Redox reducing agents

Stannate(II) ions are powerful reducing agents. Since, for tin, the stability of oxidation state -b4 is greater than that of oxidation state -b2, tin(II) always has reducing properties, but these are greater in alkaline conditions than in acid (an example of the effect of pH on the redox potential, p. 101). 

As the above redox potentials indicate, only in the presence of very powerful oxidising agents does hydrogen peroxide behave as a reducing agent. For example ... 

Unlike the reactions that we have already considered, the equilibrium position of a redox reaction is rarely expressed by an equilibrium constant. Since redox reactions involve the transfer of electrons from a reducing agent to an oxidizing agent, it is convenient to consider the thermodynamics of the reaction in terms of the electron. 

Quantitative Calculations The stoichiometry of a redox reaction is given by the conservation of electrons between the oxidizing and reducing agents (see Section 2C) thus... 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Reducing agents are employed to return the Fe to Fe . By starting at a lower temperature, the heat of reaction can be balanced by the sensible heat of the water in the emulsion. Temperature profiles from 20 to 70°C are typical for such systems. Care must be taken when working with redox systems to... 

Hydroperoxides are generally used with reducing agents, eg, iron salts, in redox emulsion polymerization systems. 

Hydroquinone [123-31 -9] represents a class of commercially important black-and-white chemical reducing agents (see Hydroquinone,RESORCINOL, AND catechol). The following scheme for silver haUde development with hydroquinone shows the quantitative importance of hydrogen ion and haUde ion concentrations on the two half-ceU reactions that describe the silver—hydroquinone redox system ... 

The requirements of a developer moiety for incorporation into a dye developer are well fulfilled by hydroquinones. Under neutral or acidic conditions hydroquinones are very weak reducing agents and the weakly acidic phenoHc groups confer tittle solubility. In alkali, however, hydroquinones are readily soluble, powerful developing agents. Dye developers containing hydroquinone moieties have solubility and redox characteristics in alkali related to those of the parent compounds. 

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

Redox reactions are particularly instructive. If all thermodynamically allowed reactions in liquid NH3 were kinetically rapid, then no oxidizing agent more powerful than N2 and no reducing agent more powerful than H2 could exist in this solvent. Using data for solutions at 25° ... 

Mino and Kaizerman [12] established that certain. ceric salts such as the nitrate and sulphate form very effective redox systems in the presence of organic reducing agents such as alcohols, thiols, glycols, aldehyde, and amines. Duke and coworkers [14,15] suggested the formation of an intermediate complex between the substrate and ceric ion, which subsequently is disproportionate to a free radical species. Evidence of complex formation between Ce(IV) and cellulose has been studied by several investigators [16-19]. Using alcohol the reaction can be written as follows ... 

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