Half Cell junction potentials

The intercept voltage, Vq, is principally the aggregate of the anode and cathode half-cell reversible potentials, the anode and cathode overpotentials, and the liquid-junction potential at the membrane (usually small). For a typical new cell with activated cathodes, Vq is about 2.4 V. With uncoated cathodes, this increases to about 2.7 V. 

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. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

In assembling cells for making thermodynamic measurements, one should try not to combine half-cells in a manner that results in a junction potential. Figure 9.7 is a schematic representation of the Daniell cell, which is one with a junction potential. The half-cell reactions are... 

A liquid junction potential E-f forms when the two half-cells of a cell contain different electrolyte solutions. The magnitude of Ej depends on the concentrations (strictly, the activities) of the constituent ions in the cell, the charges of each moving ion, and on the relative rates of ionic movement across the membrane. We record a constant value of j because equilibrium forms within a few milliseconds of the two half-cells adjoining across the membrane. 

The second method of minimizing the junction potential is to employ a swamping electrolyte S. We saw in Section 4.1 how diffusion occurs in response to entropy effects, themselves due to differences in activity. Diffusion may be minimized by decreasing the differences in activity, achieved by adding a high concentration of ionic electrolyte to both half-cells. Such an addition increases their ionic strengths I, and decreases all activity coefficients y to quite a small value. 

Most common reference electrodes are silver-silver chloride (SSC), and saturated calomel electrode (SSC, which contains mercury). The reference electrode should be placed near the working electrode so that the W-potential is accurately referred to the reference electrode. These reference electrodes contain concentrated NaCl or KC1 solution as the inner electrolyte to maintain a constant composition. Errors in electrode potentials are due to the loss of electrolytes or the plugging of the porous junction at the tip of the reference electrode. Most problems in practical voltammetry arise from poor reference electrodes. To work with non-aqueous solvents such as acetonitrile, dimethylsulfoxide, propylene carbonate, etc., the half-cell, Ag (s)/AgC104 (0.1M) in solvent//, is used. There are situations where a conventional reference electrode is not usable, then a silver wire can be used as a pseudo-reference electrode. 

In fact, the movement of ions in and out of a half cell (for example, across a semipermeable membrane or frit) gives rise to an additional form of potential, the so-called junction potential . Such potentials are discussed in Section 3.6.5. 

Up until now, all values of the emf have only comprised two half-cell potentials. The junction potential is an additional source of potential, so our fundamental relationship (equation (3.3)) now becomes ... 

Worked Example 3.17. Consider the simple cell SCE 11 Cujj j Cu(s> . From a measurement of the emf and a knowledge of sce. die electrode potential, cu2+,Cu> was determined to be 0.300 V. We will consider three possible situations, and calculate the activity of the copper ion in each case (i) Ej = 0, (ii) Ej = 30 mV, with the junction adding to the electrode potential for the copper half cell, and (iii) Ej = 30 mV, with the junction subtracting from the electrode potential for the copper half cell. As usual, we will assume a value of a(Cu) = 1 throughout and E 2+ = 0.340 V. 

As with all electrochemical cells, the emf represents the separation between two half-cell potentials (see equation (3.3)). In accepting this statement, we are implying that the cell has neither a junction potential nor any IR drop. The latter assumption is valid since the solution in our example is acidified and hence its ionic strength I is high. Solutions with high ionic strengths can be assumed to have a minimal solution resistance R. 

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]. 

Whenever dissimilar electrolyte solutions are in contact, a voltage difference called the junction potential develops at their interface. This small voltage (usually a few millivolts) is found at each end of a salt bridge connecting two half-cells. The junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, because we usually do not know the contribution of the junction to the measured voltage. 

In potentiometric measurements, the indicator electrode responds to changes in the activity of analyte, and the reference electrode is a self-contained half-cell with a constant potential. The most common reference electrodes are calomel and silver-silver chloride. Common indicator electrodes include (1) the inert Pt electrode, (2) a silver electrode responsive to Ag+, halides, and other ions that react with Ag+, and (3) ion-selective electrodes. Unknown junction potentials at liquid-liquid interfaces limit the accuracy of most potentiometric measurements. 

LIQUID JUNCTION. To avoid the unknow n liquid junction potential in measuring the potential of a half-cell against a reference electrode, the two hall-cells are frequently connected via a sail bridge, usually a concentrated solution of potassium chloride. Since its anion and cation have almost the same velocity, a negligible ddlusittn potential is set up across the liquid junctions at the ends of llte bridge. 

The cell reaction for cells without liquid junction can be written as the sum of an oxidation reaction and a reduction reaction, the so-called half-cell reactions. If there are C oxidation reactions, and therefore C reduction reactions, there are C C — 1) possible cells. Not all such cells could be studied because of irreversible phenomena that would take place within the cell. Still, a large number of cells are possible. It is therefore convenient to consider half-cell reactions and to associate a potential with each such reaction or electrode. Because of Equation (12.88), there would be (C - 1) independent potentials. We can thus assign an arbitrary value to the potential associated with one half-cell reaction or electrode. By convention, and for aqueous solutions, the value of zero has been assigned to the hydrogen half-cell when the hydrogen gas and the hydrogen ion are in their standard states, independent both of the temperature and of the pressure on the solution. 

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... 

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. 

In the simplest cells, the barrier between the two solutions can be a porous membrane, but for precise measurements, a more compbcated arrangement, known as a salt bridge, is used. The salt bridge consists of an intermediate compartment filled with a concentrated solution of KC1 and fitted with porous barriers at each end. The purpose of the salt bridge is to minimize the natural potential difference, known as the junction potential, that develops (as mentioned in the previous section) when any two phases (such as the two solutions) are in contact. This potential difference would combine with the two half-cell potentials so as introduce a degree of uncertainty into any measurement of the cell potential. With the... 

Table 2.1 lists half-reactions for electrodes of the second type and their potential for unit activities. These electrodes have, in the majority of cases, their own electrolyte associated with them. So, to calculate the potential of a cell in relation to the standard hydrogen electrode it is necessary to take the liquid junction potential between the two electrolytes into account (Section 2.10). 

When measuring an unknown electrode potential the half cell under examination M M+ is combined with a reference electrode (e. g. a calomel one) in the manner illustrated in Fig. 13. In this figure A is the calomel electrode, B the element to be measured and C the salt bridge, which forms the electric connection between A and B and eliminates at the same time the liquid junction... 

Figure 6-2. Two half-cells, or redox couples, connected by a wire and a saltbridge to complete the electrical circuit. Electrons donated by Fe2+ to one elec bode are conducted by the wire to the other elecbode where they reduce Cu2+. Both elecb-odes (couples) are necessary before elech-ons can flow. A saltbridge, which provides a pathway along which ions can move and so helps maintain elecboneubality by avoiding the buildup of charge in either half-cell, often contains agar and KC1 the latter minimizes the diffusion potentials at the junctions between the saltbridge and the solutions in the beakers (see Chapter 3, Section 3.2B).

In a diagram of a cell a single vertical line conventionally represents a phase boundary at which a potential difference is taken into account. A double vertical line represents a liquid junction at which the potential difference is ignored or is considered to be eliminated by an appropriate salt bridge. For example, a cell consisting of zinc and copper half-cells can be expressed by... 

For cations and anions generally, the assumption that liquid-junction potentials are the same in the measurement of standards and unknowns is less likely to be valid than for pH measurements. It has been suggested that a quantity A ) expressed in pM or pA units be included in (13-26) and (13-27) to correct for changes injunction potential arising from differences in ionic strengths of standard and test solutions. Alternatively, these effects could be eliminated through the use of two reference half-cells composed of electrodes without liquid-junction potentials. For example, if the test solution contained chloride ion, both reference half-cells could be Ag/AgCl, and the liquid-junction potential would be eliminated. In practice, external reference half-cells without liquid junction are not always convenient. 

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