Ferrocyanide oxidation kinetics

Example 16.3-2 The rate of ferrocyanide oxidation We are studying electrochemical kinetics using a flat platinum electrode 0.3 cm long immersed in a flowing aqueous solution. In one series of experiments, the solution is 1-M KCl containing traces of potassium ferrocyanide. The ferrocyanide is reduced by means of the reaction... 

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 aqueous ferricyanide oxidation of 2-mercaptoethanol to the disulphide is also complex kinetically" . In the pH range used (l.S. l) no complication from ionisation of the thiol is expected. Individual decays of oxidant concentrations are initially second-order but eventually become almost zero-order. For both second-and zero-order paths the rate depends on the first power of the thiol concentration and the former path is retarded by increasing the acidity, an approximately inverse relation existing above pH 3.2. Addition of ferrocyanide transforms the kinetics the rapid, second-order path is inhibited and the zero-order path is accelerated until, at 10 M ferrocyanide, the whole of the disappearance of oxidant is zero-order. Addition of Pb(C104)2, which removes product ferrocyanide, greatly enhances the oxidation rate and the consumption of oxidant becomes rs/-order. Two routes are considered to co-exist (taking due account of the acidity of ferrocyanic acid), viz. 

Keith WG, Powell RE (1969) Kinetics of decomposition of peroxynitrous acid. J Chem Soc (A) 90 Keyer K, Imlay JA (1997) Inactivation of dehytratase [4F3-4S] clusters and disruprion of iron home-stasis upon cell exposure to peroxynitrite. J Biol Chem 272 27652-27659 Khaikin Gl, Alfassi ZB, Huie RE, Neta P (1996) Oxidation of ferrous and ferrocyanide ions by peroxyl radicals. J Phys Chem 100 7072-7077... 

Relaxation Kinetics. The details of the experimental procedure have been described earlier (14). 0.1 M phosphate buffer, pH 7.0, containing 2 X 10"5 M EDTA was used in all relaxation experiments. These were performed with solutions of different initial reagent composition— either ferrocyanide was added to oxidized azurin or ferricyanide to reduced azurin. Temperature jumps of 2.9° or 4.7° were applied to the reaction solution. The subsequent transmission changes were monitored at 625 nm (absorption of oxidized azurin) or 420 nm (absorption of ferricyanide ). Each plotted value of the relaxation time or amplitude represents the average of at least four measurements. 

As a final example, we should perhaps look at the oxidation and reduction of the ferri/ferrocyanide couple, which is often used as a "test reaction" in electrode kinetics ... 

Gerken, Dekker, Schlodder and Witt S utilized the same protocols that were used for isolating the AA(Qa -Qa) difference spectrum to indirectly obtain the difference spectrum just for the oxidation of the secondary donor Y, i.e., AA[Yz -YzJ. To isolate the difference spectrum, AA(Qa -Qa)5 electron transfer in the PS-II complex was blocked between Yz and P680 by Tris treatment and after Qa by DCMU. A ferri-/ferro-cyanide couple was added to the reaction mixture to intervene with oxidized Yz and reduced Qa following photo-excitation. Then, after a certain time after the flash, say, at 800 ms, the oxidized Yz would have been completely re-reduced by ferrocyanide rather than by the Mn-cluster, but Qa re-oxida-tion by ferricyanide would have occurred to a slight extent due to slower kinetics. When the absorbance... 

In most amperometric cytochrome b2 electrodes the reaction is followed by anodic oxidation of ferrocyanide at a potential of +0.25 V or above. The first of such sensors was assembled by Williams et al. (1970), who immobilized the enzyme (from baker s yeast) physically at the tip of a platinum electrode within a nylon net of 0.15 mm thickness. The large layer thickness resulted in a response time of 3-10 min. Owing to the low specific enzyme activity used, the sensor was kinetically controlled. Therefore the linear measuring range extended only up to 0.1 Km-A similar sensor has been applied by Durliat et al. (1979) to continuous lactate analysis. The enzyme was contained in a reaction chamber of 1 pi volume in front of the electrode. This principle has also been employed in the first commercial lactate analyzer using an enzyme electrode (Roche LA 640, see Section 5.2.3.3X With a sensor stability of 30 days and a C V below 5%, 20-30 samples/h can be processed with this device. 

Kinetic studies of reactions of horseradish peroxidase (HRP) using stopped-floiv and temperature-jump techniques are summarized. The reactions were studied intensively as a function of pH to establish the presence or absence of pH-dependences in the reaction rates. Minimum mechanisms are presented which cannot he proven to be correct. However, simpler mechanisms will not fit the data within experimental error. The reactions which have been studied are fluoride and cyanide binding by (dissociation from) HRP and the oxidation of ferrocyanide to ferricyanide by HRP Compounds I and II. From the pH profiles of the reaction rates, the pK values of acid groups which influence the rates are deduced. Trends in pK values can be explained qualitatively in terms of electrostatic effects. 

These studies led us to begin kinetic investigations of the enzymatic reaction of HRP the first of these is the oxidation of ferrocyanide. 

With the exception of a study carried out with a partially characterized multicopper oxidase isolated from tea leaves (85), there has been very little detailed work concerned with the steady state kinetic behavior of laccases. Early work on the transient kinetics indicated, however, that (1) enzyme bound Cu + was reduced by substrate and reoxidized by O2, and (2) substrate was oxidized in one-electron steps to give an intermediate free radical in the case of the two electron donating substrates such as quinol and ascorbic acid. The evidence obtained suggested that free radicals decayed via a non-enzymatic disproportionation reaction rather than by a further reduction of the enzyme (86—88). In the case of substrates such as ferrocyanide only one electron can be donated to the enzyme from each substrate molecule. It was clear then that the enzjmie was acting to couple the one-electron oxidation of substrate to the four-electron reduction of oxygen via redox cycles involving Cu. 

The kinetics of the oxidation of ferrocyanide by compounds I and II from horse-radish peroxidase (HRP-I and HRP-II) and of iodide by HRP-II have been reported. In the latter case the rate constant decreases linearly from 2.3 x 1(P to 0.11 mol s with increasing pH over the... 

The rate of electron transfer for basal plane sites has been reported to correspond to 10 cm s for the oxidation of ferrocyanide and is considered to be possibly even zero [4-6]. How does one know that this is actually correct As shown in Fig. 3.9a, a strangely distorted voltammogram would be observed in the limit of very low defect density [23]. Due to the fact that two peaks have never been observed experimentally, it is generally accepted that edge plane electron transfer kinetics are anomalously faster over that of basal plane the latter is sometimes referred to as being inert [5,6,23]. Interested readers are directed to the elegant work of Davies et al. and Ward et al. to further appreciate this work [5,24]. 

Xiong, L., Batchelor-McAuley, C., Ward, K. R. et al. 2011. Voltammetry at graphite electrodes The oxidation of hexacyanoferrate (11) (ferrocyanide) does not exhibit pure outer-sphere electron transfer kinetics and is sensitive to pre-exposure of the electrode to organic solvents. J. Electroanal. Chem. 661 144—149. 

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