Reference electrode first kind

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

Electrodes such as Cu VCu which are reversible with respect to the ions of the metal phase, are referred to as electrodes of the first kind, whereas electrodes such as Ag/AgCl, Cl" that are based on a sparingly soluble salt in equilibrium with its saturated solution are referred to as electrodes of the second kind. All reference electrodes must have reproducible potentials that are defined by the activity of the species involved in the equilibrium and the potential must remain constant during, and subsequent to, the passage of small quantities of charge during the measurement of another potential. 

This procedure of using a single measurement of electrode potential to determine the concentration of an ionic species in solution is referred to as direct potentiometry. The electrode whose potential is dependent upon the concentration of the ion to be determined is termed the indicator electrode, and when, as in the case above, the ion to be determined is directly involved in the electrode reaction, we are said to be dealing with an electrode of the first kind . 

The interfacial tension always depends on the potential of the ideal polarized electrode. In order to derive this dependence, consider a cell consisting of an ideal polarized electrode of metal M and a reference non-polarizable electrode of the second kind of the same metal covered with a sparingly soluble salt MA. Anion A is a component of the electrolyte in the cell. The quantities related to the first electrode will be denoted as m, the quantities related to the reference electrode as m and to the solution as 1. For equilibrium between the electrons and ions M+ in the metal phase, Eq. (4.2.17) can be written in the form (s = n — 2)... 

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

Another form of redox reference electrode is similar to the electrode of the first kind. In this case the inert metal (e.g., Pt, Au, or C) is used as the inner electrode and a stable and soluble redox couple is placed inside the inner reference electrode compartment. A normal liquid junction is used in this type of reference electrode. Unlike the electrode of the first kind, the redox reference electrode is relatively immune to changes in concentration inside the reference electrode compartment because it is the ratio of the reduced/oxidized form of redox couple that determines the potential and not the absolute concentrations. However, redox reference electrodes are sensitive to changes of concentration of oxygen and other redox species. 

Reference electrodes can be classified into several types (1) electrodes of the first kind a metallic or soluble phase in equilibrium with its ion... 

Open-circuit potential (OCP) — This is the - potential of the - working electrode relative to the - reference electrode when no potential or - current is being applied to the - cell [i]. In case of a reversible electrode system (- reversibility) the OCP is also referred to as the - equilibrium potential. Otherwise it is called the - rest potential, or the - corrosion potential, depending on the studied system. The OCP is measured using high-input - impedance voltmeters, or potentiometers, as in - potentiometry. OCP s of - electrodes of the first, the second, and the third kind, of - redox electrodes and of - ion-selective membrane electrodes are defined by the - Nernst equation. The - corrosion po-... 

The reference electrodes have been divided according to the electrode reaction responsible for maintaining the constant potential In electrodes of the first kind, the potential of the metal electrode is determined by the concentration of the metal ions in electrodes of the second kind, the potential is determined by the concentration of an anion that forms an insoluble salt with the metal cation and in redox electrodes, the potential of an indifferent electrode is determined by the relative concentrations of the two components of a redox system. 

There are lots of systems, especially for electroplating and electrosynthesis, in which electrodes of the first kind can be used, without any liquid junctions (the example is liquid A1 in AlFs-containing melts). More universal systems for melts of various kinds are a chlorine electrode in equimolar NaCl + KCl melt and Ag/Ag+ electrodes with the range of Ag" " concentrations (0.01-10 mM) corresponding to usual solubility values. Reference electrodes of the second kind can hardly be used in melts because of the high solubility of the majority of inorganic solids. 

Metal electrodes corresponding to these halfcells are referred to as electrodes of the first kind. The activity of the metal, Zn or Ag, is unity, as will be that of every solid component (as well as H2O) of a half-cell reaction. 

In order to gain more fiexibihty in the detection of gaseous chemical species other than the mobile species of the electrolyte, electrodes of the second kind are used. The second kind of electrodes occurs when the analyte reacts reversibly with the mobile ions in the electrolyte, forming an intermediate phase, separate or dissolved phase in the electrolyte. An equilibrium is thus established between the gas and the intermediate phase. For the reference electrode, an electrode of the first kind is utilized. Typical examples of the so-called type II sensors (Fig. 3b) are CO2 and SO3 sensors with K2CO3 [11] and Ag2S04 [7] solid electrolytes, respectively. 

Depending on the type of the reference electrode, various modifications of Eq. (7.7) can be obtained. If it is the first kind electrode, reversible with respect to metal ions, then Z/ = Zg, a. = and, having used the condition that l al/ o = obtain ... 

As explained in previous chapters (see Chaps. 1 and 2), an electrode of the first kind is one based on atoms or molecules and their corresponding cation or anion in solution. This type of electrode is most commonly used within the ionic liquids fields. Requirements for this class of reference electrode are that the atom or molecule used does not react with the ionic liquids. Additionally the corresponding cation or anion should be stable in the solvated form within the IL and should not have any corresponding undesirable chemical reactions with the IL components. If the reference electrode is based on a metal MImetal ion M" electrode system, and assuming the activity of the metal is 1 (which may not be true for specific cases), then the corresponding potential of the half-ceU is given by ... 

As stated in Sect. 7.2.1 of this chapter, for electrodes of the first kind, the metal ions should not react with the ILs. Basile et al. have noted that the presence of water results in the formation of silver nanoparticles for solutions of Ag" ions dissolved into [Cqmpyr] [TFSI] [31]. These authors postulate that this occurs via chelation of Ag" by the [TFSI] anion and a subsequent disproportionation reaction in the presence of water to form a Ag -[TFSI] complex and Ag° [31]. Thus, the choice of a reference electrode based on AglAgOTfl[C4mpyr][TFSI] may be problematic. The authors did note that over a short period of time, when chemical reaction rates were low, the FcIFc" reversible potential was close to that reported by Snook et al. [27, 31]. According to Snook et al. [27] the AglAgOTfl[C4mpyr][TFSI] reference electrode is only stable for 3 weeks before the solution needs to be replaced and the electrode remade. Therefore, if the AglAg" couple is to be used in other ILs, then any potential chemical reactions of Ag irais within this IL should be explored prior to use, as well as the stability of the reference electrode over time. 

The most commonly used reference electrodes in these IL types have used a fritted compartment with an aluminium wire dipped into a melt with a fixed N value. Literature examples [96-103] commonly use an aluminium wire in an N = 0.60 (acidic melt) ionic liquid, which is also referred to as a 1.5 1.0 QAm" AICI3 ratio. A slight variation in the reference half-cell with 66.7 mol% of AICI3 was used to match the melt of interest in work by Xu et al. [95]. In all cases, a major aluminium species in the melt is [AI2CI7] from which aluminium can be deposited and stripped off from the wire. This means the electrode is a reference electrode of the first kind (MIM +) or (AllAl ). 

Table 10.1 summarizes the characteristics of common ISEs and a number of new sensors in this field. We have not included in this table the liquid or polymer membrane-based electrodes which are selective, but rather fragile (for more details on such membranes see References 58,59). ISEs of the first kind are not very numerous, e.g., F -ISE (monocrystalhne membrane based on LaFj), Ag" -ISE (silver salts), or Na" -ISE (Na alumino-silicate glass or polyciystalline NASICON [Na super ionic conductor] membranes). Most of the ISEs are of the second kind and are based on insoluble silver salts for example, halide ISEs (CE, Br, I"), Cd ", Pb ", Cu ", etc. Such ISEs use mixtures of insoluble salts based on silver sulfide or silver selenide. Recently, Vlasov etal. and Neshkova have proposed several glasses sensitive to transition metals. Typical ISE devices are shown in Figure 10.5. Thin-layer chemical sensors based on chalcogenide glasses have also been developed. ... 

Electrodes are devices with which one can detect the movement and separation of charges occurring at phase boundaries, as well as induce and vary such processes by means of a forced current flow. An electrode can be a piece of some sufficiently inert conductor (such as Pt, Ag, Cu, etc.). If such an electrode is immersed in an electrolyte solution which contains ions of the electrode material, a potential difference will develop between electrode and solution which depends on the activity of this particular metal ion in solution. This is referred to as an electrode of the first kind. 

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