Hydrogen, activation potential

Fig. 4. Catalytic activities of metals (as potentials measured at 10-4 A.cm-2) for anodic oxidation of different reductants. Er thermodynamic oxidation-reduction potentials of reductants. H2 reversible hydrogen electrode potential in solution used to study oxidation of each reductant. Adapted from ref. 38.

Quantum mechanical approaches have been successfully used to predict hydrogen abstraction potentials and likely sites of metabolism of drug molecules [78-81]. AMI, Fukui functions, and density functional theory calculations could identify potential sites of metabolism. Activation energies for hydrogen abstraction were calculated by Olsen et al. [81] to be below 80 kj/mol, suggesting most CH groups can be metabolized which particular one depends on steric accessibility and intrinsic reactivities. 

We measure the acidity or alkalinity of a substance by using the pH scale. pH stands for potential of hydrogen, which makes sense because acidity has to do with hydrogen activity. Acids and bases are on opposite sides of the pH scale. An acid has a pH between 0 and 7, and a base has a pH between 7 and 14. A pH of 7 is neutral.The lower the number, the more hydrogen ions there are and the more acidic the substance is. The higher the number, the more basic, or alkaline, the substance is. 

Thus, a more appropriate question to ask is Is it possible to measure the absolute potential of the hydrogen reaction, /H+(abs) Actually it is possible. Remembering the definition of a standard hydrogen electrode potential (see Section 6.3.4), this was defined as the potential obtained when a metal comes in contact with a solution containing H+ under thermodynamically reversible conditions at unit activity, and H2 at 1 atm, at 298 K. As to the identity of the metal base, it can in principle be any metal at which it is possible to observe the reaction H2 H+ + e taking place at equilibrium. In practice, the metals used as substrates can only be noble metals because most other metals enter into equilibria with their own species in solution. Usually platinum is the metal chosen. 

Electronic interactions between catalytic species and support have been reported to increase the hydrogenation activity (52). Carbon, titania, and zirconia may have potential in this area and should be considered for further investigation (124). 

The brackets indicate the molar concentrations of the various molecular species. The empirical quantity a is defined by pH = —log a. In sea water, pH measurements do not yield a thermodynamic hydrogen ion activity due to liquid junction and asymmetry potentials a only approximates the hydrogen activity an+. For sea water of 33%>o salinity at 20 °C and at pH 8, 87% of the inorganic phosphate exist as HPO4-, 12% as PO4-, and 1% as H2POj. Of the PO - species 99.6% is complexed with cations other than Na+. The equilibrium relationship for the system is shown in Fig. 15. 

TABLE 8.4 Activation Potentials and Current Sensitivities for the Voltanunetric Oxidation of Dissolved Hydrogen at Activated Platinum Electrodes in Five Solvents (Scan Rate 0.1 V s 1) Solubilities and Diffusion Coefficients for H2 in Several Solvents... 

In electrochemistry we have customarily employed, instead of the absolute electrode potential / abs scale, a relative scale of the electrode potential, E yila scale, referred to the standard or normal hydrogen electrode potential E m at which the hydrogen electrode reaction, 2H + 2e dox = H2(gas), is at equilibrium in the standard state unit activity of the hydrated proton, the standard pressure of 101.3 kPa for hydrogen gas, and room temperature of 298 K. Since Eniie is + 4.44 V (or + 4.5 V) in the absolute electrode potential scale, we obtain Eq. 9.9 for the relation between abs scile and [Refs. 4 and 5.] ... 

When working with hydrogen at a pressure of 1 atm., In pH2 = 0 and the hydrogen electrode potential only depends on the activity of the hydrogen ions in the solution ... 

Many of the classical electrolytic reactions occur at a potential which is either more negative (reduction) or more positive (oxidation) than the decomposition potentials of the media. The mechanism of such reactions must be investigated in each case, but it can usually be classified as one of the following three cases (1) a direct electron transfer from electrode to substrate (A2), (2) a formation of solvated electrons which, in turn, reduoe the substrate (Bl), or (3) a formation of an active species in the electrochemical step (adsorbed hydrogen, active metals, hydrogen peroxide, hydroxyl radicals, halogens, etc.) which reacts chemically with the substrate (B2). 

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