The amalgam electrode

These are mainly used in the study of reactions involving alkali metals. Because of the extreme reactivity of alkali metals it is impossible to have a reversible electrode of the type M(s) M+(aq) or M(s) MX(s) X (aq). This can be overcome by dissolving the alkali metal in Hg(l) which gives an alkali metal amalgam which is attacked much less vigorously by water. The Na(Hg) Na (aq) electrode is a common example of this type of electrode which behaves reversibly. 

This electrode is reversible with respect to Na (aq) since this is the species in solution. 

A similar electrode is the cadmium amalgam electrode, Cd(Hg) Cd (aq) where  

Write down the reactions occurring when the following electrodes act as  

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In the experiments to be described, in each case the half-cell was coupled to a Ag/AgOl half-cell, which in each case formed the positive electrode of the cell that is to say, on closing the external circuit, electrons flowed in the external circuit from the amalgam electrode to the silver electrode. This was the situation whether the solvent was water, or methanol, or a mixture of methanol and water. 

This figure demonstrates that also under potentiometric conditions (- no external current flow) electrochemical net reactions occur. The EMF of the zinc-amalgam in a given Zn2 -ion solution depends on the current-voltage characteristic of other ions (in this example, Cu2 and Pb2 are interfering ions with respect to the Zn2 equilibrium potential) at the amalgam electrode. EMF drifts are thus explainable. 

In order to get the potential of the sodium electrode in aqueous 0.100M NaCl against a calomel electrode in the same solution, one measures the emfs of cells (1) and (2). Cell (1) is to measure the potential of the amalgam electrode (Na(Hg)) and cell (2) to measure the potential of the Na electrode against the Na(Hg) electrode. The sum of the emfs of the two cells (-3.113 V) corresponds to the emf of the hypothetical cell (3) and is equal to the potential of the Na electrode in the aqueous solution. 

The theoretical decomposition voltage of sodium chloride may be calculated as the sum of the reversible oxidation potential of the chlorine electrode and the reversible reduction potential of the amalgam electrode. 

As the amalgam electrode is connected by a conductor to the electrode at which the hydrogen is liberated (Fe or graphite extends into both amalgam and electrolyte), the current passing through the cell is... 

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. 

As reference electrode [211] for polarographic work, an aqueous or methanolic calomel electrode, connected to the DMSO solution with a suitable salt bridge to avoid contamination, has been used. The most stable reference electrodes in DMSO seem to be the amalgam electrodes, such as Tl(Hg)/TlCl [214] or Li(Hg)/LiCl [207]. 

In investigating systems of metal complexes, the latter method was also used by Gerischer [5-7]. With a sufficient excess of cyanide, Zn(II) complexes containing four cyanide ions prevail in solutions therefore, the net reaction taking place on the amalgam electrode can be written as follows ... 

The amalgam electrode is a variation of a metal-metal ion electrode in which the metal is in the form of an amalgam (i.e. dissolved in mercury) rather than in the pure form a platinum wire is used to make electrical contact. The reaction occurring is the same as that at a metal electrode the mercury plays no chemical role. In general, the potentials of amalgam electrodes are more reproducible than the potentials of solid metal electrodes, which are sensitive to surface impurities and mechanical strains in the crystalline solid. The contribution to the cell... 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

Mercury layers plated onto the surface of analytical electrodes serve as Hquid metal coatings. These function as analytical sensors (qv) because sodium and other metals can be electroplated into the amalgam, then deplated and measured (see Electro analytical techniques). This is one of the few ways that sodium, potassium, calcium, and other active metals can be electroplated from aqueous solution. In one modification of this technique, a Hquid sample can be purified of trace metals by extended electrolysis in the presence of a mercury coating (35). 

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

Electrodes and Galvanic Cells. In connection with Fig. 9 in See. 11 we discussed the removal of a positive atomic core from a metal. The same idea may be applied to any alloy that is a metallic conductor. When, for example, some potassium has been dissolved in liquid mercury, the valence electron from each potassium atom becomes a free electron, and we may discuss the removal of a K+ core from the surface of the amalgam. The work to remove the K+ into a vacuum may be denoted by Ycr When this amalgam is in contact with a solvent, we may consider the escape of a K+ into the solvent. The work Y to remove the positive core into the solvent is much smaller than Yvac. 

The Alkali Chlorides as Solutes. In order to make a similar study of the transference of KC1, NaCl, and LiCl between water and methanol-water mixtures, the hydrogen electrode was replaced by an amalgam electrode, as described in Sec. 111. The arrangement when two cells having potassium amalgam electrodes are placed back to back may be written... 

In an Evans diagram 89> the mixed potential can easily be found and also be verified by measuring the open circuit potential of a zinc-amalgam electrode in a Cu2+-ion solution. Even the complication by the simultaneous presence of another reducible species, e.g., Pbz+ can be graphically demonstrated for different limiting conditions... 

As has been shown 82 85 88), the behavior of amalgam electrodes under conditions of cementation is very similar to that of liquid and glass membrane electrodes under stationary state conditions. Here, Eq. (2) should be written in the following way ... 

The silver reductor has a relatively low reduction potential (the Ag/AgCl electrode potential in 1M hydrochloric acid is 0.2245 volt), and consequently it is not able to effect many of the reductions which can be made with amalgamated zinc. The silver reductor is preferably used with hydrochloric acid solutions, and this is frequently an advantage. The various reductions which can be effected with the silver and the amalgamated zinc reductors are summarised in Table 10.11. ... 

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 ... 

P9.3 The potentials (emfs) of lithium amalgam electrodes in the cell... 

The dropping electrode, in partieular used with mercury and with amalgams, has been frequently employed in kinetic studies [66But3]. Data obtained with this electrode/method are labelled DME, if no specific technique is applied that is more helpful in qualifying the obtained data. 

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