Titration oxidation-reduction reactions

A titration in which the reaction between the analyte and titrant is an oxidation/reduction reaction. 

The majority of potentiometric titrations involve chemical reactions which can be classified as (a) neutralisation reactions, (b) oxidation-reduction reactions, (c) precipitation reactions or (d) complexation reactions, and for each of these different types of reaction, certain general principles can be enunciated. 

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... 

Most oxidation reactions are between specific metal cations or metal oxy-anions and cations. The problem that arises when applying oxidation-reduction reactions to soils is that all soils contain a complex mixture of oxidizable and reducible cations, anions, and organic matter, which means that it is impossible to determine which is being titrated. An exception to this is the oxidation of organic matter where an oxidation-reduction titration is routinely carried out. Organic matter determination will be discussed in Section 10.3. 

In addition, potentiometric titration methods exist in which an electrode other than an ion-selective electrode is used. A simple platinum wire surface can be used as the indicator electrode when an oxidation-reduction reaction occurs in the titration vessel. An example is the reaction of Ce(IV) with Fe(II) ... 

Kolthoff, I.M. Belcher, R., Volumetric Analysis. Titration. Methods Oxidation Reduction Reaction. Interscience Publishers, Inc., NY 1957. Vol. Ill Koppenol, W.H. Liebman, J. J. Phys. Chem. 1984, 88, 99-101. 

As we have covered, titration is used to determine the concentration of an unknown substance in acid-base reactions through the use of a known concentration in a solution. It can also be used in oxidation-reduction reactions for the same analytical... 

A redox titration is based on an oxidation-reduction reaction between analyte and titrant. In addition to the many common analytes in chemistry, biology, and environmental and materials science thai can be measured by redox titrations, exotic oxidation states of elements in uncommon materials such as superconductors and laser materials are measured by redox titrations. For example, chromium added to laser crystals to increase their efficiency is found in the common oxidation states +3 and +6, and the unusual +4 state. A redox titration is a good way to unravel the nature of this complex mixture of chromium ions. 

An oxidation-reduction titration or redox titration is an oxidation-reduction reaction involving a transfer of electron(s) between two substances in solutions. A substance is said to be oxidized when it loses electron(s) and reduced when it gains electron(s). Examples of oxidation-reduction reactions are illustrated below ... 

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. 

Potentiometric titration can determine the end point more accurately than the color indicators. Thus, the quantitative consumption of a titrant in an acid-base neutralization, oxidation-reduction reaction, or complex formation reaction can be determined precisely and very accurately by potentiometric titration. The titration involves the addition of large increments of the titrant to a measured volume of the sample at the initial phase and, thereafter, adding smaller and smaller increments as the end point approaches. The cell potential is recorded... 

The violet complexes Mn(H2P 07)73 and Mn(C204)73 (both presumably chelates) are also known. The latter, when heated or illuminated strongly, undergoes an internal oxidation-reduction reaction, yielding C02 and an oxalate complex (or complexes) of Mn(II). The stability of the Mn(C204)73 complex at room temperature makes it necessary to carry out the familiar titration of oxalate with permanganate at elevated temperature (>60° 0). 

Redox titration — A - titration method in which electrons are transferred between the - titrant and the - analyte. Usually, the - end point of oxidation/reduction reactions is measured by chemical or potentiometric methods. The chemical method involves an - indicator that usually has a change in color at the end point, while the other method is a - potentiometric titration [i]. 

Ferroin With the introduction of Ce(IV) as an oxidant and the evaluation of the formal potential of the Ce(rV)-Ce(III) couple, the need for indicators with higher electrode potentials became evident. The indicator ferroin, tris(l,10-phenanthroline)-iron(II), was discovered by Walden, Hammett, and Chapman, and its standard potential was evaluated at 1.14 V. Hume and KolthofiF found that the formal potential was 1.06 V in 1 M hydrochloric or sulfuric acid. The color change, however, occurs at about 1.12 V, because the color of the reduced form (orange-red) is so much more intense than that of the oxidized form (pale blue). From Figure 15-1 it can be seen that ferroin should be ideally suited to titrations of Fe(II) and other reductants with Ce(lV), particularly when sulfuric acid is the titration medium. It has the further advantages of undergoing a reversible oxidation-reduction reaction and of being relatively stable even in the presence of oxidant. 

Kolthoff, I. M., Belcher, R. Volumetric Analysis, Vol. Ill, Titration Methods. Oxidation-Reduction Reactions, 2nd ed.. Interscience, New York, 1957. 

Table 4 Coulorimetric titrations of oxidation-reduction reactions...

Applications of coulometric titrations involving oxidation-reduction reactions are shown in Table 4. Electrogenerated oxidizing agents such as bromine have proved to be useful, especially in organic analysis. 

Chemical reactions are frequently carried out in solution, and their description requires modifications to the rules of stoichiometry described in Chapter 2. We illustrate these modified rules by the important analytical techniques of titration in acid-base and oxidation-reduction reactions. 

As we have just seen, system for oxidation/reduction titration is usually independent of dilution. Consequently, titration curves for oxidation/reduction reactions are usually independent of analyte and reagent concentrations. This characteristic is in distinct contrast to that observed in the other types of titration curves we have encountered. 

All the methods of end point detection discussed in the previous paragraphs are based on the assumption that the titration curve is symmetrical about the equivalence point and that the inflection in the curve coiresponds to this point. This assumption is valid if the titrant and analyte react in a 1 1 ratio and if the electrode reaction is reversible. Many oxidation/reduction reactions, such as the reaction of iron(II) with permanganate, do not occur in equimolar fashion. Even so, such titration curves are often so steep at the end point that vei little error is introduced by assuming that the curves are symmetrical. 

Summary of Coulometric Titrations Involving Oxidation/Reduction Reactions ... 

The most common titrations are based on acid-base neutralisation (acid-base titration), or oxidant-reductant reaction (redox titration) principles. With these two titration methods, many textile chemicals can be analysed. The common indicators used in these titrations are listed in Table 4.U and 4.2. For an accurate titration, the consumption of the standard solution is ideally between 35 and 45 ml in a 50 ml burette. 

The discussion of potentiometric titrations will deal first with acidimetry, by the direct and differential procedures, after which methods depending upon precipitations and oxidation-reduction reactions will be considered. 

Another important amperometric titrant is bromine solution, which undergoes stoichiometric oxidation-reduction reactions with many substances such as As(III), Sb(III), ammonium salts, and others. Often the titration involves adding an excess of KBr to an acidified solution of the substance to be oxidized and then titrating it with potassium bromate solution. Bromine is thereby generated in situ. 

A 20.00 mL sample of oxalic acid solution, H2C2O4, was titrated with 0.256 M KMn04 solution. What is the molarity of the oxalic acid solution if it took 14.6 mL of the KMn04 solution to completely react with the oxalic acid The oxidation-reduction reaction is ... 

Titrations can be conducted using neutralization, precipitation, or oxidation-reduction reactions. T FIGURE 4.19 illustrates a typical neutralization titration, one between a HCl solution of unknown concentration and a NaOH solution we know to have a concentration of 0.100 M (our standard solution). To determine the HCl concentration, we first add a specific volume of the HCl solution, 20.0 mL in this example, to a flask. Next... 

A sample of an iron ore is dissolved in acid, and the iron is converted to Fe. The sample is then titrated with 47.20 mL of0.02240 M Mn04 solution. The oxidation-reduction reaction that occurs during titration is... 

Titrations based on oxidation-reduction reactions enjoy wide use. Permanganate, dichromate, and iodine and iron(II), tin(II), thiosulfate, and oxalate are commonly used oxidizing and reducing titrants, have been employed to determine components in both inorganic and organic analysis. As we saw in Chapter 7, solvent water does not play as central a role as in acid-base titrations. Oxidants or reductants strong enough to decompose water are not practical as titrants. 

Titrations can be conducted using neutralization, precipitation, or oxidation-reduction reactions. Figure 4.18 illustrates a typical neutralization titration, one... 

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