Sulfite, redox potential

Aqueous solutions of sodium sulfite are alkaline and have a pH of ca 9.8 at 1 wt %. The solutions are oxidized readily by air. The redox potential is a function of pH, as would be expected from the foUowing equation ... 

These enzymes catalyze the two-electron oxidation of purines, aldehydes and pyrimidines, sulfite, formate and nicotinic acid in the general reaction shown in equation (49). These enzymes show some differences in properties. Xanthine oxidase, xanthine dehydrogenase and aldehyde oxidase all have relatively low redox potentials and a unique cyanolyzable sulfur atom, and so will be discussed together. 

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

Many samples have redox potentials such fiiat fiiey can be oxidized by iodine. Therefore, file iodine in file titrant may be consumed by readily oxidizable samples fiiat will give a false high value for file water content. Some common substances fiiat can be oxidized by iodine are ascorbic acid, arsenite (As02 ), arsenate (As04 ), boric acid, tetraborate (3407 ), carbonate (COs ), disulfite (8205 ), iron(ll) salts, hydrazine derivatives, hydroxides (OH ), bicarbonates (HCOs"), copper(l) salts, mercaptans (RSH), nitrite (N02 ), some metal oxides, peroxides, selenite (SeOs "), silanols (RsSiOH), sulfite (SOs ), tellurite (TeOs ), fiiiosulfate (8203 ), and tin(ll) salts. For situations such as fiiese where file material under analysis reacts wifii iodine, an oven can be used to liberate fiie moisture from file sample, which is fiieii carried into file reaction vessel and titrated wifiiout interference. 

D.gigas Fdll and Fdl have been implicated in two important methabolic pathways of this sulfate reducing organism48 49), the phosphoroclastic reaction6) and the sulfite reductase system 39) schematically shown in Fig. 11. The structural studies described above should be analysed in perspective to the redox potentials and physiological activities of the two oligomeric forms ... 

As the redox potential of sulfite reductase is about - 110 mV and that of pyruvate dehydrogenase is about — 600 mV it is reasonable that the [3 Fe—xS] centers, which have less negative redox potentials are the most efficient in ii) and that only the... 

Mo(Tp )(E)(bdt)j, [Mo(Tp )(E)(tdt)], [Mo(Tp )(E)(bdtCl2)]82 83 (E = O, NO), and [MoO(qdt)(Tp )]84 have been investigated as models for various pyranopterin Mo enzyme active sites, including sulfite oxidase. Solution redox potentials and heterogeneous electron transfer rate constants for these species have been also reported.85 The interactions between the sulfur tt-orbitals of arene dithiolates and high-valent Mo in [MoO(Tp )(bdt)] have been investigated by gas-phase photoelectron spectroscopy and DFT methods in order to understand the properties of the active site of pyranopterin Mo-W enzymes.86 Temperature-dependent measurements of potential and electron-transfer rate constants are also reported for electrochemical reduction of a series of [MoO(Tp )(X,Y)] complexes.87 The molecular and electronic structures of the SO active site [MoO(Tp )(bdt)] have been also reported.88... 

Soil redox potential (Eh) and the pH parameters are closely related. Production of carbon dioxide, an end product of the reduction of oxygen, has considerable influence on the soil s pH. When a reducing wetland soil system becomes oxidized, its pH may decrease drastically due to the oxidation of iron to Fe(lll) and the subsequent hydrolysis of the iron or the oxidation of sulfite to sulfate, which is accompanied by the release of protons. Lowering of the Eh of the soil due to flooding will result in a rise of pH, because many reduction reactions (such as the reduction of sulfate to sulfide, Ee to Fe, and Mn + to Mn +) involve the uptake of protons or the release of hydroxyls. 

Both reactions require close pH control and the completeness of the processes is determined by measuring the redox potential of the CI2/CI couple at a platinum electrode. When all cyanide has been entirely oxidized the redox potential of the solution is determined by the CI2/CI couple. Chromate ions in the process effluents can also be eliminated by reducing them to chromium(in) by sulfite ions ... 

The system described by Martin and Kiser [229], as well as many of those in chlorine plants that are based on sulfites, is a simple in-line addition with nearby downstream measurement. This requires good mixing and rapid reaction for its success. As pointed out above, hydrogen peroxide may not react so rapidly, and some lag time may be necessary. Mannig and Scherer [225] described such a control system and presented their data on redox potential as a function of pH at two different concentrations of NaOCl (Fig. 7.102). At pH 10, the presence of 10 ppm of NaOCl shifted the potential 400 mV. This is enough to provide close control of the process. O Brien [208] described a system in which the controller was arranged as above, with measurement close to the addition point. This was done to avoid problems with dead time in the control loop. The controller then could be reset from an ORP measurement made sufficiently far downstream that the completion of the reaction could safely be assumed. The need for such an elaboration should be determined by tests on plant brine when possible. In wastewater applications, it is conunon to use this technique with 15-30 min retention between the peroxide addition point and the control measurement [230]. 

---