Rate constants ferrocyanide

Hydroxyl radicals. The acid ionization constant of the short-lived HO transient is difficult to determine by conventional methods but an estimate can be made because HO, but not its conjugate base, O -, oxidizes ferrocyanide ions HO + Fe(CN) — OH- + Fe(CN)g . Use the following kinetic data26 for the apparent second-order rate constant as a function of pH to estimate Ka for the acid dissociation equilibrium HO + H20 =... 

The non-steady-state optical analysis introduced by Ding et al. also featured deviations from the Butler-Volmer behavior under identical conditions [43]. In this case, the large potential range accessible with these techniques allows measurements of the rate constant in the vicinity of the potential of zero charge (k j). The potential dependence of the ET rate constant normalized by as obtained from the optical analysis of the TCNQ reduction by ferrocyanide is displayed in Fig. 10(a) [43]. This dependence was analyzed in terms of the preencounter equilibrium model associated with a mixed-solvent layer type of interfacial structure [see Eqs. (14) and (16)]. The experimental results were compared to the theoretical curve obtained from Eq. (14) assuming that the potential drop between the reaction planes (A 0) is zero. The potential drop in the aqueous side was estimated by the Gouy-Chapman model. The theoretical curve underestimates the experimental trend, and the difference can be associated with the third term in Eq. (14). 

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

Second-Order Rate Constants for the Oxidation of Ferrocenes and Ferrocyanide by Compounds I (k6) and II (k7) and for the HRP-Catalyzed Steady-State Oxidation ( ) at [H202] 2.4 x 1 4 M (pH 6, 25 °C)... 

Pulsed-current techniques can furnish electrochemical kinetic information and have been used at the RDE. With a pulse duration of 10-4 s and a cycle time of 10-3 s, good agreement was found with steady-state results [144] for the kinetic determination of the ferri-ferrocyanide system [260, 261], Reduction of the pulse duration and cycle time would allow the measurement of larger rate constants. Kinetic parameter extraction has also been discussed for first-order irreversible reactions with two-step cathodic current pulses [262], A generalised theory describing the effect of pulsed current electrolysis on current—potential relations has appeared [263],... 

Fig. 2.3. Plot of the second order rate constants for homogeneous oxidation of NADH by a variety of different mediators as a function of their electrode potential. The graph uses results taken from work by Miller and coworkers [21-26] and from [28]. The different mediator groups are ( ) aminopyrimidines (A) 1,4-diaminobenzenes (A) 1,2-diaminob-enzenes ( ) o-quinones (O) p-quinones ( ) ferrocenes (+) ferrocyanide and (O) heteropolyanions.

At the pH attained here (around 10), the reduction of ferri-cyanide to ferrocyanide is supposed to be complete and fast, and the value of the related rate constant could be estimated 3 moHdm s". ... 

Altschul et al. (1, 2) originally discovered that cytochrome c peroxidase reacts with a stoichiometric amount of hydroperoxide to form a red peroxide compound, which will be referred to hereafter as Compound ES. It has a distinct absorption spectrum, as shown in Fig. 2. The formation of Compound ES from the enzyme and hydroperoxides is very rapid (fci > 10 10 sec"M. No intermediate, which precedes Compound ES, has been thus far detected. In the absence of reductants, or S2, Compound ES is highly stable. The rate constant of its spontaneous decay is of the order of 10 sec 22). The primary peroxide compound (Compound I) of horseradish peroxidase decays much faster at a rate of 10 sec (6). This unusual stability of Compound ES allows one to determine various physical and chemical parameters quantitatively and reliably. Titrations of Compound ES with reductants such as ferrocjHio-chrome c Iff, 20) and ferrocyanide 18, 34) have established that Compound ES is two oxidizing equivalents above the original ferric nnzyme. The absorption spectrum of Compound ES is essentially identical to that of Compound II of horseradish peroxidase which contains one oxidizing equivalent per mole in the form of Fe(IV). In addition, EPR examinations have revealed that Compound ES contains a stable free radical, the spin concentration of which is approximately one equivalent per mole (Fig. 3). Therefore, it is reasonable to conclude that two oxidiz-... 

The oxidized form of superoxide reductase formed in this reaction is reduced back by rubredoxin, dependent ultimately on reduced pyridine nucleotides via intermediate electron carriers [65]. The reaction of SOR with superoxide is also very fast, the reaction rate constant being of an order of 10 M s. It has been demonstrated that CuZnSOD can also function as superoxide reductase reducing superoxide at the expense of oxidation of ferrocyanide, or as superoxide oxidase, oxidizing superoxide at the expense of reducing ferricyanide [66]. Both ferri- and ferrocyanide are unphysiological substrates but the enzyme can also act as superoxide reductase with nitroxyl anion oxidizing it to nitric oxide [67]. 

Figure 20. Potential dependence of the forward and reverse rate constants for electron transfer between TCNQ and ferrocyanide. (Reprinted with permission from Ref. 164 copyright 2001, Elsevier SA.)...

In the ferricyanide-formate-02 solutions and ferricyanide-ethyl alcohol—02 systems, Gred is derived by assuming that all the OH radicals react with the organic solute and that none react with the ferrocyanide. These systems are further complicated by the fact that OH radicals can convert into O" radicals, whose rate constants with the solutes are not known. 

Table 3.2. Second-Order Rate Constants for the Reduction of Arenediazonium Tetrafluoroborate Salts by Potassium Ferrocyanide and Decamethylferrocene ...

Two reportshave appeared recently on the Landolt oscillating reaction. Where the oxidation of sulfite and ferrocyanide by iodate takes place in a continuously stirred reaction vessel, large-amplitude oscillations in pH at constant [I ] are observed. The nature of the intermediates and elementary steps have been discussed together with the detailed mechanistic profile. The overall processes may be described as shown in equations (31)-(37). Individual rate constants for the rate-determining steps in the above reactions have been identified. 

Once again a good deal of bromine chemistry is covered in the section on oscillatory reactions. The species BrJ may be involved in olefin brominations at high concentrations of bromine. The kinetics and equilibria of reaction (58) have been studied, and rate constants for (59) and (60) evaluated. The mechanism of oxidtion of ferrocyanide by bromate involves [Fe(CN)sH20] " as an intermediate. The uncatalyzed and the vanadium(V) catalyzed reaction of bromate with methylene blue have been reported. ... 

B served as a titrant for C and the change of the collector current compared to the collector current in the absence of C was an indicator of (1) the amount of C present in solution and (2) the rate constant of reaction (12.137). Rajantie et al. [328] used ferrocyanide as a titrant which was generated from ferricyanide galvanostatically and detected at the collector band amperometrically. The analyte was ascorbic acid. Good agreement was found between simulation and experiment. 

Calculations of electron transfer rate constants from the energies of intervalence transfer (IT) bands in mixed-valence complexes have provided some interest (Table 1.1). The ion pair formed from paraquat (l,r-dimethyl-4,4 -bipyridine " ), and ferrocyanide, [PQ ", Fe(CN)6 ], shows an IT band from which the activation energy for thermal electron transfer within the ion pair can be derived (Figure 1.1) using Hush s theory to compare spectroscopic and kinetic data [equation (5)]. 

The question of what might be termed outer-sphere isomerism arises in the reaction [Co(NHg)6py] + + [Fe(CN)6]. In the ion pair the pyridine ligand may be directed towards or away from the ferrocyanide complex, and the more stable of the two arrangements is not necessarily the most favourable for electron transfer. The fact that insertion of substituents into the pyridine ring makes little difference to the first-order rate constant is taken to imply that the effective configuration has the ring in the remote position. ... 

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