Aqueous reactions oxidation-reduction

Typically reported topics are solvolytic reactions, oxidations, reductions, and C-C coupling reactions. The saponification of activated esters in aqueous micelles is a typical model for an enzyme mimetic reaction. The influence of the micellar medium on the reaction rate has been investigated, as well as the alteration of the stereoselectivity. Models of metalloenzymes were developed with the ligands 1-3 [8]. 

There is apparently an almost endless variety of free radicals capable of initiating polymerization which can be readily formed in aqueous media Oxidation-reduction (redox) reactions, in which a one-electron transfer step produces a radical intermediate are probably the most numerous. Two of the most widely used are cited below. 

The only oxidation state of importance In aqueous solution Is the +2 state. Compounds of the +4 plumbic Ion are well known but are Invariably unstable or Insoluble in aqueous solution. Due to the dominant stability of the +2 state In aqueous solution oxidation -reduction reactions are of minor Importance In lead separations and determinations. The notable exceptions are anodic oj ldatlon to the dioxide and reduction to the metal either cathodloally or by Internal electrodeposltlon. 111000 reactions have been used for the separation and determination of lead and are dlsoussed In detail In section IV-10 of this report. 

Spin trapping Aqueous/ solid Short-lived radicals nuy be studied. Possibility for assignment of radicals. Only a relative measure of radicals. Introduction of foreign substances with a risk of side reactions (oxidation, reduction). Verification by different spin traps or alternative analytical methods needed for identification. 

Manganese is the third most abundant transition metal, and is widely distributed in the earth s crust. The most important ore is pyrolusite, manganese(IV) oxide. Reduction of this ore by heating with aluminium gives an explosive reaction, and the oxide Mn304 must be used to obtain the metal. The latter is purified by distillation in vacuo just above its melting point (1517 K) the pure metal can also he obtained by electrolysis of aqueous manganese(II) sulphate. 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

The peroxodisulfate ion in aqueous solution is one of the strongest oxidising agents known. The standard oxidation—reduction potential for the following reaction is 2.08 V (77,78). 

The optical absorption spectra of Pu ions in aqueous solution show sharp bands in the wavelength region 400—1100 nm (Fig. 4). The maxima of some of these bands can be used to determine the concentration of Pu ions in each oxidation state (III—VI), thus quantitative deterrninations of oxidation—reduction equiUbria and kinetics are possible. A comprehensive summary of kinetic data of oxidation—reduction reactions is available (101) as are the reduction kinetics of Pu + (aq) (84). 

NH2OH can exist as 2 configurational isomers (cis and trans) and in numerous intermediate gauche conformations as shown in Fig. 11.7. In the crystalline form, H bonding appears to favour packing in the trans conformation. The N-O distance is 147 pm consistent with its formulation as a single bond. Above room temperature the compound decomposes (sometimes explosively) by internal oxidation-reduction reactions into a complex mixture of N2, NH3, N2O and H2O. Aqueous solutions are much more stable, particularly acid solutions in which the compound... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Oxidation-reduction reactions involving perchlorates have been mentioned in several of the preceding sections and the reactivity of aqueous solutions is similar to that of aqueous solutions of perchloric acid. 

Another common type of reaction in aqueous solution involves a transfer of electrons between two species. Such a reaction is called an oxidation-reduction or redox reaction. Many familiar reactions fit into this category, including the reaction of metals with acid. 

Many oxidation-reduction reactions (nicknamed redox reactions) take place in aqueous solution. One of these was mentioned in Section 11-2.1 when we characterized acids ... 

As a third oxidation-reduction example, suppose a strip of metallic zinc is placed in a solution of copper nitrate, Cu(N03)j. The strip becomes coated with reddish metallic copper and the bluish color of the solution disappears. The presence of zinc ion, Zn+2, among the products can be shown when the Cu+2 color is gone. Then if hydrogen sulfide gas is passed into the mixture, white zinc sulfide, ZnS, can be seen. The reaction between metallic zinc and the aqueous copper nitrate is... 

In quantitative analysis we are chiefly concerned with reactions which take place in solution, i.e. ionic reactions. We shall therefore limit our discussion of oxidation-reduction to such reactions. The oxidation of iron(II) chloride by chlorine in aqueous solution may be written ... 

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

The reaction of magnesium metal with aqueous strong acid, which appears in Figure 4-12. illustrates the fundamental principles of oxidation-reduction. When a piece of magnesium is dropped into a solution of hydrochloric acid, a reaction starts almost Immediately. The metal dissolves, and gas bubbles from the solution. The gas is H2, and analysis of the solution reveals the presence of ions. A list of chemical species before and after the reaction... 

Most of the chemical processes discussed before (acid-base equilibria, precipitation-dissolution, neutralization, complexation, and oxidation-reduction) are interrelated that is, reactions of one type may influence other types of reactions, and consequently must be integrated into aqueous- and solution-geochemistry computer codes. 

Keeping in mind the above studies of multivalent cations (Fe, Cr and Mn) in aqueous medium, some experiments involving redox or complexometric reactions of these metal ions have been carried out, using ultrasound (20 kHz) and its effect on the precipitation, oxidation, reduction and decomposition of complex have been evaluated. An Ultrasonic Processor model P2 with a titanium tip of diameter 12 mm and 250 watts power was used. In the subsequent sections details of some of the interesting experiments, carried out in aqueous solutions of salts of Fe, Cr and Mn in their different oxidation states, have been discussed. 

Both of these reactions are reductions, another is the use of Fe2 to catalyse the oxidation reactions of aqueous hydrogen peroxide solution ... 

The inorganic elements in aqueous solution reactions, both acid-base complex formation, precipitation and oxidation/reduction, frequently come rapidly to equilibrium when no more reactions are possible. The implication is that in the environment and in organisms many of their properties cannot change unless circumstances change, for example the introduction of new components. 

Enthalpies, Entropies, and Gibb s Energies of Transition Metal Ion Oxidation-Reduction Reactions with Hydrogen Peroxide in Aqueous Solution (T = 298 K) [23]... 

Both models apply the same chemical scheme of mercury transformations. It is assumed that mercury occurs in the atmosphere in two gaseous forms—gaseous elemental HgO, gaseous oxidized Hg(II) particulate oxidized Hgpart, and four aqueous forms—elemental dissolved HgO dis, mercury ion Hg2+, sulphite complex Hg(S03)2, and aggregate chloride complexes HgnClm. Physical and chemical transformations include dissolution of HgO in cloud droplets, gas-phase and aqueous-phase oxidation by ozone and chlorine, aqueous-phase formation of chloride complexes, reactions of Hg2+ reduction through the decomposition of sulphite complex, and adsorption by soot particles in droplet water. 

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