Chlorine reduction reaction

Titanium is stable in chlorine-containing brine solutions. However, it is prone toward crevice corrosion in the sealing areas and gaps between welded titanium structures, as titanium can be anodically oxidized to TiOj either from Ti or UCh or to the corresponding cathodic reaction being the chlorine reduction reaction or the hydrogen... 

A promising approach to selective recovery of chlorine from the inerts in the vent gas is to electrochemically reduce chlorine to chloride at the cathode and oxidize the ion to chlorine at the anode. The selectivity arises because of the fast kinetics of the chlorine reduction reaction compared to that of the oxygen and carbon dioxide reduction... 

An asymmetric chlorination/reduction reaction for the synthesis of p-chloroalcohols has been developed using MacMillan s resolved catalyst and NCS as the chlorine source (Scheme 7.17) [31]. The reaction was highly selective and enantiomeric ratios of >95 5 were common. 

While ethyl chloride is one of the least toxic of all chlorinated hydrocarbons, CE is a toxic pollutant. The off-gas from the reactor is scrubbed with water in two absoiption columns. The first column is intended to recover the majority of unreacted ethanol, hydrogen chloride, and CE. The second scrubber purifies the product fiom traces of unreacted materials and acts as a back-up column in case the first scrubber is out of operation. Each scrubber contains two sieve plates and has an overall column efficiency of 65% (i.e., NTP = 1.3). Following the scrubber, ethyl chloride is finished and sold. The aqueous streams leaving the scrubbers are mixed and recycled to the reactor. A fraction of the CE recycled to the reactor is reduced to ethyl chloride. This side reaction will be called the reduction reaction. The rate of CE depletion in the reactor due to this reaction can be approximated by the following pseudo first order expression ... 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

Titanium tetrachloride is produced on an industrial scale by the chlorination of titanium dioxide-carbon mixtures in reactors lined with silica. During the reactor operation, the lining comes into contact not only with chlorine but also with titanium tetrachloride. There appears to be no attack on silica by either of these as the lining remains intact. However, the use of such a reactor for chlorinating beryllium oxide by the carbon-chlorine reduction chlorination procedure is not possible because the silica lining is attacked in this case. This corrosion of silica can be traced to the attack of beryllium chloride on silica. The interaction of beryllium chloride with silica results in the formation of silicon tetrachloride in accordance with the reaction... 

The reactions that this sodium-chlorine case typifies are called oxidation-reduction reactions. The term oxidation refers to the loss of electrons, while the term reduction refers to the gain of electrons. A number of oxidation-reduction reactions (nicknamed redox reactions) are useful in titrimetric analysis, and many are encountered in other analysis methods. 

This scheme implies formation of one mole of acetone per mole of nitrobenzene being consumed, which agrees with the experimental results. Nitrosobenzene is an attractive intermediate, since it had been shown independently to undergo chlorination/reduction to the product pattern given above in a dark reaction 18,34). 

If a chemical reaction can make electricity it should not be surprising to learn that electricity can make a chemical reaction. Using an electric current to cause a chemical reaction is called electrolysis, a technique widely used to win elements from their compounds. For example, pure sodium metal (Na) and chlorine gas (CI2) are obtained by passing electricity through molten sodium chloride (NaCl). The study of the interplay of electricity and oxidation-reduction reactions is called electrochemistry. 

An oxidation-reduction reaction occurs when sodium and chlorine react to form sodium chloride, as shown in Figure 11.1. The equation for this reaction is... 

We have seen that oxidation is electron loss and reduction is electron gain. Electrons are real particles and cannot just be lost. Therefore, whenever a reaction involves the oxidation of a species, it must be accompanied by the reduction of another species. Oxidation and reduction taken separately are like one hand clapping one must occur in conjunction with the other for reaction to take place. For instance, in the reaction between chlorine and sodium bromide, the bromide ions are oxidized and the chlorine molecules are reduced. Because oxidation is always accompanied by reduction, chemists speak of redox reactions rather than simply oxidation reactions or reduction reactions. 

The reductive reaction is slow under anaerobic conditions, because iron may be oxidized by oxygen. Chlorinated contaminants possess an oxidizing... 

The most established technology for treating contaminants by abiotic reduction reactions relies on zero-valent iron metal (82). In addition to effecting hydrogenolysis (Equation (4)) and reductive elimination (Equation (5)), Fe° readily reduces nitro aromatics (Equation (6)), azo dyes, nitrate, chromate, chlorine residual, and some radionuclides. Recently, an investigation of soils contaminated with the herbicide alachlor provided evidence for reductive N-dealkylation (as well as dechlorination) by Fe° (83). 

An electrolysis reaction in which the oxidation reaction is not the reverse of the reduction reaction is the electrolysis of molten sodium chloride. Molten sodium chloride (NaCl), with calcium chloride (CaCy added to decrease the melting point, is electrolyzed in a Downs cell, in which the sodium and chlorine products are separated so that sodium chloride does not reform. The following reactions occur ... 

Phenylarsine oxide, C6H5As = O, is as effective as sodium thiosulfate in reducing iodine. It is more stable than thiosulfate. An advantage is that it is stable even in dilute solution. This substance is, however, highly toxic and is a suspected carcinogen. Because of its toxicity, its application is limited. One such application is in the amperometric titration of residual chlorine. The oxidation-reduction reaction of PAO is similar to thiosulfate. Its equivalent weight in iodine reaction is 168. 

Oxidation-reduction reactions, even complex ones, can be balanced using either the half-reaction method or the oxidation number method. The half-reaction method will be discussed first, using the reaction of iron with chlorine to produce iron chloride. 

Chemical/physical treatment processes are those in which a chemical reaction is used to alter or destroy a hazardous waste component. Chemical treatment techniques can be applied to both organic and inorganic wastes, and may be formulated to address specific target compounds in a mixed waste. Typical chemical treatment processes include oxidation-reduction reactions such as ozonation, alkaline chlorination, electrolytic oxidation and chemical dechlorination. Physical treatment processes separate waste component by either applying physical force or changing the physical form of the waste. Various physical processes include adsorption, distillation, or filtration. Physical treatment is applicable to a wide variety of waste streams but further treatment is usually required. 

Chlorine and other oxidants are removed using activated carbon by an oxidation/reduction reaction. Chlorine oxidizes the carbon while the chlorine is being reduced. Chlorine ends up forming hydrochloric acid via equation 8.1. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Each of the photosystems ejects an electron from the excited chlorin complex to a quinone within a nanosecond, followed by electron transfer along chains leading out of the charge separation center within 100 ns. The high potential reaction of Tyr and Mn in PSII is quite rapid, beginning in the simulations on the same time scale as the quinone reduction reaction. However, it has been suggested that tyrosine oxidation may not be rate limited by tunneling, but by H+ transfer (Diner et al., 2001). 

In this reaction, solid sodium, which contains neutral sodium atoms, reacts with chlorine gas, which contains diatomic Cl2 molecules, to form the ionic solid NaCl, which contains Na+ and Cl ions. This process is represented in Fig. 4.19. Reactions like this one, in which one or more electrons are transferred, are called oxidation-reduction reactions or redox reactions. 

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