In redox titrations

Equations for the principal methods for the redox determinations of the elements are given in Table 11.29. Volumetric factors in redox titrations for the common titrants are given in Table 11.28. 

Consistent with the two-electron donor nature of H2, the reaction behaved as an n=2 Nernst redox reaction. It showed a pH dependence of 66mV per pH unit, so again one proton was taken up for each electron. It is not known where all incoming protons are localized in the enzyme. The reaction shows that in addition to the light-sensitive hydrogen species bound to the active site in the Nia-C " state, a second hydrogen can react at the active site and deliver its two electrons to the enzyme. We hence proposed that the active site of the A. vinosum enzyme has two sites where hydrogen can bind. If H2 is completely removed, the Nia-C state persists for hours this is unlike the situation in redox titrations in the presence of redox mediators. As the active site in the Nig-SR state has one electron more than that in the Nia-C state, an Fe-S cluster has to be involved in this reaction with H2. 

Aqueous solutions of iodic acid serve as strong oxidizing agents. The acid also is used in redox titrations. 

Oxidation states higher than + 3 are exhibited by Ce, Pr, and Tb, but only Ce4+ is stable (kinetically) in water. It is a very strong oxidizing agent in aqueous solution ( ° = 1.74 V) and is used as a volumetric standard in redox titrations. Some of its salts [e.g., cerium(IV) ammonium nitrate, cerium(IV) sulfate] find application in... 

Although coordination compounds have sometimes been used as titrants, e.g. ferrocyanide ions for the titration of zinc(II), and permanganate or dichromate ions in redox titrations, probably the only outstanding reaction in classical titrimetry that involves the reversible formation of a coordination compound is the Liebig-Deniges reaction. 

Potassium dichromate is another common oxidizing reagent employed in redox titrations. The titration is carried out in the presence of 1MHC1 or H2S04. The half-reaction is as follows ... 

Many other powerful oxidants are used in redox titrations. Often a metal ion may be present in more than one oxidation state which must be oxidized or reduced into the desired oxidation state. For example, a salt solution of iron may contain both Fe2+ and Fe3+ ions. Peroxydisulfates, bismuthates, and peroxides are often used as auxiliary oxidizing reagents to convert the ion of interest into the higher oxidation state. The half-reactions for these oxidants are as follows ... 

Its oxidized form (Phen)3Fe3+ is pale blue, while the reduced form (phen)3Fe2+ is red. The transition potential is about 1.11 V. Among the substituted derivatives of phenanthrolines, 5-methyl- and 5-nitro-1,10-phenanthroline complexes of iron have found wide applications in redox titrations. 

The sodium salt of these acids may be used to prepare aqueous solutions of indicators. Other examples of redox indicators include starch, potassium thiocyanate, methylene blue, and phenosafranine. Some selected general indicators in redox titrations are listed in Table 1.6.3. The properties of starch as an indicator in iodometric titration are discussed in the following section. 

Although mercury seldom is used as an indicator electrode in redox titrations (because it is so readily oxidized), it is used extensively for potentiometric titrations with complexing agents such as ethylenediaminetetraacetic acid... 

Another compound that has wide utility as an oxidizing agent is potassium dichromate, K2Cr207. This compound is an oxidizing agent that will oxidize a broad spectrum of materials in synthetic reactions, and it is used in redox titrations in analytical chemistry. 

Unfortunately, potassium permanganate is not obtainable in high enough purity and can undergo decomposition by exposure to sunlight. Therefore it cannot be used as a primary standard (p. 143). However, it can be used in redox titrations provided it is standardized with sodium oxalate (which is available in high purity). The redox reaction involving oxalate is as follows ... 

Table 24.1 Common oxidizing and reducing agents used in redox titrations...

Although these materials are still too costly to be used in commercial processing they do possess potential in this respect and may be employed as reagents in redox titrations. An interesting exception is afforded by the mineral glauconite which when impregnated with manganese(IV) oxide acts as a redox couple, thus ... 

An intermediate oxidation level can be identified in redox titrations below physiological pH this is P, which has di S= Ijl-Sj mixed spin state. " This may arise from mixtures of isomers in which Seri 88 (p) is bound or not, from states in which the hole is on different halves of the cluster, or in which different cysteinates are cleaved.It has been possible to stabilize this state with a mutation Serl88Cys in which the electron-rich thiolate shifts the redox potentials to lower values. The P /P redox couple is pH dependent, as noted above. Another redox state, P, has also been identified, and it also is apparently a mixed spin state with 5 = 7/2 and 1/2. " In the... 

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