Electrodes evolution from acid solutions

C. Catalyst-Coated Titanium Electrodes for Oxygen Evolution from Acid Solutions... 

This technique is applied to mixtures of metal ions in an acidic solution for the purpose of electroseparation only the metal ions with a standard reduction potential above that of hydrogen are reduced to the free metal with deposition on the cathode, and the end of the reduction appears from the continued evolution of hydrogen as long as the solution remains acidic. Considering the choice of the cathode material and the nature of its surface, it must be realized that the method is disturbed if a hydrogen overpotential occurs in that event no hydrogen is evolved and as a consequence metal ions with a standard reduction potential below that of hydrogen will still be reduced a classic example is the electrodeposition of Zn at an Hg electrode in an acidic solution. 

In the assumptions that were made in this chapter up to the beginning of this section, it was assumed that transport of charge carriers to and from the electrode played no part in rate control because it was always plentiful. Thus, in the evolution of hydrogen from acid solutions, the current density in most experimental situations is less than 10 times the limiting diffusion current and for this reason there is a negligible contribution to the overpotential due to an insufficiency of charge carriers. Like water from the tap in a normal city, the rate of supply of carriers is both tremendously important but seldom considered, for there is always plenty available. 

The primary exploitation of kinetics for effecting separation relates to hydrogen evolution, the kinetics of which varies significantly with the (metal) electrode material. Slow hydrogen evolution allows copper to be deposited from acid solutions and is the reason why such a wide range of metal ions can be reduced at mercvuy. 

The evolution of chlorine from acidic solutions is certainly an elec-trocatalytic reaction, but experimental difficulties have arisen in fundamental studies, owing to corrosion in the chloride medium. Much work has nevertheless been carried out over the past 12-15 years on the chlorine evolution reaction and its mechanisms. A comprehensive review on this topic, including the relation of electrode performance to chemical composition of oxide films and their band structures, has been given by Novak, Tilak, and Conway in the Modern Aspects of Electrochemistry series. 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

Knowledge of the variation of electron transfer rate with electrode potential is important for the understanding of electrochemical reactions. The first experiments in this area were prompted by the observation that nitrobenzenes and aromatic carbonyl compounds are reduced in acid solution with little competition from the hydrogen evolution process. This is the case even though the electrode potential is more negative than the value calculated for the reversible evolution of hydrogen in the same solution. The kinetics of hydrogen evolution have been examined in detail. 

The intermediate remains on the electrode until it is transformed into another particle during the consecutive steps that make up the overall reaction. The simplest example is (Section 7.6.2) the mechanism of hydrogen evolution, in which one possible step involves chemical recombination between adsorbed H s, put onto the electrode surface by means of the discharge of H20+ from acid or H20 from alkaline solutions. The adsorbed H is the intermediate radical. 

The dissolution reaction is Pt - Pt2+ + 2e and the value of its reversible thermodynamic potential is 1.2 V on the normal hydrogen scale. The evolution of O2 in acid solution at a current density of, say, 100 mA cm, needs an overpotential on platinum of nearly 1.0 V, i.e., the electrode potential would be >2.0 V. It follows feat at these very anodic potentials platinum would tend to dissolve, although its dissolution would be slowed down by fee fact feat it forms an oxide film at fee potentials concerned. Nevertheless, fee facts stated show feat fee alleged stability of Pt may be more limited than is often thought. This is an important practical conclusion because dissolved Pt from an anode may deposit on fee cathode of fee cell, and instead of having fee surface one started wife as fee cathode, it becomes in fact what is on its surface, platinum. 

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

The anodic partial current may be a sum of several partial currents when two or more electrode processes take place simultaneously (see partial current) for instance, the evolution of chlorine and oxygen from aqueous hydrochloride acid solutions at high positive potentials. 

Electrolytic Oxidation I. Electrode Potential.—A series of stable potentials is difficult to obtain at an anode in the presence of a depolarizer the potential generally rises rapidly from the low value, at which the anode dissolves, to the high value for passivity and oxygen evolution. Since a platinum electrode is nearly always passive, however, it is possible to obtain graded potentials to a limited extent the data quoted in Table LXXXVI were recorded for the oxidation of an acid solution of... 

The most reliable data are from studies of hydrogen evolution on mercury cathodes in acid solutions. This reaction has been studied most extensively over the years. The use of a renewable surface (a dropping mercury electrode, in which a new surface is formed every few seconds), our ability to purify the electrode by distillation, the long range of overpotentials over which the Tafel equation is applicable and the relatively simple mechanism of the reaction in this system all combine to give high credence to the conclusion that p = 0.5. This value has been used in almost all mechanistic studies in electrode kinetics and has led to consistent interpretations of the experimental behavior. It... 

Two electrodes are of importance, platinized Pt and Hg. Electrons transfer easiest from a platinized Pt electrode, so this type is preferred for cathodes. Hg resists the transfer of electrons and is preferred for metals more negative than H , because it reduces the hydrogen evolution so reductions can take place. This electrode is preferred for many electrochemical reactions. To platinize a Pt electrode, simply deposit a thin film of Pt on it, usually from a dilute chloroplatinic acid solution. 

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