Redox pairs

Acrylamide is polymerized by the conventional free radical initiators, e.g., peroxides [27,28], redox pairs [29-33], and azo compounds [34]. Electro-chemical initi-... 

Detailed kinetic studies in connection with digital simulations do confirm the RR coupling mechanism postulated in older publications as well as the oxidation of the resulting dimer D to the dication D. But the surprising drop in the height of the reduction wave for the redox pair as the concentration... 

If a system is not at equilibrium, which is common for natural systems, each reaction has its own Eh value and the observed electrode potential is a mixed potential depending on the kinetics of several reactions. A redox pair with relatively high ion activity and whose electron exchange process is fast tends to dominate the registered Eh. Thus, measurements in a natural environment may not reveal information about all redox reactions but only from those reactions that are active enough to create a measurable potential difference on the electrode surface. 

The UPD of metals onto the surface of chalcogens can be performed not only electrochemically but chemically, i.e., through the use of appropriate redox pairs in solution. Such an approach has been demonstrated by the electroless UPD of bismuth(III) onto the Te surface using the titanium(III)/(IV) redox pair in aqueous solution [96]. 

The methano-dimer of a-tocopherol (28)50 was formed by the reaction of o-QM 3 as an alkylating agent toward excess y-tocopherol. It is also the reduction product of the furano-spiro dimer 29, which by analogy to spiro dimer 9 occurred as two interconvertible diastereomers,28 see Fig. 6.23. However, the interconversion rate was found to be slower than in the case of spiro dimer 9. While the reduction of furano-spiro dimer 29 to methano-dimer 28 proceeded largely quantitatively and independently of the reductant, the products of the reverse reaction, oxidation of 28 to 29, depended on oxidant and reaction conditions, so that those two compounds do not constitute a reversible redox pair in contrast to 9 and 12. 

E° = standard potential (mV) of the redox pair (under the experimental conditions)... 

Oxi] = concentration (mol/1) of the oxidised form of the redox pair [Red] = concentration (mol/1) of the reduced form of the redox pair n = electrochemical valency. 

If the concentrations [Oxi] and [Red] are identical, the potential prevailing in solution is identical to the standard potential E° of the redox pair under the conditions of use. If the ratio of these concentrations [Oxi]/[Red] is modified by electrolysis, the potential changes accordingly ... 

A detailed theoretical study of the properties of the redox system FeS/FeS2 was carried out in the Department of Geosciences of SUNY Stony Brook (Schoonen et al., 1999). The authors conclude that the hypothetical reduction of CO2 (by the FeS/FeS2 redox pair) formulated in Wachtershauser s early work, and the carbon fixation cycle on the primeval Earth associated with it, probably could not have occurred. This judgement is made on the basis of a theoretical analysis of thermodynamic data other conditions would naturally have been involved if CO had reacted rather than C02. It is not known whether free CO existed in the hydrosphere, or if so, at what concentrations. 

Back electron transfer takes place from the electrogenerated reduc-tant to the oxidant near the electrode surface. At a sufficient potential difference this annihilation leads to the formation of excited ( ) products which may emit light (eel) or react "photochemical ly" without light (1,16). Redox pairs of limited stability can be investigated by ac electrolysis. The frequency of the ac current must be adjusted to the lifetime of the more labile redox partner. Many organic compounds have been shown to undergo eel (17-19). Much less is known about transition metal complexes despite the fact that they participate in fljjany redox reactions. 

The choice of new complexes was guided by some simple considerations. The overall eel efficiency of any compound is the product of the photoluminescence quantum yield and the efficiency of excited state formation. This latter parameter is difficult to evaluate. It may be very small depending on many factors. An irreversible decomposition of the primary redox pair can compete with back electron transfer. This back electron transfer could favor the formation of ground state products even if excited state formation is energy sufficient (13,14,38,39). Taking into account these possibilities we selected complexes which show an intense photoluminescence (0 > 0.01) in order to increase the probability for detection of eel. In addition, the choice of suitable complexes was also based on the expectation that reduction and oxidation would occur in an appropriate potential range. 

At a terminal ac voltage of 4 V and a frequency of 30 Hz we observed a weak eel which was clearly identified as the IL emission of Pt(QO)2. It is+assumed that the ac electrolysis generates a redox pair Pt(QO)2 Pt(QO)2. The subsequent annihilation leads to the formation of electronically excited Pt(QO)2. The low eel intensity may be associated with the observation that the electrochemical oxidation and reduction of Pt(QO)2 is largely irreversible. CV measurements revealed an oxidation at E1. -... 

The complex Tb(TTFA) (o-phen) underwent a reduction at E --1.5 V vs. SCE which was partially reversible. An oxidation was not observed below +2 V. All redox reactions should be ligand-based processes. The potential difference of Ae > 3.5 V is energy sufficient to generate the IL triplet at 2.56 eV. The low eel intensity could be due to a competing irreversible decay of the primary redox pair. 

Usually we know the properties of the electron donor, and we want to use EPR spectroscopy to determine those of the acceptor only. So we write down the Nemst equation for a single redox pair (replacing B with X) ... 

A flexible method for modeling redox disequilibrium is to divide the reaction database into two parts. The first part contains reactions between the basis species (e.g., Table 6.1) and a number of redox species, which represent the basis species in alternative oxidation states. For example, redox species Fe+++ forms a redox pair with basis species Fe++, and HS- forms a redox pair with SO4. These coupling reactions are balanced in terms of an electron donor or acceptor, such as 02(aq). Table 7.1 shows coupling reactions from the llnl database. 

The modeler controls which redox reactions should be in equilibrium by interactively coupling or decoupling the redox pairs. For each coupled pair, the model uses the corresponding coupling reaction to eliminate redox species from the reactions in the database. For example, if the pair Fe+++-Fe++ is coupled, the model adds the coupling reaction to the reaction for hematite,... 

Models of natural waters calculated assuming redox disequilibrium generally require more input data than equilibrium models, in which a single variable constrains the system s oxidation state. The modeler can decouple as many redox pairs as can be independently constrained. A completely decoupled model, therefore, would require analytical data for each element in each of its redox states. Unfortunately, analytical data of this completeness are seldom collected. 

The SPECE8 input script below describes the analysis of a hypothetical ground-water, assuming equilibrium with ferric hydroxide and a soil gas in which fco2 = 10-2. In the script, we decouple a number of redox pairs so that we can constrain the amounts of several elements in two or more redox states. 

The potential of selected half-reactions listed in order of increasing potential are shown in Table 2.1. This table is often referred to as the electron tower. A low potential means that there is a high tendency to produce electrons (oxidation), and a high potential means a corresponding relative low tendency, i.e., preference for reduction. As an example, if the redox pair number (3) in Table... 

Table 2.1 states the redox relations at standard conditions. Extended information on the distribution of the redox pairs — still under equilibrium conditions but under varying redox potential and pH — is given in a Pourbaix diagram. Figure 2.4 is an example of such a diagram for the binary sulfur and oxygen system in water at 1 atm and 25°C with the sum of the concentrations of... 

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