Hydrogen oxidation-reduction potential

Direct reaction of oxygen with most organic materials to produce radicals (eq. 13) is very slow at moderate temperatures. Hydrogen-donating antioxidants (AH), particularly those with low oxidation—reduction potentials, can react with oxygen (eq. 14), especially at elevated temperatures (6). 

Fig. 4. Catalytic activities of metals (as potentials measured at 10-4 A.cm-2) for anodic oxidation of different reductants. Er thermodynamic oxidation-reduction potentials of reductants. H2 reversible hydrogen electrode potential in solution used to study oxidation of each reductant. Adapted from ref. 38.

The oxidation-reduction potential or redox potential ( h) is a measure of the tendency of a solution to be oxidizing or reducing. Oxidation and reduction are basically electrical processes that are readily measiued by an electrode potential. All measurements are referred to die standard hydrogen electrode, the potential of which is taken as 0.00 V at 298 K, the H2 pressure as 101325 N/m (1 atm) and activities of H2 and as unity. When the half-cell reaction is written as an oxidation reaction ... 

The mechanism(s) by which lactic acid bacteria inhibit or inactivate other bacteria is not totally clear. Daly et al. (1972), Speck (1972), and Gilliland and Speck (1972) have cited evidence which suggests that the following may be involved (1) production of antibiotics such as nisin, diplococcin, acidophilin, lactocidin, lactolin, and perhaps others (2) production of hydrogen peroxide by some lactic acid bacteria (3) depletion of nutrients by lactic acid bacteria, which makes growth of pathogens difficult or impossible (4) production of volatile acids (5) production of acid and reduction in pH (6) production of D-leucine and (7) lowering the oxidation-reduction potential of the substrate. 

Oxidative coenzymes with structures of precisely determined oxidation-reduction potential. Examples are NAD+, NADP+, FAD, and lipoic acid. They serve as carriers of hydrogen atoms or of... 

Buffer Capacities of Natural Waters. Natural waters are buffered in different ways and to varying degrees with respect to changes in pH, metal ion concentrations, various ligands, and oxidation-reduction potential. The buffer capacity is an intensive variable and is thermodynamic in nature. Hydrogen-ion buffering in natural waters has recently been discussed in detail by Weber and Stumm (38). Sillen (32) has doubted... 

Eh is a measure of oxidation-reduction potential in the solution. The chemical reactions in the aqueous system depend on both the pH and the Eh. While pH measures the activity (or concentration) of hydrogen ions in the solution, Eh is a measure of the activity of all dissolved species. Aqueous solutions contain both oxidized and reduced species. For example, if iron is present in the solution, there is a thermodynamic equilibrium between its oxidized and reduced forms. Thus, at the redox equilibrium, the reaction is as follows ... 

Variations Between Lakes. Results of a study to evaluate sulfide production variation with water depth is given in Table V. In this experiment, samples were taken from five different sediment depths over a two-day period at each lake in early October. At both lakes sulfate reduction exceeded putrefaction by a factor of approximately 2 with overall mean rates of 0.55 and 0.29 mg S L-kH1 respectively. Sulfate reduction exceeded cysteine decomposition in all samples except one collected from Third Sister Lake at 17 m. Results of this study snow a good correlation at Third Sister Lake between percent hydrogen sulfide production attributable to putrefaction and depth of sampling station (r=0.94) and oxidation-reduction potential (r=0.98). This correlation was not observed at Frains Lake. A possible factor m differences observed may be the physical nature of the sediment at Frains which was less dense and more flocculent than thatofTliird Sister. 

Because of this low oxidation-reduction potential, the number of methods available for reducing ferredoxin is limited. Apart from hydrogen gas, ferredoxin may be reduced with organic reductants, such as pyruvate or hypoxanthine in the presence of the appropriate enzymes. Ferredoxin can be reduced nonenzymically with sodium hydrosulfite (dithionite) (Tagawa and Arnon (99) Fry et al. (45)), potassium borohydride (D Eustachio and Hardy (40)), and formamidine sulfinic acid (Shashoua (90)). It can be reduced also by illuminated chloroplasts (Whatley, Tagawa, and Arnon (114)) and, under these conditions, the reduction of ferredoxin is most complete (Bachofen and Arnon (12)). 

The need for oxidants can be determined by measuring the oxidation reduction potential (ORP) of the water to be treated. If the ORP measures above negative 170 millivolts, Filox can be used without the use of additional oxidants. Lower than negative 170 millivolts will require additional oxidants. Air, hypochlorite, hydrogen peroxide, ozone, and potassium permanganate are all suitable oxidants to use with Filox. Note that weaker oxidants, such as air and hypochlorite will be sufficient for most applications. 

Oxidation-reduction potential (or redox potential, E) is the potential of compounds to accept electrons and is by convention measured relative to that of hydrogen. Thus E is very negative for NADPH (a strong reductant) but positive for 02 (a strong oxidant). Standard redox potentials (Eo values in volts) refer to standard conditions (1M redox components) at neutral pH (pH 7). The standard free energy change at pH 7 for a particular redox reaction (AGo ) is given by ... 

CONCLUSIONS DRAWN FROM THE TABLES OF OXIDATION-REDUCTION POTENTIALS From the values of oxidation-reduction potentials we can easily find out whether a particular oxidation-reduction reaction is feasible or not. We have already seen the rules that govern the displacement of metals by one another, and the feasibility of dissolving metals in acid with the liberation of hydrogen. Those conclusions can now be extended and generalized. It can be said that the more positive the oxidation-reduction... 

Sometimes the use of quantum chemically determined indices (1, 24, 57) or oxidation-reduction potentials (58) instead of the Hammett-Taft constants, the use of chromatographically obtained hydrophobicity parameter, ARm, (5, 59) instead of w or log P, or the addition of other variables such as those for hydrogen bonding (60), dipole moments (61),... 

Oxidation-reduction potentials, like the other types discussed in the preceding chapter, are generally e pres.sed on the hydrogen scale, so that for the system... 

Oxidation-Reduction Indicators.—A reversible oxidation-reduction indicator is a substance or, more correctly, an oxidation-reduction system, exhibiting different colors in the oxidized and reduced states, generally colored and colorless, respectively. Mixtures of the two states in different proportions, and hence corresponding to different oxidation-reduction potentials, will have different colors, or depths of color every color thus corresponds to a definite potential which depends on the standard potential of the system, and frequently on the hydrogen ion concentration of the solution. If a small amount of an indicator is placed in another oxidation-reduction system, the former, acting as a potential mediator, will come to an equilibrium in which its oxidation-reduction potential is the same as that of the system under examination. The potential of the given indicator can be estimated from its color in the solution, and hence the potential of the system under examination will have the same value. 

Since the eye, or even mechanical devices, are capable of detecting color variations within certain limits only, any given oxidation-reduction indicator can be effectively employed only in a certain range of potential. Consider, for example, the simple case of an indicator system for which n is unity the oxidation-reduction potential at constant hydrogen ion concentration is given approximately by... 

In some cases the oxidation-reduction potential is close to the potential at which another process, e.g., hydrogen evolution, can occur in this event both reactions will take place simultaneously. For example, reduction of the titanic-titanous system in hydrochloric acid commences at about + 0.05 volt, while in the same solution the reversible hydrogen potential is approximately zt 0.0 volt. It follows, therefore, that if a platinized platinum cathode, which has almost zero hydrogen overvoltage, is employed, reduction of the titanic ions and evolution of hydrogen will take place at the same time. The reduction eflSciency will then be small. If a high overvoltage cathode is employed, however,... 

Table 2 Standard oxidation-reduction potentials 25° C, volts Di. hydrogen electrode...

---