Titrimetry redox titrations

As with acid-base and complexation titrations, redox titrations are not frequently used in modern analytical laboratories. Nevertheless, several important applications continue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin, however, with a brief discussion of selecting and characterizing redox titrants, and methods for controlling the analyte s oxidation state. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Controllcd-Currcnt Coulomctry The use of a mediator makes controlled-current coulometry a more versatile analytical method than controlled-potential coulome-try. For example, the direct oxidation or reduction of a protein at the working electrode in controlled-potential coulometry is difficult if the protein s active redox site lies deep within its structure. The controlled-current coulometric analysis of the protein is made possible, however, by coupling its oxidation or reduction to a mediator that is reduced or oxidized at the working electrode. Controlled-current coulometric methods have been developed for many of the same analytes that may be determined by conventional redox titrimetry. These methods, several of which are summarized in Table 11.9, also are called coulometric redox titrations. 

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. 

This chapter provides in troductory material that applies to all types of titrimetric methods of analysis, using precipitation titrimetry to illustrate the various theoretical aspects of the titration process. Chapters 14, 15, and 16 are devoted to the various types of neutralization titrations, in which the analyte and titrants undergo acid/base reactions. Chapter 17 provides information about titrations in which the analytical reactions involve complex formation. These methods are of particular importance for the determination of a variety of cations. Finally, Chapters 18 and 19 are devoted to volumetric methods, in which the analytical reactions involve electron transfer. These methods are often called redox titrations. Some additional titration methods are explored in later chapters. These methods include ampero-metric titration, in Section 23B-4, and spectrophotometric titrations, in Section 26A-4. 

Organic compounds may be determined by redox titrimetry, including glucose and other reducing sugars, vitamin C, thiols such as cysteine, and many other organic chemicals of biological importance. There are thousands of potentiometric redox titration methods published in the chemical literature. 

In resolving complex metal-ion mixtures, more than one masking or demasking process may be utilized with various aliquots of the sample solution, or applied simultaneously or stepwise with a single aliquot. In favorable cases, even four or five metals can be determined in a mixture by the application of direct and indirect masking processes. Of course, not all components of the mixture need be determined by chelometric titrations. For example, redox titrimetry may be applied to the determination of one or more of the metals present. 

One of the most important applications of redox titrimetry is in evaluating the chlorination of public water supplies. In Method 9.3 an approach for determining the total chlorine residual was described in which the oxidizing power of chlorine is used to oxidize R to 13 . The amount of 13 formed is determined by a back titration with 8203 . 

Redox (reduction-oxidation) titrimetry is used primarily for nitrate detns. Five systems are in current use ferrous sulfate—dichromate, io dome trie, periodic acid oxidation (NaOH titrant), K permanganate, and titanous chloride-ferric ammonium sulfate. The ferrous sulfate— dichromate system is used for MNT DNT detns (Vol 2, C162-Lff Vol 6, F17-Rff Ref 17). In the iodometric procedure, the sample (ie, NG) is treated in a C02 atm with a satd soln of Mn chloride in coned HC1, the vol reaction products are bubbled thru a K iodide soln, and the liberated iodine is titrated with standard thiosulfate soln (Refs 1 17). The periodic... 

Titrimetric luminescence methods record changes in the indicator emission of radiation during titration. This change is noted visually or by instruments normally used in luminescence analysis. Most luminescence indicators are complex organic compounds which are classified into fluorescent and chemiluminescent, compounds according to the type of emission of radiation. As in titrimetry with adsorption of colored indicators, luminescence titration makes use of acid-base, precipitation, redox, and complexation reactions. Unlike color reactions, luminescence indicators enable the determination of ions in turbid or colored media and permit the detection limit to be lowered by a factor of nearly one thousand. In comparison with direct luminescence determination, the luminescence titrimetric method is more precise. 

Luminescence titrimetry has been developed chiefly for acid-base titrations. Therefore, fluorescence pH-indicators are now widely used. Their application is based on changes of fluorescence spectrum upon the addition of a proton or its loss. At present, over 200 fluorescence pH-indicators are available the structural formulae of the most the widely applied indicators are given in Table 8. Some of them (No. 2, 8, 9, 12, 16, 17, 23, 25 and 29) and also, primuline, tripaflavine, and rhodamine 6G are widely used as adsorption fluorescence indicators. The titration end point can be detected in this case because of the differences in of the indicator in the adsorbed state and in solution. Redox fluorescence indicators including rhodamines B and 6 G, 3,6-dihydroxy-phthalic acids, complexes of Ru(II) with 2,2 -dipyridyl or 1,10-phenanthroline and other... 

Ultraviolet-visible spectrophotometry has also been applied to titrimetry. In this case the variation in the absorbance of the analyte with addition of titrant is used to obtain a spectrophotometric profile from which titration end points and/or equilibrium constants, etc., can be determined. This has been applied to the whole range of titrations in which a chromophore is generated. These include acid-base, redox, and complexometric titrations. 

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