transfer, working in the potentiometric mode at Cl^? concentrations and ionic strengths, I, below 0.1 mol kg^?1, for assigning primary pH values to reference pH buffer solutions. This work reports on experiments performed at higher I and Cl^? solutions up to 0.7 mol kg^?1, aiming at addressing seawater conditions with results of equally high quality. In the course of measurements, the occasional occurrence of highly unstable potentials denoted electrode malfunction; Pt metal surfaces observed by SEM/EDS and XRD exhibit strong Ag and Cl peaks corresponding to the presence of AgCl crystals deposited at both surfaces.     

Glucose Oxidase Electrode Based on Graphite and Methylthio Tetrathiafulvalene as Mediator

Abstract

A glucose sensor based on glucose oxidase and a new mediator - 4,5-dimethyl-4′-methylthio-Δ 2,2′-bi-1,3-dithiole (MTTTF) is described. The background for sensor action is the effective MTTTF cation interaction (apparent bimolecular constant (2.0+/-0.5)?10⁶ M^?1 s^?1 at 25°C and pH 7.0) with reduced glucose oxidase and the high electrochemical rate of mediator transformation.

A glucose sensor was prepared by adsorbing mediator (MTTTF) and glucose oxidase on graphite rods. The sensor responds to glucose at electrode potentials higher than 50 mV vs SCE, but the maximal activity is obtained at a potential of 250 mV. In air saturated solution the electrode shows a non-linear calibration curve with a half-saturation concentration 10.4 mM and Hill coefficient 2.08 at 250 mV. Sensor response changes little at pH 6.5–8.0. The energy of activation of the sensor response calculated from the Arrhenius equation was 64.5 kJ/mol, and the temperature coefficient at 25°C was 9.2%.     

Electrochemical investigation of a glassy carbon electrode modified with carbon nanotubes decorated with (poly)crystalline gold

Multiwalled carbon nanotubes with nanosized sputtered gold were used to modify a glassy carbon electrode (GCE). The substrate was characterized by scanning electron microscopy (SEM), X-ray diffraction, cyclic voltammetry and amperometry. SEM micrographs indicated an uniform coverage of the carbon nanotubes with nanosized (poly)crystalline gold. Cyclic voltammetry reveals that peak separation of the unmodified GCE in the presence of 1?mM ferricyanide is 131?mV, but 60?mV only for the modified GCE. In addition, the oxidation of NADH (1?mmol?L^?1 solution) begins at negative potentials (around ?100?mV vs. Ag/AgCl), and the anodic peak potential (corresponding to the irreversible oxidation of NADH) is found at +94?mV. The effect of pH on the electrocatalytic activity was studied in the range from 5.4 to 8.0. The relationship between the anodic peak potential and the pH indicated a variation of ?33.5?mV/pH which is in agreement with a two-electron and one-proton reaction mechanism. Amperometry, performed at either ?50 or +50?mV vs. an Ag/AgCl reference electrode, indicates that the modified electrode is a viable amperometric sensor for NADH. At a working potential of +50?mV, the response to NADH is linear in the concentration range from 1 to 100???mol?L^?1, with an RSD of 6% (n?=?4).

Study on the capacities,heats of dilution and properties of concentrated potassium carnallite solutions at 298.15 K

The heats of dilution of the infinitesimally dilute potassium carnallite solutions at 298.15 K have been studied by continuous titration from 1.8942 to 0.01044 mol·kg^?1, and an equation for the curve of heats of dilution has been fitted. It was shown that the enthalpy of dilution for the same concentration of the carnallite solution is equal to the sum of those of KCl and MgCl₂ solutions. The equation for the curve of enthalpy of dilution corresponds to that of natural carnallite.     

Re-evaluation of Activity Coefficients in Dilute Aqueous Hydrobromic and Hydriodic Acid Solutions at Temperatures from 0 to 60 °C

Simple two-parameter Hückel equations can be used for the calculation of the activity coefficients in aqueous hydrobromic and hydriodic acid solutions at temperatures from 0 to 60 °C and from 0 to 50 °C, respectively, at least up to a molality of 0.5 mol·kg^?1. The data measured by Macaskill and Bates (J. Solution Chem. 12:607–619, 1983) at 25 °C and those measured by Hetzer et al. (J. Phys. Chem. 68:1929–1933, 1964) at various temperatures on galvanic cells without a liquid junction were used in the parameter estimations for the hydrogen bromide (HBr) and hydrogen iodide (HI) solutions, respectively. The latter data consist of sets from 0 to 50 °C at intervals of 5 °C. The parameter values for HBr solutions were also tested using the numerous galvanic cell points from the other three data sets existing in the literature for hydrobromic acid solutions and covering wide range of temperatures from 0 to 60 °C. It was observed in the parameter estimations and tests that all of the estimated parameters are independent of the temperature. The recommended parameter values were additionally tested using the isopiestic data of Macaskill and Bates (see the citation above) and those of Harned and Robinson (Trans. Faraday Soc. 37:302–307, 1941) for dilute HBr and HI solutions at 25 °C, respectively. In more concentrated solutions up to a HBr molality of 4.5 mol·kg^?1 and up to a HI molality of 3.0 mol·kg^?1, an extended Hückel equation was used, which contains an additional quadratic term with respect to the molality. The parameters for the extended Hückel equations were determined from these isopiestic data and tested using these data and the existing galvanic cell data. The activity and osmotic coefficients calculated from the resulting equations are recommended in the present study for the more concentrated solutions. The recommended values are compared to the activity values reported in several previous tabulations.     

Amineptine Plastic Membrane Electrode Based on its Ion Associate with Tetraphenylborate and Phosphomolybdic Acid

New amineptine hydrochloride (Am-Cl) ion-selective electrodes (conventional type) based on amineptinium-tetraphenylborate (I) and amineptinium-phosphomolybdate (II) were prepared. The electrodes exhibited mean slopes of calibration graphs of 57.9?mV and 53.8?mV per decade of (Am-Cl) concentration at 25?°C for electrodes (I) and (II), respectively. The electrodes can be used within the concentration range 3.16×10^?5?10^?2?M (Am-Cl) at a pH range of 2.0–3.9 for both electrodes. The standard electrode potentials were determined at different temperatures and used to calculate the isothermal coefficients of the electrodes, which were 0.00172?V?°C^?1 and 0.00091 V?°C^?1 for (I) and (II) electrodes, respectively. The electrodes showed a very good selectivity for (Am-Cl) with respect to a number of inorganic cations and sugars. The standard addition method is successfully applied to determine (Am-Cl) in pure solutions and in amineptine-containing tablets.     

Physical–chemical study of water in contact with a hydrophilic polymer: Nafion

The study detailed in this paper is about the determination of the physical–chemical parameters of water, after keeping it in prolonged contact with the Nafion polymer. The parameters under study are: electrical conductivity, χ (μS cm^?1); heat of mixing with acid (HCl), ΔQ _mix ^HCl (J kg^?1) or basic (NaOH) solutions, ΔQ _mix ^NaOH  (J kg^?1), and pH. χ increases of up to two orders of magnitude, ΔQ _mix ^NaOH  (J kg^?1) is exothermic and increases as the electrical conductivity increases, with a roughly linear trend, up to one order of magnitude. The analogous ΔQ _mix ^HCl  (J kg^?1), on the contrary, is found to be null. The pH is quite acid and shows a very good linear correlation with log χ. The linear correlations hint at a single cause for the variation of the three very different physical–chemical parameters. This complex and hard to rationalize phenomenology, finds a good theoretical support in the work hypothesis of the formation of dissipative structures within the liquid. These are far-from-equilibrium systems outside the paradigm of classical thermodynamics. The work hypothesis of the formation of molecular aggregates of water molecules (dissipative structures, aqueous nanostructures, clusters, coherence domains, etc.) is shared with two other aqueous systems obtained with different preparation protocols, so we briefly recall them here: (1) EDS (extremely diluted solutions): obtained through an iterative process of successive dilutions and agitations. (2) IFW (iteratively filtered water): obtained through an iterative process of successive filtrations through sintered glass filters. (3) INW (iteratively nafionized water): obtained through an iterative process of successive drying and wetting of the Nafion polymer. Each protocol produces water exhibiting its own peculiarities, to the point that they can be considered different, albeit with the common element of a variation of the super-molecular structure of the water solvent. The physical–chemical properties of these perturbed waters cannot be framed by the paradigm of classical thermodynamics, but rather require the use of the thermodynamics of systems far from the equilibrium and of irreversible processes.     

Determination of sulfonamide residues in cultured sea cucumber by pre-column derivatization capillary electrophoresis with fluorescence detection

A simple and mild method for the separation of sulfonamide residues based on a condensation reaction with O-phthalaldehyde solution (OPA) as labeling reagent with capillary electrophoresis has been developed. A 58.5 cm × 50 μm i.d. (50 cm effective length) untreated fused-silica capillary was used. To optimize the separation conditions, the background electrolyte concentration, pH, column temperature, voltage and other factors were evaluated. The optimal separation conditions were as follows: 20 mmol L^?1 borate buffer; pH 9.1; column temperature 20 °C; separation voltage 18 kV, pressure 50 mbar and injection time 8 s. Under the optimal conditions, 10 kinds of sulfonamide derivatives could be well-separated within 8 min, and the linear ranges were 0.35–100 μg kg^?1. The detection limit (at a signal-to-noise ratio of 3) was in the range of 0.12–0.25 μg kg^?1, and the quantification limit (at a signal-to-noise ratio of 10) was in the range of 0.35–0.70 μg kg^?1. The sulfonamide residues from cultured sea cucumber samples were determined under the optimal conditions with satisfactory results.     

Study of a Novel Bisnaphthalimidopropyl Polyamine as Electroactive Material for Perchlorate‐selective Potentiometric Sensors

In this work, the new polyamine bisnaphthalimidopropyl‐4,4’‐diaminodiphenylmethane is proposed as a new ionophore for perchlorate potentiometric sensors. The optimal formulation for the membrane comprised of 12 mmol kg^?1 of the ionophore, and 68 % (w/w) of 2‐nitrophenyl phenyl ether as plasticizer and 31 % (w/w) of high molecular weight PVC. The sensors were soaked in water for a week to allow leakage of anionic impurities and for one day in a perchlorate solution (10^?4 mol L^?1) to improve reproducibility due to its first usage. The stability constant for the ionophore‐perchlorate association in the membrane, log β_IL1=3.18±0.04, ensured a performance characterized by the slope of 54.1 (±0.7) mV dec^?1 to perchlorate solutions with concentrations between 1.24×10^?7 and 1.00×10^?3 mol L^?1. The sensors are insensitive to pH between 3.5 to 11.0, they have a practical detection limit of 7.66 (±0.42) ×10^?8 mol L^?1 and a response time below 60 s for solutions with perchlorate concentrations above 5×10^?6 mol L^?1. The accuracy of the results was confirmed by the analysis of the contaminant in a certified reference water sample.     

A Selective and Sensitive Method for Simultaneous Determination of Traces of Paracetamol and p‐Aminophenol in Pharmaceuticals Using Carbon Ionic Liquid Electrode

A reliable and simple electrochemical method has been proposed for the simultaneous determination of paracetamol (PAR) and p‐aminophenol (PAP) in pharmaceutical formulations. The oxidation and reduction peak potentials in cyclic voltammetry (CV) for PAR on carbon ionic liquid electrode (CILE) were occurred at 370 and 225 mV vs. Ag/AgCl, respectively at pH 7.0, while those for PAP on CILE appeared at 128 mV and 68 mV, respectively at the scan rate of 0.05 V s^?1. In comparison to the conventional carbon paste electrode, the apparent reversibility and kinetics of the electrochemical reactions of PAR and PAP were significantly improved on CILE. In differential pulse voltammetric technique, the peak potentials for PAR and PAP appeared at 345 and 130 mV, respectively, with the peak separation of 215 mV, sufficient for their simultaneous determination in samples containing these two species. The proposed method was used for simultaneous determination of PAR and PAP in tablets. PAR and PAP can be determined in the ranges of 2.0×10^?6–2.2×10^?3 M and 3.0×10^?7–1.0×10^?3 M, with the detection limits of 5.0×10^?7 and 1.0×10^?7 M (calculated by 3σ), respectively. The relative standard deviations for the determination of PAR and PAP were less than 2%.     

Ultra‐sensitive Voltammetric Aluminum Determination on a Solid Bismuth Electrode for Water Filter Verification

This work presents a new ultra‐sensitive method of Al(III) as Al‐cupferron complex voltammetric determination, in 0.04 M ammonium sulfate of pH=6.1 and 0.003 M cupferron by environmentally friendly and durable Bismuth Bulk Annular Band Electrode (BiABE). The optimal measuring parameters include: potential window from ?700 to ?1250 mV versus Ag|AgCl, preconcentration time and potential of 120 s, ?700 mV, respectively. The electrode surface can be activated by fast in situ method: applying short conditioning by the potential of ?1300 mV. The best obtained analytical parameters are: range of linearity 0.2–1.2 μg L^?1 with high sensitivity of 1.333 μA/μg L^?1, limit of detection 0.04 μg L^?1 and repeatability below 2.3 %. The described procedure was verified using various CRMs, i. e. surface waters, waste waters and tea leaves. Satisfactory recovery values were obtained in the interval 99.2–103.5 %. Developed DPV procedure was used to determine Al in tap and natural waters in aspect of recovery of aluminum, which was added to the samples. For the first time, we tested also Al removal efficiency from water by the water filters mounted in the trip bottle.     

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

The values of the second dissociation constant, pK ₂, and related thermodynamic quantities of 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) have already been reported over the temperature range 5 to 55 °C including 37 °C. This paper reports the pH values of four NaCl-free buffer solutions and four buffer composition containing NaCl salt at I=0.16 mol⋅kg⁻¹. Conventional pa _H values are reported for all eight buffer solutions. The operational pH values have been calculated for four buffer solutions recommended as pH standards, at 25 and 37 °C after correcting the liquid junction potentials with the flowing junction cell.     

Re-evaluation of the Activity Coefficients of Aqueous Hydrochloric Acid Solutions up to a Molality of 16.0 mol·kg<Superscript>−1</Superscript> Using the Hückel and Pitzer Equations at Temperatures from 0 to 50 °C

The simple three-parameter Pitzer and extended Hückel equations were used for calculation of activity coefficients of aqueous hydrochloric acid at various temperatures from 0 to 50 °C up to a molality of 5.0 mol·kg^?1. A more complex Hückel equation was also used at these temperatures up to a HCl molality of 16 mol·kg^?1. The literature data measured by Harned and Ehlers J. Am. Chem. Soc. 54, 1350–1357 (1932) and 55, 2179–2193 (1933) and by Åkerlöf and Teare [J. Am. Chem. Soc. 59, 1855–1868 (1937)] on galvanic cells without a liquid junction were used in the parameter estimations for these equations. The latter data consist of sets of measurements in the temperature range 0 to 50 °C at intervals of 10 °C, and data at these temperatures were used in all of these estimations. It was observed that the estimated parameters follow very simple equations with respect to temperature. They are either constant or depend linearly on the temperature. The values for the activity coefficient parameters calculated by using these simple equations are recommended here. The suggested new parameter values were tested with all reliable cell potential and vapor pressure data available in literature for concentrated HCl solutions. New Harned cell data at 5, 15, 25, 35, and 45 °C up to a molality of 6.5 mol·kg^?1 are reported and were also used in the tests. The activity coefficients obtained from the new equations were compared to those calculated by using the Pitzer equations of Holmes et al. [J. Chem. Thermodyn. 19, 863–890 (1987)] and of Saluja et al. [Can. J. Chem. 64, 1328–1335 (1986)] at various temperatures, and by using the extended Hückel equation of Hamer and Wu [J. Phys. Chem. Ref. Data 1, 1047–1099 (1972)] at 25 °C.