Nernst chain

This remarkable influence is explained by Chapman f in terms of a degradation of the energy of the activated chlorine molecules by collision with oxygen molecules. On the basis of the Nernst chain hypothesis the effect can also be explained by supposing that the oxygen removes the atomic hydrogen by combining with it and thus interrupts the chain of reactions. J... 

The situation is different for a mixture of hydrogen with chlorine. The reaction mechanism is known—the Nernst chain subsidiary reactions with admixtures and recombinations become of secondary importance at high temperatures in a detonation wave ... 

Taken together, these results all verify the importance of the Nernst chain mechanism for the production of HCI, reactions (2) and (3), and the inhibitory influence of HCI and O2. Again it is well to keep in mind the complexity of this system owing to the chain character of the process which in turn arises because of the reactivity of Cl atoms. 

Zerdovich found for two reactions (hydrogen-chlorine and hydrogen-oxygen) the effect of the reaction mechanisms on the detonation velocity [530]. The first reaction follows a simple chain mechanism involving the basic alternating steps of the Nernst chain... 

Chain reactions ivere discovered around 1913, ivhen Bodenstein and Dux found that the reaction betv een H2 and CI2 could be initiated by irradiating the reaction mixture ivith photons. They ivere surprised to find that the number of HCl molecules per absorbed photon, called the quantum yield, is around 10 Nernst explained this phenomenon in 1918 the photon facilitated the dissociation of CI2 into Cl radicals (the initiation step), which then started the following chain process ... 

Therefore we expect Df, identified as the fast diffusion coefficient measured in dynamic light-scattering experiments, in infinitely dilute polyelectrolyte solutions to be very high at low salt concentrations and to decrease to self-diffusion coefficient D KRg 1) as the salt concentration is increased. The above result for KRg 1 limit is analogous to the Nernst-Hartley equation reported in Ref. 33. The theory described here accounts for stmctural correlations inside poly electrolyte chains. 

The energy required for the water dissociation can be calculated from the Nernst equation for a concentration chain between solutions of different pH values. It is given by ... 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

Fig. 3.1. A, The respiratory chain. Q and c stand for ubiquinone and cytochrome c, respectively. Auxiliary enzymes that reduce ubiquinone include succinate dehydrogenase (Complex II), a-glycerophosphate dehydrogenase and the electron-transferring flavoprotein (ETF) of fatty acid oxidation. Auxiliary enzymes that reduce cytochrome c include sulphite oxidase. B, Thermodynamic view of the respiratory chain in the resting state (State 4). Approximate values are calculated according to the Nernst equation using oxidoreduction states from work by Muraoka and Slater, (NAD, Q, cytochromes c c, and a oxidation of succinate [6]), and Wilson and Erecinska (b-562 and b-566 [7]). The NAD, Q, cytochrome b-562 and oxygen/water couples are assumed to equilibrate protonically with the M phase at pH 8 [7,8]. E j (A ,/ApH) for NAD, Q, 6-562, and oxygen/water are taken as —320 mV ( — 30 mV/pH), 66 mV (- 60 mV/pH), 40 mV (- 60 mV/pH), and 800 mV (- 60 mV/pH) [7-10]. FMN and the FeS centres of Complex I (except N-2) are assumed to be in redox equilibrium with the NAD/NADH couple, FeS(N-2) with ubiquinone [11], and cytochrome c, and the Rieske FeS centre with cytochrome c [10]. The position of cytochrome a in the figure stems from its redox state [6] and its apparent effective E -, 285 mV in...

The first coordination shell of Fe in Fib is identical to that of Mb, but there are other major differences that influence the spectroelectrochemical/Nernst plot profile of Hb and make it distinct from that of Mb. Hb is a tetrameric protein with four heme-containing subunits (a2p2), each of which is redox-active. Differences between Mb and Hb include amino-acid sequence, which results in different redox potentials for the a and (3 chains, plus subunit-subunit interactions that lead to allostery in Hb. This allosteric interaction leads to cooperative electron transfer that gives rise to a non-Nernstian redox profile that requires special consideration for data analysis and interpretation of results. ... 

Carbon activities in alkali metals are also estimated by electrochemical meters. These are based on the activity differences between two carbon bearing electrodes separated by a carbon ions conducting electrolyte. The electrolyte is a molten salt mixture, consisting of the eutectic of lithium and sodium carbonate, melting at approximately 500 °C. The molten salt mixture has to be kept free from any impurities or humidity. The mixture, acting as liquid electrolyte is kept in an iron cup. The iron wall is in contact with both the liquid electrolyte and the liquid metal. Thus, it exchanges carbon with both up to the equilibrium. Iron, with the same carbon potential as the liquid metal, acts as one electrode. The reference electrode of graphite or any other material with a well defined and stable carbon activity is immersed in the molten electrolyte. The Nernst equation defines the potential of the electrochemical chain ... 

CI2. The free radical mechanism was clarified by Nernst It is initiated by chlorine free radical (Cl). Each cycle of the mechanism starts with one Cl and regenerates one Cl. The chain-propagating steps for the reaction of hydrogen and oxygen include the following ... 

Nernst (1918) suggested that free radicals take part in chemical reactions and postulated a radical chain mechanism for the combination of H2 and CI2. Paneth and coworkers (1929) first demonstrated the existence of alkyl free radicals by decomposition of metal alkyls. Norrish (1931) suggested that free radicals could occur as intermediates in the photolysis of carbonyl compounds. Rice and Herzfield (1934) produced radicals from the dissociation of hydrocarbons. Up until relatively recently, radicals were regarded as highly reactive species, whose reactions were unselective and difficult to control (remember the radical chlorination of methane). The last 20 years have seen the field developed to such an extent that it is now recognized that radicals can take part in highly useful and selective reactions. 

Fluctuations in ion density near a polyelectrolyte impose fluctuating electrostatic forces on the chain that can affect the chain s diffusion. As described by Sedl (207), polyelectrolyte diffusion in dilute solution is fast diffusion because coim-terions cause more rapid motion in a polyelectrolyte than would be noted in an equivalent neutral chain. Several investigators have modified equation 50 to include this Nernst-Hartley-type diffusion in a consistent manner (203,206). Tivant and co-workers suggested that in a monovalent electrolyte solution at large I the enhancement of Dm by polyelectrolyte-electrolyte coupling should be expressed as(208)... 

We shall treat each of these four contributions separately in the following sections, along with discussions of their effects in the context of experimental results. The same mathematical procedures described in Chapters 6 and 8, namely, the Fokker-Planck formalism, first passage times, Poisson-Nernst-Planck formalism, and the Goldman-Hodgkin-Katz equations, are implemented (Muthukumar 2010) to obtain the steady-state flux of the polymer chains and the probability of successful barrier crossing. 

When Nk is small this mean value is not very different from the geometric mean. The boundary values are readily determined from the initial value P(t=0) and the steady state voltage using the Nernst equation (see page 446) . The above equation is also useful if conductivity measurements are carried out in the stationary state of a chemical polarization and can be used to correct the first order approximation in the case of chains including a reversible and a blocking electrode [555]. 

---