Standard potential significance

The ultimate goal of PSM is improved safety performance. In addition, over and above the mandatory compliance with OSHA standards, there are potentially significant business benefits to implementing PSM, including improved efficiency, cost savings, and competitive advantages. 

Standard potentials Ee are evaluated with full regard to activity effects and with all ions present in simple form they are really limiting or ideal values and are rarely observed in a potentiometric measurement. In practice, the solutions may be quite concentrated and frequently contain other electrolytes under these conditions the activities of the pertinent species are much smaller than the concentrations, and consequently the use of the latter may lead to unreliable conclusions. Also, the actual active species present (see example below) may differ from those to which the ideal standard potentials apply. For these reasons formal potentials have been proposed to supplement standard potentials. The formal potential is the potential observed experimentally in a solution containing one mole each of the oxidised and reduced substances together with other specified substances at specified concentrations. It is found that formal potentials vary appreciably, for example, with the nature and concentration of the acid that is present. The formal potential incorporates in one value the effects resulting from variation of activity coefficients with ionic strength, acid-base dissociation, complexation, liquid-junction potentials, etc., and thus has a real practical value. Formal potentials do not have the theoretical significance of standard potentials, but they are observed values in actual potentiometric measurements. In dilute solutions they usually obey the Nernst equation fairly closely in the form ... 

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. 

This analogy to a surface redox mediated process is significant. In a way very similar to the reaction sequence (1.14), the standard potential of the redox surface system Pt(H20)/Pt-0Hads (0.80 V with respect to RHE) determines the active (reduced) site population at any cathode potential E, and consequently is the critical parameter in determining the ignition potential for the ORR process. 

Since such correlations belong to a series of treatments which are commonly identified as Linear Free Energy Relationships (LFER), and as only the standard potential is an electrochemical quantity directly linked with free energy (AG° = -n F AE°), one can make use of these mathematical treatments only in cases of electrochemically reversible redox processes (or in the limit of quasireversibility). Only in these cases does the measured redox potential have thermodynamic significance. 

There are no large differences between the reactivities of PhS , (EtO)2PO and CHjCOCHj" with the same aryl radical, but CN appears to be significantly less reactive. It is not easy to evaluate the respective role of the bond dissociation free energy and of the Nu-/Nu" standard potential in equation (13) in this connection because of the paucity of available data concerning these two quantities. An explanation of the low reactivity of CN" should thus await the availability of such data as well as that of a precise expression of the intrinsic barrier in a model of these intramolecular concerted electron-transfer-bond-breaking (or forming) reactions. 

The reaction of durene with H NOj under the conditions just mentioned is another specific but principally important case. The reaction is accompanied with a strong enhanced absorption in N NMR spectrum with respect to a signal belonging to the product, that is, nitrodurene. Durene and naphthalene have very similar standard potentials (2.07 and 2.08 V, respectively, in AN see Ridd 1991). A significant difference between them is that, with durene, much of the nitration supposedly arises from ipso attack followed by rearrangement according to Scheme 4.41. 

The use of AG as a criterion for deciding the direction of a chemical change has been briefly mentioned. AG"" refers to the reaction with both products and reactants in their standard states and industrial processes are, of course, often carried out under conditions far removed from standard. The significance of AG"" in relation to the technical feasibility of a reaction is illustrated in Table 1 where it is seen that AG = + 20 kJ mol" means an equilibrium constant of 3 x 10 at 298 K. With such a small equilibrium constant, a reaction is unlikely to be of much industrial value. At 2000 K, however, the same value of AG"" leads to an equilibrium constant of 0.3, potentially a very much more attractive situation. 

In an aqueous medium, where water is the only significant ligand, the halfreaction and standard potential for the cobalt(II)-cohalt(0) couple can be written... 

R. Parsons, The Single Electrode Potential Its Significance and Calculation and Standard Electrode Potentials Units, Conventions and Methods of Determination, in Standard Potentials in Aqueous Solution, A. J. Bard, R. Parsons, and J. Jordan, eds. Chs. 1 and 2, Marcel Dekker, New York (1985). 

When the total isolated trans fat levels are relatively low (below 10%), a potentially significant interference may be found in products containing conjugated unsaturation, e.g., 1 % in ruminant fat or 5% in tung oil (Firestone and Sheppard, 1992). This is due to the fact that conjugated trans,trans (near 990 cm ) and/or cis/trans (near 990 and 950 cm"1) double bonds exhibit absorption bands that are sufficiently close to 966 cm"1. This interference can be eliminated by applying a standard addition procedure (Mossoba et al., 2001 a). This procedure also applies to interferences from the food matrix. 

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