Redox potential monitoring

Material (or Component) Potential and Process Redox Potential Monitoring... 

Material (or Component) potential and process redox potential monitoring... 

Various methods have been used to determine the redox potentials (Table XI). Very commonly, EPR-monitored chemical redox titration is performed, which can be used to measure the redox potential not only in isolated complexes but also in membrane preparations. In general, there is good agreement between redox potentials determined in membranes, isolated complexes, or isolated Rieske proteins or fragments the only exception is the water-soluble Rieske fragment from spinach bef complex where differences of more than 50 mV have been observed by the same group but using different methods (31). 

The enzyme poised at well-defined redox potentials appears to be in rather homogenous IR states 65, 84). Curiously, in corresponding EPR-monitored experiments, the Ni signal generally corresponds to significantly less than one spin/mole, indicating that the sample is heterogeneous with respect to it (77). 

The one-electron reduction of the Ni-C form results in the diamagnetic species Ni-R. From the redox titration studies of Lindahl s group, a plausible catalytic cycle can be postulated where the enzyme in the Ni-Sl state binds H2 (77) and becomes the two-electron more reduced Ni-R state. Sequential one-electron oxidations from Ni-R to Ni-C and then to Ni-Sl will close the cycle (Fig. 6). The various redox states differ not only in the extent of their reduction, but also in their protonation, as shown by the pH dependence of their redox potentials (87). It is remarkable that both EPR (which monitors the magnetic... 

Redox titrations monitored by visible and EPR spectroscopies show that the clusters have very different midpoint redox potentials approximately 0 mV for center I, and < - 300 mV for center II (139). 

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

It has not been possible to monitor the DO at these levels but its control can be achieved indirectly from measuring the redox potential. At 1% of saturation the redox potential remains about 0 to 50 mV Ecal (pH 7.8). There is an exponential fall in redox potential as DO falls to zero at about -400 to -450 mV Ecal (pH 7.8). 

The presence or absence of Ca + ions in one or both sites also appears to effect the reduction potential of the high-potential heme. In equilibrium redox titrations monitored spectroscopically, done in the presence of Ca + ions, this is shifted positive by about 50 mV PP = - - 226 mV) compared with titrations done in the presence of a chelator (IP = -1-176 mV) (52). This former value is close to the reduction potentials reported for the high-potential heme in the CCP from P. aeruginosa (51), but about 200 mV lower than reported for the high-potential hemes in the enzymes from N. europea (46) and Methylococcus capsulatus Bath (80). In contrast, the reduction potential of the peroxidatic heme is unaffected by the presence or absence of Ca + ions (16, 52). 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

The value of the critical nuclearity allowing the transfer from the monitor depends on the redox potential of this selected donor S . The induction time and the donor decay rate both depend on the initial concentrations of metal atoms and of the donor [31,62]. The critical nuclearity corresponding to the potential threshold imposed by the donor and the transfer rate constant value, which is supposed to be independent of n, are derived from the fitting between the kinetics of the experimental donor decay rates under various conditions and numerical simulations through adjusted parameters (Fig. 5) [54]. By changing the reference potential in a series of redox monitors, the dependence of the silver cluster potential on the nuclearity was obtained (Fig. 6 and Table 5) [26,63]. 

Redox potentials for copper systems have been based on a variety of approaches including (i) redox titrations, (ii) potentio-static methods involving spectral monitoring, (iii) cyclic voltammetry (CV), and (iv) pulsed methods. Of these, CV measurements are by far the most prevalent. No effort has been made in this treatise to identify the method used for a specific reported potential value unless the method itself appeared to be pertinent. 

Oxidation-Reduction Potential Oxidation-reduction potential (ORP) is measured by an ORP probe, which is effective to monitor the redox potential of a bioreactor operated under microaerobic conditions that cannot be successfully measured by a DO probe. The measurements of redox are sometimes influenced by changes in the pH and mineral concentrations of a culture broth. 

Controls and monitors liquid level, pH, dissolved oxygen, reduction-oxidation (Redox) potential, air rate, temperature, optional automatic sterilization cycle control, rupture disk on vessel, relief valve on jacket... 

In addition to providing redox potentials, cyclic voltammetry can provide a tremendous amount of information about the reactions occurring prior or subsequent to the actual redox processes. This is obtained by monitoring the change in, for example, peak potentials and/or peak currents with scan rate and also by noting the appearance or disappearance of peaks with variation in scan rate or when the potential range is scanned successively. 

Nonetheless, there are running plants at laboratory and pilot-scale levels at institutes/universities and industry where process control is already exerted. Usually this is done in a rather conventional fashion, e.g. using commercial pressure hold valves and temperature determination at the in- and outlets and process-specific concentration monitoring outside the micro reactor. For example, an analysis of the redox potential was used for process monitoring for continuous azo pigment production at Clariant (see Figure 4.68) [99],... 

The detection of (specifically) dopamine is hindered by the presence in the extracellular fluid of several compounds having redox potentials close to that of dopamine. The technique most likely to succeed here is fast scan cyclic voltammetry (Section 8.6) because the voltamogram provides characteristics that are indicative of the individual compound being monitored. The microelectrodes used have radii of 5 pm, but even this is not small enough to be able to determine dopamine from just one cell. The reacting compounds come from several nerve endings. Nevertheless, the fast scan cyclic voltammetry technique her sufficient time and resolution to allow information to be obtained on the part played by dopamine in neurotransmission in the brain. For example, it answers such questions as does the released dopamine stay at the synapse or does it diffuse in the extracellular fluid to contact other neurons ... 

NMR can help to monitor energization, see, e.g. [121, 273], especially the levels of 31P-containing metabolites, e.g. [45, 366], enzyme kinetics, compartmentalized intracellular ion activities, the fate of 3H-, 2H-, 13C-, 15N-, or 19F-labeled tracers, e.g. [108,109], 02 tension, compartmentalized redox potential, membrane potential, cell number or cell volume, see [133], and even pH. Major drawbacks are the cost of the equipment, the low intrinsic sensitivity and the interpretation of spectra [430]. 

To monitor process streams on line. Erratic fluctuations of pH and redox potential might serve as alert signals for anomalous sulfur speciation. 

---