Cells with amalgam electrodes

Such a cell is formed by two metal amalgam electrodes of different metal activity dipping into a common solution of a soluble salt of the metal, e.g. 

It should be noted that Equation (5.25) gives the instantaneous cell e.m.f. which will fall as the ratio 2/ 1 decreases due to transfer of material. Xhus, overall, the cell reaction involves the passage of thallium from the higher to the lower activity. When these activities become equal, the potentials of both electrodes are the same and the cell ceases to operate. For the passage of 1 faraday, the free energy change accompanying the movement of 1 equivalent of thallium from 2 to ai is 

2 Cells with gas electrodes operating at different pressures 

Here we may consider a cell consisting of two hydrogen electrodes operating at different pressures dipping into a common solution of hydrochloric acid, e.g. 

the free energy change for the passage of 1 faraday is, 

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Fia. 64. Concentration cell with amalgam electrodes (Maclnnes and Parker)... 

No experiments appear to have been made with such cells, although the equation has been verified with oxygen at different partial pressures in admixture with nitrogen, with platinum electrodes and hot solid glass as electrolyte (Haber and Moser). A similar case is that of two amalgams of a metal, of different concentrations, as electrodes, and a solution of a salt of the metal as electrolyte (G. Meyer, 1891). Here we must take the osmotic pressures of the metals in the amalgams, Pi, P2, and, for an 7i-valent metal ... 

Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold amalgam electrode. When this electrode dips into a solution containing the analyte together with a small amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by (b) and (c). 

In this cell, two amalgams with different mole fractions of lead act as electrodes in a common electrolyte solution containing a lead salt. The activities of lead in these amalgams can be calculated from emf measurements with this cell. 

The Kolbe reaction is earned out in an undivided cell with closely spaced platinum electrodes. Early examples used a concentrated, up to 50 %, aqueous solution of an alkali metal salt of the carboxylic acid and the solution became strongly alkaline due to hydrogen evolution at the cathode. Ingenious cells were devised with a renewing mercury cathode, which allowed removal of alkali metal amalgam. These experimental conditions have been replaced by the use of a solution of the carboxylic acid in methanol partially neutralised by sodium methoxide or trieth-... 

Various attempts have been made to circumvent these problems and to eliminate junction potentials, including (1) extrapolation procedures designed to eliminate the difference between the compositions of the two solutions in the appropriate limit, (2) separation of the two solutions by means of a doublejunction salt bridge, (3) the use of double cells with dilute alkali metal amalgam connectors, and (4) the use of glass or other types of ion-specific electrodes as bridging reference electrodes. 

The solution to the right of the glass electrode is usually a buffer solution of KH2P04 and Na2HP04, with 0.1 mol dm"3 of NaCl. The reference electrode is usually a calomel electrode, silver/silver chloride electrode, or a thallium amalgam/thallous chloride electrode. The emf of this cell depends on a(H + ) in the solution X in the same way as that of the cell with the Pt H2 electrode, and thus the same procedure is followed. 

The cell consists of a small beaker with a top that will accommodate two electrodes as shown in Fig. 1. The cadmium electrode is made by plating cadmium onto a platinum wire that is sealed through the bottom of a small glass tube. The amalgam electrode is made by placing a small quantity of the cadmium amalgam in the cup of a special J-shaped glass tube with a platinum wire sealed into it. Electrical contacts are made by copper wires spot-welded to the platinum lead wires. 

Concentration Cells with a Single Electrolyte Amalgam Concentration Cells.—In the concentration cells already described the e.m.p. is a result of the difference of activity or chemical potential, i.e., partial molal free energy, of the electrolyte in the two solutions it is possible, however, to obtain concentration cells with only one solution, but the activities of the element with respect to which the ions in the solution are reversible are different in the two electrodes. A simple method of realizing such a cell is to employ two amalgams of a base metal at different concentrations as electrodes and a solution of a salt of the metal as electrolyte thus... 

The use of a sodium amalgam electrode [Na(Hg)/NaC104(s)] in DMF has been reported [205] and a similar lithium amalgam electrode has been employed in DMSO [207]. The potential of the cell, Li(Hg)/Li Cr (DMSO), has been measured for LiCl concentrations from 0.01 to 1.0 M and the Nernst relation was verified within 1 mV the Li(Hg) electrode obeys the Tafel equation with the transfer coefficient a = 0.5 over... 

What would be the b.m.f. at 321° C of a cell consisting of two bismuth amalgam electrodes, with mole fractions 0.1 and 0.9 of bismuth, respectively, in the same electrolyte (Use results of Exercise 6.)... 

The following e.m.f. results were obtained in measurements on cells with potassium amalgam electrodes at 25 C in each case one electrode consisted of an amalgam in which the mole fraction of potassium (nJ) was 0.01984, while the composition (N2) of the other was varied ... 

Clark cell. Standard cell for measuring electrical potential with mercury and zinc amalgam electrodes in zinc sulfate solution. 

Another important area of experimental investigation involves concentration cells. These devices derive their EMF from the fact that a given cell component has a different concentration in two parts of the cell. A simple example of a concentration cell is one involving the electrodes. Eor example, one may set up a cell with two amalgam electrodes containing the same metal at different concentrations ... 

This chapter is concerned with the determination of activity coefficients with the aid of various types of concentration cells, and with the comparison of such activity coefficients with the predictions of the Debye-Hiickel theory, developed in the previous chapter. The types of cells discussed are (a) cells without transference, including those containing amalgam electrodes, (b) cells with transference, and (c) cells without transference containing mixtures of electrolytes. 

Weston cadmium cell A standard cell that produces a constant e.m.f. of 1.0186 volts at 20°C. It consists of an H-shaped glass vessel containing a negative cadmium-mercury amalgam electrode in one leg and a positive mercury electrode in the other. The electrolyte - saturated cadmium sulfate solution - fills the horizontal bar of the vessel to connect the two electrodes. The e.m.f. of the cell varies very little with temperature, being given by the equation = 1.0186 - 0.000 037 (T - 293), where T is the thermodynamic temperature. 

D7.7 Electrode combinations that produce identical cell compartments with differing concentrations only (electrolyte concentration cells) have a cell potential dependence upon the liquid junction potential and the concentration difference. If the cell has identical compartments with either gaseous or amalgam electrodes (electrode concentration cell), the cell potential will depend upon the gas pressure differences or the amalgam concentration differences but will not have a liquid junction potential. Other electrode combinations produce cells for which the cell potential depends upon the half-reaction reduction potentials. 

Daniell cell A type of primary voltaic cell with a copper positive electrode and a negative electrode of a zinc amalgam. The zinc-amalgam electrode is placed in an electrolyte of dilute sulphuric acid or zinc sulphate solution in a porous pot, which stands in a solution of copper sulphate in which the copper electrode is immersed. While the reaction takes place ions move through the porous pot, but when it is not in use the cell should be dismantled to prevent the diffusion of one electrolyte into the other. The e.m.f. of the cell is 1.08 volts with sulphmic acid and 1.10 volts with zinc sulphate. It was invented in 1836 by the British chemist John Daniell (1790-1845). 

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

Since metallic sodium is unstable in aqueous solutions, the measurement of the reversible potential for reaction (44) is difficult. This was, however, overcome by measuring the voltage of a cell with sodium metal and amalgam as the electrode in an aprotic electrolyte, to ensure stability of sodium, as ... 

Daniell cell A type of primary cell invented by British chemist John Daniell (1790-1845) in 1836. It consists of two electrodes in different electrolytes separated by a porous pot. The positive electrode is copper immersed in copper(II) sulfate solution and the negative zinc-mercury amalgam electrode is in either dilute sulfuric acid or a zinc sulfate solution. The porous pot prevents mixing of the electrolytes, but allows ions to pass. With sulfuric acid the e.m.f. is about 1.08 volts with zinc sulfate it is about 1.10 volts. 

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