Reference half-cells

Together, the oxidized and reduced forms of the substance are referred to as a redox couple.) Such a sample half-cell is connected to a reference half-cell... 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Reference half-cell Indicator half-cell... 

It is impossible to measure the potential of a half-cell directly and a reference half-cell must be used to complete the circuit. The hydrogen electrode (Figure 4.3) is the standard reference electrode against which all other halfcells are measured and is arbitrarily attributed a standard electrode potential of zero at pH 0. Because it is difficult to prepare and inconvenient to use, the... 

Many scales of measurement have zero values that are arbitrary. For example, on Earth, average sea level is often assigned as the zero of altitude. In this ThoughtLab, you will investigate what happens to calculated cell potentials when the reference half-cell is changed. 

Measnrements of Ea are usually made with a platinum electrode placed in the soil solntion together with a reference half cell electrode of known potential. The platinnm electrode transfers electrons to and from the soil solution withont reacting with it. Reducing half reactions in the soil tend to transfer electrons to the platinum electrode and oxidizing half reactions to remove them. At eqnilibrinm no electrons flow and the electric potential difference between the half cell comprising the platinnm electrode and the soil solntion and the half cell comprising the reference electrode is recorded. 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

Marcus has tabulated AG° for a variety of ions, from water to a variety of solvents [34]. Values for the alkali metal ions (AGj (M+,w s)) are included in Table 7. AGj (Ag+, w CH3CN) are included for comparative purposes because the Ag/Ag+ couple is a reliable reference half-cell in a number of different solvents. AGj (H+, w s) are included because they are needed to determine the influence that pH might have on half-cells containing alkali metals. These numbers are also useful in revealing important periodic trends. To provide a frame of reference in comparing the various ions, entries in Table 7 are sorted by AG°(Na+, w s). 

Chapter 2) apply. The standard reference half-cell is reaction 15.6, the standard hydrogen electrode (SHE), and the standard conditions are those listed in Section 2.3, although for our purposes the molar concentration scale (mol L 1) can generally be used without significant loss of precision. We will simplify matters further, for illustrative purposes, by equating activities with molar concentrations our numerical results will therefore be only approximate, except where concentrations are very low. A thermodynamically acceptable treatment would require the calculation or measurement of ionic activities or, at the very least, maintenance of constant ionic strength, as outlined in Section 2.2. 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

If cells are constructed by making different combinations of half-cells, then the cell emf values obey an additive law. It is therefore convenient to combine half-cells with a single reference half-cell and thus obtain a series of related emf values compared to the reference half-cell taken as zero. 

The universally accepted primary reference half-cell is the standard hydrogen electrode. The electrode consists of a noble metal (platinized platinum) dipping into a solution of hydrogen ions at unit activity and saturated with hydrogen gas at 1 bar (i.e. 1 X 105 Pa, which in practical terms may be taken to be equal to 1 atmosphere). In practice such a standard electrode cannot be realized, but the scale it defines can. 

If we could determine E° values for individual half-reactions, we could combine those values to obtain E° values for a host of cell reactions. Unfortunately, it s not possible to measure the potential of a single electrode we can measure only a potential difference by placing a voltmeter between two electrodes. Nevertheless, we can develop a set of standard half-cell potentials by choosing an arbitrary standard half-cell as a reference point, assigning it an arbitrary potential, and then expressing the potential of all other half-cells relative to the reference half-cell. Recall that this same approach was used in Section 8.10 for determining standard enthalpies of formation, A H°f. 

The numerical value of an electrode potential depends on the nature of the particular chemicals, the temperature, and on the concentrations of the various members of the couple. For the purposes of reference, half-cell potentials are taken at the standard states of all chemicals. Standard state is defined as 1 atm pressure of each gas (the difference between 1 bar and 1 atm is insignificant for the purposes of this chapter), the pure substance of each liquid or solid, and 1 molar concentrations for every nongaseous solute appearing in the balanced half-cell reaction. Reference potentials determined with these parameters are called standard electrode potentials and, since they are represented as reduction reactions (Table 19-1), they are more often than not referred to as standard reduction potentials (E°). E° is also used to represent the standard potential, calculated from the standard reduction potentials, for the whole cell. Some values in Table 19-1 may not be in complete agreement with some sources, but are used for the calculations in this book. 

A voltmeter can replace the traditional indicator in titrations by making the titration vessel a half-cell (with an appropriate electrode) and connecting it to a reference half-cell via a salt bridge. To follow the titration in Problem 18-57, a silver/silver chloride electrode is inserted into the halide solution, and the reference half-cell is a silver/silver chloride electrode immersed in LOOM KC1. The reference electrode goes to the positive terminal of the voltmeter. Calculate the voltage reading at each of the five points in the titration specified in Problem 18.56. 

Until recently, the most popular reference half-cell for potentiometric titrations, polarography, and even kinetic studies has been the saturated aqueous calomel electrode (SCE), connected by means of a nonaqueous salt bridge (e.g., Et4NC104) to the electrolyte under study. The choice of this particular bridge electrolyte in conjunction with the SCE is not a good one because potassium perchlorate and potassium chloride have a limited solubility in many aprotic solvents. The junction is readily clogged, which leads to erratic junction potentials. For these practical reasons, a calomel or silver-silver chloride reference electrode with an aqueous lithium chloride or quaternary ammonium chloride fill solution is preferable if an aqueous electrode is used. 

Reference half-cells The fact that individual half-cell potentials are not directly measurable does not prevent us from defining and working with them. Although we cannot determine the absolute value of a half-cell potential, we can still measure its value in relation to the potentials of other half cells. In particular, if we adopt a reference half-cell whose potential is arbitrarily defined as zero, and measure the potentials of various other electrode systems against this reference cell, we are in effect measuring the half-cell potentials on a scale that is relative to the potential of the reference cell. 

In that diagram A and B represent both electrodes. Q is the concentration of the electrolyte 1 in contact with the electrode A. C2 is the salt bridge electrolyte concentration. C3 is the concentration of the electrolyte 3 in contact with the electrode B. The electrodes are joined through metallic conductors MA and MB connected to a - potentiostat. The cell under study A-Q is kept at a temperature TA and the reference half-cell B-C3 is maintained at a temperature To. For the determination of the temperature coefficient, the temperature in the half-cell A-Ci is varied, while the temperature T0 is kept constant. 

Thermocell (thermogalvanic cell) — is a cell that comprises a reference -> half-cell maintained at constant temperature, and another half-cell under study whose temperature is varied in a controlled manner [i, ii]. The variation of the temperature for the half-cell under study allows determining the -> temperature coefficient. However, temperature differences between both half-cells may induce other undesired additional effects to the electrochemical reaction. The following figure shows a schematic diagram of a commonly used thermocell. 

We are assuming for the moment that the changes in our reference half-cell are of negligible importance or else compensated, so that all of the changes take place in the working half-cell. 

What property would you look for in designing a reference half-cell that would produce a potential relatively stable with respect to temperature (See Exercise 39.)... 

The reduction potential is an electrochemical concept. Consider a substance that can exist in an oxidized form X and a reduced form X . Such a pair is called a redox couple. The reduction potential of this couple can be determined by measuring the electromotive force generated by a sample half-cell connected to a standard reference half-cell (Figure 18.6). The sample half-cell consists of an electrode immersed in a solution of 1 M oxidant (X) and 1 M reductant (X ). The standard reference half-cell consists of an electrode immersed in a 1 M H+ solution that is in equilibrium with H2 gas at 1 atmosphere pressure. The electrodes are connected to a voltmeter, and an agar bridge establishes electrical continuity between the half-cells. Electrons then flow from one half-cell to the other. If the reaction proceeds in the direction... 

Thus, electrons flow from the sample half-cell to the standard reference half-cell, and the sample-cell electrode is taken to be negative with respect to the standard-cell electrode. The reduction potential of the X X couple is the observed... 

The standard hydrogen electrode, which is the reference half-cell electrode, defined as 0.0 V at all temperatures, against which values of E" are expressed. H2 gas at 1 atmosphere pressure is bubbled over a platinum electrode immersed in an acid solution with an activity of unity. This electrode is rarely used for analytical work, since it is unstable and other reference electrodes are easier to construct and use. 

Reference solutions In an ion-selective electrode assembly the internal reference solution (Figure 13-2) performs a dual function. It must contain one ionic species to provide a stable electrode potential for the internal reference half-cell and another to provide a stable membrane potential at the inner solution-membrane interface. In the usual pH glass electrode assembly the inner reference half-cell is Ag/AgCl the potential is stable because the internal reference solution is 0.1 M in chloride ion. The potential of the inner membrane surface also is stable because the internal filling solution is 0.1 ilf in hydrogen ion (or a buffered solution). The single-electrolyte solution 0.1 Af HCl serves admirably to provide a high and stable concentration of both ions. 

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