Electrode potentials, standard agents

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

Generally the solubility of a given metal halate decreases from chlorate(V) to iodatef and many heavy metal iodates(V) are quantitatively insoluble. Like their parent acids, the halates(V) are strong oxidising agents, especially in acid solution their standard electrode potentials are given below (in volts) ... 

Reductions. Hydrazine is a very strong reducing agent. In the presence of oxygen and peroxides, it yields primarily nitrogen and water with more or less ammonia and hydrazoic acid [7782-79-8]. Based on standard electrode potentials, hydrazine in alkaline solution is a stronger reductant than sulfite but weaker than hypophosphite in acid solution, it falls between and Ti ( 7). 

Oxidation Reactions. Potassium permanganate is a versatile oxidizing agent characterized by a high standard electrode potential that can be used under a wide range of reaction conditions (100,133—141). The permanganate ion can participate in a reaction in any of three distinct redox couples. 

As may be seen from the diagram, silver in highly alkaline solution corrodes only within a narrow region of potential, provided complexants are absent. It is widely employed to handle aqueous solutions of sodium or potassium hydroxides at all concentrations it is also unaffected by fused alkalis, but is rapidly attacked by fused peroxides, which are powerful oxidising agents and result in the formation of the AgO ion Table 6.6 gives the standard electrode potentials of silver systems. 

The standard electrode potential is a quantitative measure of the readiness of the element to lose electrons. It is therefore a measure of the strength of the element as a reducing agent in aqueous solution the more negative the potential of the element, the more powerful is its action as a reductant. 

To obtain comparative values of the strengths of oxidising agents, it is necessary, as in the case of the electrode potentials of the metals, to measure under standard experimental conditions the potential difference between the platinum and the solution relative to a standard of reference. The primary standard is the standard or normal hydrogen electrode (Section 2.28) and its potential is taken as zero. The standard experimental conditions for the redox... 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

From a practical viewpoint, the choice of the most appropriate redox agent follows the same criteria applied to the choice of the working potential in controlled potential electrolysis. The redox agent must possess a standard electrode potential 100-200 mV more positive... 

Thus, the overall reaction [Eq. (8.2)] is the outcome of the combination of two different partial reactions, Eqs. (8.4) and (8.5). As mentioned above, these two partial reactions, however, occur at one electrode, the same metal-solution interphase. The equilibrium (rest) potential of the reducing agent, E eq,Red [Eq. (8.5)] must be more negative than that of the metal electrode, E eq,M [Eq. (8.4)], so that the reducing agent Red can function as an electron donor and as an electron acceptor. This is in accord with the discussion in Section 5.7 on standard electrode potentials. 

Lacking a table of standard electrode potentials, or one that is adequate, what guidelines can be used to identify oxidizing and reducing agents, and to estimate their relative strengths Here are a few. 

Reaction (13.42) has a reversible electrode potential under standard conditions of 0.4033 V. This potential was calculated from standard Gibbs energy data, and its value indicates that iron can be used as the reducing agent. 

As indicated by the high value of its standard electrode potential (E° = 1.98 V) Ag2+ is a powerful oxidizing agent. When Ag+ is anodically oxidized in nitric acid [63,68], the dark-brown complex AgN03+ is formed ... 

The standard electrode potentials for all the rare earths have similar values and are comparable with the redox potentials of alkaline earth metals [144], Thus the lanthanides are strong reducing agents, and form trivalent ions easily. Both europium and samarium can exist in both trivalent and divalent states and the divalent states are not stable in aqueous solutions. Cerium can exist in both tetravalent and trivalent states in solution but Ce(III) is the most stable. 

The selectivity inherent to electrochemical detection is derived from the differences between the oxidation or reduction half-wave potentials exhibited by different analytes. Even when two or more analytes have nearly the same half-wave potentials, complexing agents or alterations in mobile-phase composition can be used to differentiate between analytes. In order to carry out the electrochemical quantitation of an analyte, the potential difference between the working microelectrode and the reference electrode is maintained at a value that lies on the plateau of the oxidation or reduction wave (voltamogram) of the analyte of interest. The diffusion current thus measured, which is due to the oxidation or reduction of the analyte, is proportional to the area under the analyte peak eluted. In order for absolute quantitation to be effected, the diffusion current of a standard sample of the analyte must also be measured for comparison with that of the unknown sample. 

Potassium dichromate solutions are indefinitely stable, can be boiled without decomposition, and do not react with hydrochloric acid. Moreover, primary-standard reagent is available commercially and at a modest cost. The disadvantages of potassium dichromate compared with cerium(IV) and permanganate ion are its lower electrode potential and the slowness of its reaction with certain reducing agents. 

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