Recent development of non-faradaic potentiometry

Non-faradaic potentiometry has been plagued by a great many fundamental errors and a lack of conceptualization. Of greatest concern is the second Nernst equation hiatus. Potentiometry may be generally classified as faradaic and non-faradaic. The former deals with the redox reactions using the Nernst equation to explain the potential origin. The latter deals with the non-redox reactions using the Boltzmann and modified Boltzmann equations to explain the origin of electrode potential. Redox faradaic potentiometry has been well described in the textbooks. However, non-faradaic potentiometry has been almost completely neglected in the literature. Many well-known electrodes, such as the pH glass electrode, common reference electrodes, and ion selective electrodes (ISE) have been mistakenly interpreted as redox reactions or ion exchange reactions. New theories and experimental results show their mechanisms to be non-faradaic in nature. Furthermore, the reaction mechanisms for ISE have been confused in textbooks with redox reactions and the Nernst equation. The ISE potentials originating from adsorption of ions or charged particles based on surface charge density will be explained using the double and counterion triple layers concept. The new counterion triple layer concept may be applied to the potential development of sensors. The reason for a new concept, theory, or mechanism is to better explain the phenomena. Examples will be given of how our new concept explains the capacitor, counterion triple layer, surface adsorbed layers interactions, and the interface structure. We will also discuss the new sensor development based on the new adsorption concept. For the first time a new type of Ag/AgCl reference electrode for non-faradaic potentiometry will be presented, one without a liquid junction and with a Pt wire instead of a salt bridge. They will help open up a new horizon for electrochemical sensor research and may be used under unusual conditions, such as high temperature and high pressure, stirring, etc.     

Acid dissociation of hexaamminemetal nitrates in liquid ammonia

The proton dissociation of hexaamminemetal nitrates of Cr(III), Co(II), Co(III), Ni(II), and Cu(II) in liquid ammonia has been investigated at a temperature of –39°C. This could be realized by using a specially constructed thin-walled glass electrode combined with a Ag/Ag(I) (in liquid ammonia) reference electrode. The potential of the couple used has been found to obey the Nernst equation in liquid ammonia. The pK _a values of the hexaamminemetal nitrates in liquid ammonia are 7.27 [Cr(III)], 6.99 [Co(II)], 7.78 [Co(III)], 8.95 [Ni(II)], and 8.24 [Cu(II)]. The temperature dependence of the pK _avalues falls within the experimental error.     

Determination of the number of electrons by chronoamperometry at small electrodes

The number of electrons transferred, n, was determined by taking the ratio of the square of the slope to the intercept of Cottrell plots by chronoamperometry at a small disk electrode without knowing values of diffusion coefficients. The intercept is ascribed to the contribution of spherical diffusion. This technique is useful for estimating reaction mechanisms of unstable species of which lifetime is shorter than the period of bulk electrolysis. It takes advantage not only of getting the simultaneous acquisition of two independent variables (the slope and the intercept) in one chronoamperometric curve but also of evaluating the dependence of n for multi-step charge transfer reactions on electrode potentials. Ferrocene and N,N′-bis(4-methoxyphenyl)-N,N′-diphenyl-1,4-phenylenediamine were used for redox species at n = 1 and 2, respectively. Chronoamperometric currents were obtained at the platinum disk electrode 50 μm in radius for the time less than 3 s, responding to the various stepped potential values. Values of n were obtained at each potential even for sluggish charge transfer reactions.     

Response mechanism of platinum electrode to uncoupled ions(Ⅰ)——Response of platinum electrode to Pb~(2+),Cd~(2+),Ca~(2+) and Mg~(2+)

The transient response mechanism of the platinum electrode to the uncoupled ions may be interpreted with the mixed phase formation (MPF) model of the transient response of precipitate-based ion-selective electrodes to interfering tons for Kxy<<1 It is discovered that the peak height of the transient signal is related to the solubility of M(OH)2 and hydration heat of M2+ The relation between the positive peak height of transient signal of Pb2+ or Cd2+ and lgaM obey tne Nernst equation,while that of Ca2+ or Mg2+ does not.The equilibrium potential is not of Nernst response for all ions.     

Sensing mechanism of non-equilibrium solid-electrolyte-based chemical sensors

Solid electrolytes can be used in several different types of chemical sensors. A common approach is to use the equilibrium potential generated across a solid electrolyte given by the Nernst equation as the sensing signal. However, in some cases, stable electrode materials are not available to establish equilibrium potentials, so non-equilibrium approaches are necessary. The sensing signal generated by such sensors is often described by the mixed potential theory, in which a pair of electrochemical reactions establishes a steady state at the electrode, such that the electrons produced by an oxidation reaction are consumed by a reduction reaction. The rates of both reactions depend on several factors, such as electron exchange, active area, and gas phase diffusion, so establishment of the steady-state potential is complex and alternative explanations have been proposed. This paper will review and discuss the mechanisms proposed to explain the sensor response of non-equilibrium-based electrochemical sensors.     

Response mechanism of platinum electrode to uncoupled ions(I)

The transient response mechanism of the platinum electrode to the uncoupled ions may be interpreted with the mixed phase formation (MPF) model of the transient response of precipitate-based ion-selective electrodes to interfering ions for K_xy ≪ 1. It is discovered that the peak height of the transient signal is related to the solubility of M(OH)₂ and hydration heat of M²⁺. The relation between the positive peak height of transient signal of pb²⁺ or cd²⁺ and lga_m obey the Nernst equation, while that of Ca²⁺ or Mg²⁺ does not. The equilibrium potential is not of Nernst response for all ions. Project supported by the National Natural Science Foundation of China.     

基于碳纳米管/Ag/MoS2转导层的全固态钙离子选择电极

构建了一种电势稳定性好的全固态钙离子选择电极(Ca²⁺-ISE),采用碳纳米管(CNT)/Ag/MoS₂为转导层,自制的三脚架化合物作为钙离子载体,制备固体接触式Ca²⁺-ISE。 系统分析了全固态Ca²⁺-ISE的稳定性、能斯特斜率、响应范围、选择性系数等主要性能,发现制备的固体接触式Ca²⁺-ISE在钙离子浓度为1×10^-6~1×10^-1 mol/L范围内呈现线性能斯特响应,响应斜率为28.1 mV/decade。 CNT/Ag/MoS₂的引入有利于提高固体接触式Ca²⁺-ISE的离子浓度线性响应范围,缩短电势平衡时间,降低测试能斯特斜率与理论值差,对于Ca²⁺-ISE的长期在线检测有重要研究意义。     

Direct determination of benzylnitriles using the cyanide ion selective electrode

The response characteristics of the solid state cyanide ion selective electrode towards some benzylnitriles are investigated. In 10M KOH solution, the electrode exhibits nearly Nernstian response over the concentration range of 10^–2 to 10^–4 M of various substituted benzylnitriles with an anionic slope of 53–59 mV/concentration decade. The response time varies from 10 to 20 min depending on both the nature of the substituent group and the concentration of the nitrile compound. Direct potentiometric measurement of some nitrile compounds at the concentration level of 0.01 to 1 mg/ml shows an average recovery of 98.2% and a mean standard deviation of 2.3%. Many nitrogen functional groups do not interfere.     

Electroviscous effects on pressure-driven flow of dilute electrolyte solutions in small microchannels

A fundamental understanding of the flow characteristics of electrolyte solutions in microchannels is critical to the design and control of microfluidic devices. Experimental studies have shown that the electroviscous effect is appreciable for a dilute solution in a small microchannel. However, the experimentally observed electroviscous effects cannot be predicted by the traditional theoretical model, which involves the use of the Boltzmann distribution for the ionic concentration field. It has been found that the Boltzmann distribution is not applicable to systems with dilute electrolyte solutions in small microchannels because it violates the ion number conservation condition. A new theoretical model is developed in this paper using the Nernst equation and the ion number conservation, instead of the Boltzmann distribution, to obtain the ionic concentration field. The ionic concentration field, electrical potential field, and flow field in small microchannels are studied using the model developed here. In order to verify this model, the model-predicted dP/dx (applied pressure gradient) Re (Reynolds number) relationship is compared with the experimentally determined dP/dx approximately Re relationship. Strong agreement between the model predictions and the experimental results supports this model.     

Measurement of surface potential at silver chloride aqueous interface with single-crystal AgCl electrode

The surface potential at the silver chloride aqueous interface was measured by means of a single-crystal silver chloride electrode (SCr-AgCl). The measurements were conducted by titration of the KCl solution with AgNO3, and vice versa. The SCr-AgCl electrode potentials were converted to surface potentials psi(0) by setting zero at the point of zero charge at pCl = 5.2. The psi(0)(pCl) function was linear, with a slope 12% lower with respect to the Nernst equation. It was demonstrated that the surface potential at the silver halide aqueous interface could be interpreted by means of the surface complexation model, originally developed for metal oxides.     

The deviation potential of pH glass electrode

Summary Change of the effective membrane surface area generates a deviation potential (Ed) from the conventional Nernst potential (E0). ThisEd is an exponential function of the fraction of the membrane surface area that is covered (b) as (1–e ^–2b ). There is an isosbestic pH (pH_iso) at which no deviation can be detected, even if the effective surface area of the membrane is reduced, and at this pH the conventional Nernst equation requires no modification. This is attributed to the isoelectric point of the glass membrane. An extended Nernst equation is proposed.

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Potentialabweichung einer pH-Glas-Elektrode

Zusammenfassung Eine Veränderung der effektiven Membranoberfläche verursacht eine Potentialabweichung (E _d ) von dem üblichen Nernst-Potential (E0). DiesesE _d ist eine exponentielle Funktion der bedeckten Membran-Oberfläche (b) und beträgt 1–e ^–2b . Es gibt einen isosbestischen pH-Wert (pH_iso), bei dem keine Abweichung festgestellt werden kann, auch wenn die effektive Membranoberfläche verkleinert wird; bei diesem pH erfordert die übliche Nernst-Gleichung keine Modifikation. Dies ist abhängig vom isoelektrischen Punkt der Glasmembran.-Eine erweiterte Nernst-Gleichung wurde vorgeschlagen.

     

Potentials of Silver and Gold Electrodes of ZrO2-Based Cells in N2+ O2+ CO2+ CO Gas Mixtures

Steady-state electrode potentials in cells with a ZrO₂+ 10 mol % Y₂O₃electrolyte are measured at 400 to 500°C in nonequilibrium N₂+ O₂+ CO₂+ CO gas mixtures containing 0–3 and 0–10 vol % of CO and O₂, respectively. In CO-free mixtures, the potentials obey the Nernst equation. The CO-caused deviation of the potential from its equilibrium value depends on the O₂content and temperature and is due mainly to the CO adsorption at electrodes.     

Beyond potentiometry: robust electrochemical ion sensor concepts in view of remote chemical sensing

For about 100 years, potentiometry with ion-selective electrodes has been one of the dominating electroanalytical techniques. While great advances in terms of selective chemistries and materials have been achieved in recent years, the basic manner in which ion-selective membranes are used has not fundamentally changed. The potential readings are directly co-dependent on the potential at the reference electrode, which requires maintenance and for which very few accepted alternatives have been proposed. Fouling or clogging of the exposed electrode surfaces will lead to changes in the observed potential. At the same time, the Nernst equation predicts quite small potential changes, on the order of millivolts for concentration changes on the order of a factor two, making frequent recalibration, accurate temperature control and electrode maintenance key requirements of routine analytical measurements. While the relatively advanced selective materials developed for ion-selective sensors would be highly attractive for low power remote sensing application, one should consider solutions beyond classical potentiometry to make this technology practically feasible. This paper evaluates some recent examples that may be attractive solutions to the stated problems that face potentiometric measurements. These include high-amplitude sensing approaches, with sensitivities that are an order of magnitude larger than predicted by the Nernst equation; backside calibration potentiometry, where knowledge of the magnitude of the potential is irrelevant and the system is evaluated from the backside of the membrane; controlled current coulometry with ion-selective membranes, an attractive technique for calibration-free reagent delivery without the need for standards or volumetry; localized electrochemical titrations at ion-selective membranes, making it possible to design sensors that directly monitor parameters such as total acidity for which volumetric techniques were traditionally used; and controlled potential coulometry, where all ions of interest are selectively transferred into the ion-selective organic phase, forming a calibration-free technique that would be exquisitely suitable for remote sensing applications.     

Iterative computational method for the rapid analysis of iron and plutonium by controlled potential coulometry and some interesting observations on the coulogram of the Pu(III) /Pu(IV/ couple

An iterative computational method for the determination of metal ions in aqueous solutions which form reversible couples such as Fe(II)/Fe(III), Pu(III)/Pu(IV) etc. by controlled potential coulometry has been developed. The method involves carrying out the electrolysis to about 95–97% and calculating the total amount present in the sample by an iterative computational method. The method utilizes the direct application of the Nernst equation. The important criterion to be met is that the coulogram of the couple should strictly obey the Nernst equation. The validity of the method has been checked by analyzing about 50 samples of a standard iron solution. Results of analysis of mixtures of Pu and Fe by the iterative technique show that the interference of Fe can almost entirely be eliminated. However, analysis of Pu samples by this procedure gives results about 2–3% lower than the expected value. A careful examination of the experimental coulograms of Pu in lM HClO₄ indicates a slight deviation from the theoretical coulogram, where as those of Fe match exactly.     

纳米Fe_2O_3修饰涂碳型硫酸沙丁胺醇选择电极的研制与应用

制备了以纳米Fe_2O_3为修饰剂的涂碳型硫酸沙丁胺醇选择电极,采用电位分析方法对其各项性能进行测定。结果表明,该纳米Fe_2O_3修饰电极有很好的能斯特响应,其线性范围为1.0×10~(-6)~0.1 mol/L,级差电位为59 mV/pC,与普通电极相比,响应时间较短(10 s),检出限更低(2.8×10~(-7)mol/L)。将修饰电极应用于猪肉样品中硫酸沙丁胺醇含量的测定,结果与标准方法结果相符。     

非配对型电极上的示波沉淀滴定

本文研究了非配对型铂电极上的示波沉淀滴定,提出了一种处理电极的新方法即硝酸-王水法,讨论了非配对型电极上示波滴定的机理。     

Discrete Helmholtz model: a single layer of correlated counter-ions. Metal oxides and silica interfaces,ion-exchange and biological membranes

The mechanism by which interfaces in solution can be polarised depends on the nature of the charge carriers. In the case of a conductor, the charge carriers are electrons and the polarisation is homogeneous in the plane of the electrode. In the case of an insulator covered by ionic moieties, the polarisation is inhomogeneous and discrete in the plane of the interface. Despite these fundamental differences, these systems are usually treated in the same theoretical framework that relies on the Poisson–Boltzmann equation for the solution side. In this perspective, we show that interfaces polarised by discrete charge distributions are rather ubiquitous and that their associated potential drop significantly differs from those of conductor–electrolyte interfaces. We show that these configurations, spanning liquid–liquid interfaces, charged silica–water interfaces, metal oxide interfaces, supercapacitors, ion-exchange membranes and even biological membranes can be uniformly treated under a common “Discrete Helmholtz” model where the discrete charges are compensated by a single layer of correlated counter-ions, thereby generating a sharp potential drop at the interface.

Electrolytes in solution are strongly correlated with discrete charges at insulating interfaces inducing a behavior significantly different from that of conducting interfaces.     

Multi-flow-through sensor for simultaneous cation determination

The activity of several metal ions can be detected potentiometrically using glass electrode measuring chains, which have normally a cylindrical design. To analyse different cations simultaneously, on one hand several single indicator electrodes can be integrated in a prefabricated flow-through equipment, where they can be measured versus electrochemical reference electrodes. On the other hand, by principle it is possible to construct tubular flow-through electrodes. In the present case, glass electrodes are double-walled, so that the liquid internal reference system consisting of reference solution and reference element can be placed.In the case of multi-analysis, for constructional reasons it would be necessary to combine several of such flow-through electrodes by complicated connecting elements. In this paper a multi-flow-through electrode is described, which can be fabricated in one piece by substitution of the liquid by a solid reference half cell. The contact of ion-selective glasses with semiconducting zinc oxide leads to a reversible phase interface, so that a high stability of the individual half cell potentials is achieved. Nearly all electrode properties are in a good agreement with those of traditional cation selective electrodes. The pH electrodes, for example, exhibit almost linear response in a range of pH 1–10 with a slope of 57–59 mV/pH at θ = 25 °C.     

Model for measuring excitation temperatures on a relative basis without transition probabilities

A variation of the conventional slope or two-line method is proposed for determining excitation temperature (T_exc) by atomic emission. T_exc is measured relative to some known reference value, which can be derived either from the emission source itself or from some reference line source of known temperature. Measurement of T_exc with a relative error of 1–10% should be possible for reasonable experimental uncertainties in T_ref and the measured line intensities. The proposed method is useful for some interesting and important experiments with emission lines for which the transition probabilities are either unknown or of questionable accuracy. Such experiments include measuring the dependence of T_exc on observation position or operating parameters, determining whether or not different emitting species have the same T_exc, and identifying possible deviations from a Boltzmann distribution of excited state populations within a particular emitting species.     

Use of Butler–Volmer treatment to assess the capability of the voltammetric ion sensors: Application to a PPy/DBS film for cations detection

In this work, the Butler–Volmer formalism has been used to obtain a new equation to assess the calibration of voltammetric ion sensors. This new method postulates a direct relationship between the mid point reversible potential, E_R, and the logarithm of the electrolyte concentration. In the same way as the other relationships based on the Nernst equation, a positive slope is expected for a cationic exchanging system, and a negative one for an anionic sensor. However, the theoretical slope proposed in this work includes the electron-transfer coefficient, which allows us to explain the slopes frequently reported in the literature non coincident with the ideal value estimated from the Nernst equation: 2.303RT/nF. Also, comments are made on results from other literature with respect to the new equation, and an application of this method to PPy/DBS films as a cation sensor is carried out.