Primary reference electrode

To obtain comparative values of the strengths of oxidising agents, it is necessary, as in the case of the electrode potentials of the metals, to measure under standard experimental conditions the potential difference between the platinum and the solution relative to a standard of reference. The primary standard is the standard or normal hydrogen electrode (Section 2.28) and its potential is taken as zero. The standard experimental conditions for the redox... 

The electrolytes Na", and Cl are second only to glucose in being the most frequently run hospital tests. Many clinical chemistry analyzers now contain an ISE module for electrolyte analysis. Most commonly the module will consist of a Na -glass electrode, a valinomycin/PVC electrode, a Ag/AgCl pellet or a quaternary ammonium ion/PVC electrode and a reference electrode. A selective electrode for the bicarbonate ion continues to elude workers in the field. An indirect measurement of HCOf must be made. The sample is usually reacted with acid to evolve carbon dioxide gas which is measured with a traditional Severinghaus type CO2 electrode. Alternatively, the sample is treated with base to convert HCO to CO3 and a carbonate ion-selective electrode is used In this manner, the complete primary electrolyte profile is obtained electrochemically. 

The three main requirements for a satisfactory reference electrode, given by Ives and Janz [107], are reversibility (non-polarizability), reproducibility, and stability. Hydrogen electrodes have been chosen as the primary reference electrode due to their excellent reproducibility [54], The electrode is represented schematically as... 

Measurement with the hydrogen electrode The SHE is the primary reference electrode, so other half-cell potentials are measured relative to its potential. In practice, if we wish to determine the value of E M then we construct a cell of the type... 

To learn that the primary reference electrode is the standard hydrogen electrode (SHE), and that the potential of all other reference electrodes (so-called secondary references) are determined with respect to the SHE. 

The SHE is chosen as the ultimate reference electrode since its value is defined. By simply making a cell in which one half cell is the SHE, then straightaway we also know the potential of the second half cell. For this reason, we say that the SHE is a reference electrode. Since all potentials are ultimately cited with respect to the SHE, the latter is the reference electrode from which all other electrode potentials are derived we say that the SHE is the primary standard. It is also called the primary reference electrode. 

The primary reference electrode for aqueous solutions is the standard hydrogen electrode (SHE), expressed by H+(a=l) H2(p=105 Pa) Pt (see 11 in Section 4.1). Its potential is defined as zero at all temperatures. In practical measurements, however, other reference electrodes that are easier to handle are used [24]. Examples of such reference electrodes are shown in Table 5.4, with their potentials against the SHE. All of them are electrodes of the second kind. The saturated calomel electrode (SCE) used to be widely used, but today the saturated silver-silver chloride electrode is the most popular. 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

In contrast, in non-aqueous solutions, the method of measuring electrode potentials has not been well established. The most serious problem is the reference electrode there is no primary reference electrode like the SHE in aqueous solutions and no reference electrode as reliable as the aqueous Ag/AgCl electrode. Thus, various reference electrodes have been employed in practical measurements, making the comparison of potential data difficult. As will be described later, various efforts are being made to improve this situation. 

In order to avoid confusion in reporting of electrochemical data, the Standard Hydrogen Electrode (SHE) has been selected as the primary reference electrode, which means that its standard potential has been arbitrarily set to zero, thus establishing the hydrogen scale of standard potentials. It is based on the equilibrium... 

The Ag/AgCl electrode is most commonly used as a secondary reference electrode. The hydrogen electrode, the primary reference electrode, is inconvenient to use in practice, since it requires H2(g) and... 

Abstract 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... 

The primary method for pH is based on the measurement of the potential difference of the electrochemical cell without a liquid junction involving a selected buffer solution, a platinum hydrogen gas electrode and a silver/silver chloride reference electrode, often also referred to as a Harned cell. 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

The hydrogen electrode. The hydrogen electrode is discussed first because it is the primary reference electrode used to define an internationally accepted scale of standard potentials in aqueous solution. By convention, the potential of an electrode half-reaction that is measured with respect to the normal hydrogen electrode (NHE also written as SHE, standard hydrogen electrode) is defined as the electrode potential of the half reaction. This convention amounts to an arbitrary assignment for the standard potential of the hydrogen electrode as zero at all temperatures. Thus, there is in effect a separate scale of electrode potentials at each temperature level. 

The equipment required for direct potentiometric measurements includes an ion-selective electrode, a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Fig. 5.1). Conventional voltmeters cannot be used because only very small currents can be drawn. The reference electrode should provide a highly stable potential for an extended period of time. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species (known as the primary or analyte ion). Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not destructive, and are very inexpensive. Ion-selective electrodes can be assembled conveniently in a variety of shapes and sizes. Specially designed cells allow flow or microliter analyses (see, e.g., Section 5-3). 

Primary reference electrodes, with their reduction potentials in H20 at... 

It is common in corrosion laboratories and in field corrosion monitoring probes to immerse two vertical rods parallel to one another in an electrolyte. In the lab, one of the rods consists of a high-density graphite counterelectrode while the other is a working electrode. A reference electrode may be placed in between the two rods. In the field, polarization resistance or electrochemical noise measurements are often made between two nominally identical rods that both consist of the material of interest. The primary current distribution is nonuniform with respect to circumferential position about each electrode when the distance between the two rods is small in comparison to the radius of the rod, Fig. 10a (16). Again, the value of Ra varies from where the rods face each other to where they... 

Unfortunately, due to technical difficulties, reference electrodes have not been widely employed in fuel cell measurements. The primary cause is the geometric restriction imposed by the thin solid electrolyte (for example, a typical electrolyte thickness is about 50, um for a Nation 112 membrane). Additional factors such as the shape and position of the reference electrode must be taken into account in order to obtain reliable and meaningful results. In reality, the requirements for a reference electrode are less stringent than expected, since the main problem is the drift of the reference potential during measurement, which can result in large error due to the strong potential dependence of the impedance. 

Cyclovoltammetry provides redox potentials for a variety of molecules. By applying a well-defined (versus a reference electrode) voltage to a dissolved substrate, it can be determined, whether a molecule can be reversibly reduced or oxidized or whether a first electron transfer is followed up by chemical transformations. Accordingly, the thermodynamic stability of the radical ion, which is formed by a primary electron transfer between the substrate and the working electrode can be measured together with the detection of electron transfer-induced follow-up reactions (Scheme 7.1). 

---