Reference electrodes second kind electrode

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. 

When considering the second kind electrodes in Section 2.3, copper covered by CujO was referred to as a prominent example. Cuprous oxide layers developed in electrochemical systems are interesting from various points of view. In the first place, such layers are known to form during copper corrosion. Research in this field are very important and large in number (see, e.g.. Ref [1-12] and references therein). Extensive investigations of photoelectrochemical phenomena in these systems provide further insights into the corrosion mechanism and may be useful in improving copper corrosion resistance. However, these problems are not the main objective of the present review. 

The mercury/mercury(I) chloride (calomel) electrode is a second-kind electrode composed typically of metallic mercury and excess of hardly soluble mercury(I) chloride with addition of a paste of mercury with calomel in contact with KCl solution [98, 100]. It was introduced by Ostwald in 1890 and is now the most popular mercury reference electrode. The scheme of this electrode is shown below ... 

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. 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

The potential of an electrode of the second kind is determined by the activity (concentration) of anions, or more correctly, by the mean ionic activity of the corresponding electrolyte [see Eq. (3.50)]. The most conunon among electrodes of this type are the calomel REs. In them, a volume of mercury is in contact with KCl solution which has a well-defined concentration and is saturated with calomel Hg2Cl2, a poorly soluble mercury salt. The value of such an electrode is 0.2676 V (aU numerical values refer to 25°C, and potentials are reported on the SHE scale). Three types of calomel electrode are in practical use they differ in KCl concentration and, accordingly, in the values of ionic activity and potential ... 

Although not essential, one often uses a previously coated AgCl-Ag electrode or a silver-plated Pt wire coated electrolytically in KC1 solution with a thin deposit of AgCl. Such dry AgCl-Ag electrodes are much in favour as reference electrodes (although in the absence of oxidants), in addition to calomel electrodes (Pt wire in contact with Hg, covered with calomel paste in contact with KC1 solution), which also belong to the second kind, viz.,... 

In practice, it is very often necessary to determine the potential of a test (indicator) electrode connected in a cell with a well defined second electrode. This reference electrode is usually a suitable electrode of the second kind, as described in Section 3.2.2. The potentials of these electrodes are tabulated, so that Eq. (3.1.66) can be used to determine the potential of the test electrode from the measured EMF. The standard hydrogen electrode is a hydrogen electrode saturated with gaseous hydrogen with a partial pressure equal to the standard pressure and immersed in a solution with unit hydrogen ion activity. Its potential is set equal to zero by convention. Because of the relative difficulty involved in preparing this electrode and various other complications (see Section 3.2.1), it is not used as a reference electrode in practice. 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

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)... 

In the field of ion-selective electrodes the potentials of electrodes of the second kind, acting as reference electrodes, are especially important. The electrode potentials of the most important reference electrodes, the silver chloride and calomel electrodes, are... 

Reference electrodes "1 and are usually electrodes of the second kind their... 

The precision and accuracy of the measurement also depend strongly on the reference electrode, which affects the results through fluctuations in its own potential and through the liquid-junction potential at the test solution-liquid bridge interface. This subject is extensively treated in [158]. Common electrodes of the second kind have sufficiently stable potentials at a constant temperature, but difficulties can be encountered due to temperature hysteresis. Silver chloride electrodes are preferable to calomel electrodes, because their temperature hysteresis is substantially smaller with a calomel electrode, potential stabilization after a change in the temperature may even take several hours. Negligible temperature hysteresis is exhibited by the thallamide reference electrode [26,... 

The differential technique described under (a) has an advantage in removal of the liquid-junction potential and of mechanical faults often encountered in work with reference electrodes of the second kind. The procedure described under (b) suppresses the potential fluctuations, but difficulties can arise from the very high resistance of a cell containing two ISEs. A differential amplifier was designed for this prupose [15]. The two ISEs used can also exhibit different slopes electrode membranes were therefore prepared by cutting a single crystal into two halves, where each half contains a chaimel for passage of the solution and functions as an ISE [163]. 

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. 

Mercury is widely used in the practice of electroanalytical chemistry, both for working electrodes and for reference electrodes (in the latter case usually as an electrode of the second kind). 

Another opportunity to realize constant activity of the potential determining ion at the reference interface appears when one chooses the solid electrolyte in such a way that the ion of the redox couple is the same as one ion of the major component of the electrolyte. In that case, the change of the activity due to the electrode reaction with the gas can be neglected against the overall constant activity of that ion in the salt. This is the solid-state reference arrangement. An example is the chlorine sensor (Fig. 6.40), in which the reference potential is set up by the constant activity of CP in the solid AgCl electrolyte. This arrangement is equivalent to a reference electrode of the second kind, discussed in Section 6.2.2.1. 

Generally, electrodes of the second kind are more accurate. The most frequently applied reference electrodes fall into this category. Typical examples are the saturated calomel electrode (SCE) and the silver/silver chloride electrode (Ag/AgCl/KCl) (see below). 

Various types of reference electrodes are used for permanent embedment in concrete. Some fall into the category of double junction electrodes of the second kind and are therefore reference electrodes in the true sense. Others are simply a piece of metal or another material put into the concrete. Although stable and accurate, SCE is not used for permanent embedment in concrete, mainly because it contains a liquid metal, which makes it difficult to manufacture in a rugged form. In addition, environmental reasons make it undesirable for permanent use in the field (poisonous mercury and mercury compounds). 

Properties of the Ideal Reference Electrode. An ideal reference electrode should show the following properties (1) it should be reversible and obey the Nemst equation with respect to some species in the electrolyte (2) its potential should be stable with time (3) its potential should return to its initial value after small currents are passed through the electrode (no hysteresis) (4) if it is an electrode of the second kind (e.g., Ag/AgCl), the solid phase must not be appreciably soluble in the electrolyte and (5) it should show low hysteresis with temperature cycling. 

Because of this tendency to form anionic complexes, from the point of view of establishing satisfactory reference electrodes of the second kind, the most significant constant is not necessarily the solubility product constant, but the equilibrium constant for the reaction... 

In the half-cell of Eq. (5.24), the concentration of AgClj" must be small compared to that of Cl-, or a liquid-junction potential will result because the mobilities of AgClJ and Cl- are not the same. Thus, for a reference electrode of the second kind to be elfective in cells without appreciable junction potentials, the equilibrium constant for the reaction of Eq. (5.25) must be smaller than unity (preferably <0.1). In water, methanol, formamide, and V-methyl-formamide, this criterion is met, but in most organic solvents the equilibrium constant for the reaction of Eq. (5.25) ranges from 30 to 100. The silver chloride electrode is not recommended for general use in organic solvents.27... 

The silver-silver chloride electrode. The silver chloride reference electrode is not generally suitable as an electrode of the second kind because of the large solubility of AgCl in many aptotic solvents from formation of anionic complexes with chloride ion. In many cases the silver chloride solubility will essentially be that of the added chloride. This contributes significantly to the junction potential in cells with liquid junction and makes the electrode unsuitable for precise potentiometric work. 

Other reference electrodes for use in polar aptotic solvents. Emphasis has been given to the use of the silver-silver ion reference electrode because it is almost universally applicable, and because standardization on the use of one reference electrode system simplifies the comparison of data between different workers. However, a number of other reference electrodes have been used (see Table 5.4), particularly those that have resulted from the vast amount of batteiy research. These include the Li/Li(solv)+ and other alkali metal electrodes that function reversibly in Me2SO, propylene carbonate, and hexa-methylphosphoramide. The thallium-thallous halide electrodes of the second kind also function reversibly in Me2SO and propylene carbonate. The cadmium amalgam-cadmium chloride reference electrode also functions reversibly in dimethylformamide and may be a useful substitute for the silver-silver ion reference electrode, which may be unstable in dimethyformamide.54... 

Electrodes of the second kind These consist of three phases. A metal is covered by a layer of its sparingly soluble salt, and immersed in a solution containing the anion of this salt. The Ag/AgCl/Ch and Hg/Hg2Cl2/Cl electrodes referred to above are of this type. 

Silver solid amalgam can be used for the preparation of a - reference electrode of second kind (-> electrode of second kind) [iv]. 

Calomel electrode — is an - electrode of the second kind. It was introduced in 1890 by Ostwald, F.W. Asa- reference electrode of fixed, well-known, and very reproducible -+potential, it is still a commonly used reference electrode in electrochemistry [i—iii]. It consists of mercury, sparingly soluble mercurous chloride (calomel), and a chloride-containing solution. The electrode net reaction can be formulated in the following way ... 

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 precipitation is especially enhanced because mostly a saturated KC1 solution is used to decrease the -> junction potential. It should be noted that by changing KC1 for NaCl the potential of the saturated calomel electrode (SCE) will be varied which is due to the different solubilities. In the case of electrodes of the second kind the effect of temperature on the solubility has to be considered, too. It is of importance to prevent to exchange electrolytes between the main and the reference compartments. For instance, a leakage of chloride ions, which is strongly adsorbed at platinum, may influ-... 

As reference electrode any electrode whose potential is well defined and constant may be used by far the most widely used reference electrodes in aqueous and partly aqueous solution are the calomel (SCE, saturated calomel electrode) and the silver/silver chloride electrodes, both of which are electrodes of second kind. In non-aqueous solutions quite a few other reference electrodes have been used besides the calomel electrode. A discussion of reference electrodes is included in standard monographs on electroanalytical techniques, and comparisons between the diiferent types of electrode have been made.45-48... 

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. 

As reference electrode, an Li/LiCl electrode, which in the presence of excess tetrabutylammonium chloride is an electrode of the second kind, may be used [413]. An Ag/ AgC104 reference electrode with an NaC104 salt bridge has been used [414] AgCl/Ag [415] and TlX/Tl [416] reference electrodes have also been employed. The purification of PC has been discussed [417]. 

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