For oxidation-reduction titrations

In Sections 10.11-10.16 it is shown how the change in pH during acid-base titrations may be calculated, and how the titration curves thus obtained can be used (a) to ascertain the most suitable indicator to be used in a given titration, and (b) to determine the titration error. Similar procedures may be carried out for oxidation-reduction titrations. Consider first a simple case which involves only change in ionic charge, and is theoretically independent of the hydrogen-ion concentration. A suitable example, for purposes of illustration, is the titration of 100 mL of 0.1M iron(II) with 0.1M cerium(IV) in the presence of dilute sulphuric acid ... 

In oxidation-reduction titrations, an electrode potential related to the concentration ratio between the oxidized and reduced forms of either of the reactants is determined as a function of the titrant volume. The indicator electrode must be responsive to at least one of the couples involved in the reaction. Indicator electrodes for oxidation-reduction titrations are generally constructed from platinum, gold, mercury, or palladium. The metal chosen must be unreactive with respect to the components of the reaction. The indicator metal is merely a medium for electron transfer. 

Twin polarized platinum microelectrodes are conveniently used for endpoint detection for oxidation-reduction titrations. Consider a titration curve for oxidation-reduction titration where both reactants behave reversibly at the electrodes. An example of this kind of titration is titration of iron(II) with cerium(IV) (Fig. 14A). At the starting point of the titration, no current is observed because no suitable cathode reactant is available. With addition of cerium(IV), a mixture of iron(II) and iron(III) is produced, which permits the passage of current. Beyond the midpoint in the titration, iron(III) becomes in excess, and the current is then regulated by decreasing iron(II) concentration. At the equivalence point, the current approaches zero because iron(III) are present, and the applied potential is not great enough to cause these to react at the electrode. Beyond the equivalence point, the current rises again because both cerium(III) and cerium(IV) are present to react at the electrodes. 

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. 

Asakai T, Kakiharaa Y, Kozukaa Y, Hossakaa S, Murayamaa M, Tanakab T (2006) Evaluation of certified reference materials for oxidation-reduction titration by precise coulometric titration and volumetric analysis. Analytica Chimica Acta 567(2) 269-276... 

Defect concentrations of Fe in Fei xO are usually measured by chemical analysis. It is impossible to determine the compositions of non-stoichiometric compounds, because the error of an ordinary quantitative analysis is about 10, while the deviation of a crystal with intrinsic defect from its stoichiometric composition is about <10. Nevertheless, it is possible for chemical analysis to determine if the metal atoms in non-stoichiometric compounds are excessive or less. Because a non-stoichiometric compound, in common, is a multi-component solid solution in which the different components have different valences, e.g., Fei xO can be viewed as a solid solution which consists of Fe +O and Fe2" 03. Deviation of those types of compounds can directly be determined by measuring of the concentration of an atom that shows an abnormal valence in it. For example, it forms the solutions containing large amounts of Fe + ion and less amounts of Fe + ion, when Fei xO (catalyst) is solved by hydrochloric acid solution under conditions with the absence of air or oxygen. Among these ions, the contents of both Fe + and Fe + can be determined by a titration of EDTA, but the Fe + needs to be oxidated to Fe + by ammonium persulfate prior to titration. The volume ratio of EDTA solutions that are consumed by Fe + and Fe + ions respectively is the ratio of Fe + and Fe + in sample, namely defect concentration of Fe, i.e., x = 1/(3 - - 2Fe +/Fe +). There is also Kulun titration or polarographic analysis except for oxidation-reduction titration which can be used for such measurement of an ion with abnormal valence in the solution of a solid sample. 

From these observations it was concluded that the major products of the reduction of niobium(V) chloride with anhydrous pyridine were tetrachlorodi-(pyridine)niobium(IV) and l-(4-pyridyl)pyridinium dichloride. Oxidation-reduction titrations indicated that this reduction accounted for approximately 70% of the reaction products. In view of the rapid reaction of tantalum(V) halides with pyridine to form 1 to 1 adducts, it was assumed that the remaining 30% of niobium (V) which was not reduced was present in the reaction mixture as pentachloro(pyridine)niobium(V). On this basis the following over-all reaction is proposed ... 

We now turn our attention to details of precipitation titrations as an illustration of principles that underlie all titrations. We first study how concentrations of analyte and titrant vary during a titration and then derive equations that can be used to predict titration curves. One reason to calculate titration curves is to understand the chemistry that occurs during titrations. A second reason is to learn how experimental control can be exerted to influence the quality of an analytical titration. For example, certain titrations conducted at the wrong pH could give no discernible end point. In precipitation titrations, the concentrations of analyte and titrant and the size of Ksp influence the sharpness of the end point. For acid-base titrations (Chapter 11) and oxidation-reduction titrations (Chapter 16). the theoretical titration curve enables us to choose an appropriate indicator. 

The complexes are useful as indicators for titrating weak bases in nonaqueous solvents and also for oxidation-reduction and aromatic diazotization titrations.4 8... 

This reaction is widely used to remove the excess of tin(II) ions, used for prior reduction, in oxidation-reduction titrations. 

The method of carrying out oxidation-reduction titrations potentio-metrically is essentially similar to that for precipitation reactions, except that the indicator electrode now consists merely of an inert metal. The determination of the end-point graphically or by some form of differential titration procedure is carried out in a manner exactly analogous to that described in Chap. VII various forms of simplified methods of oxidation-reduction titration have also been described. ... 

The shape of the curve for an oxidation-reduction titration depends on the nature of the system under consideration. The titration curve in Fig. 7 is symmetric about the equivalence point because the molar ratio of oxidant to reductant is equal to unity. An asymmetrical curve results if the ratio differs from this value. Solutions containing two oxidizing or reducing agents yield titration curves containing two inflection points if the standard potentials for the two species are different by more than approximately 0.2 V. Fig. 8 shows the titration curve for a mixture of iron(II) and titanium(III) with cerium(rV). The first additions of cerium are used by more readily oxidized titanium(III) ion, thus, the first step in the titration curve corresponds to titanium and the second to iron. 

The endpoint may be detected by addition of colored indicators, provided the indicator itself is not electroactive. Potentiometric and spectrophotometric indication is used in acid-base and oxidation-reduction titrations. Amperometric procedures are applicable to oxidation-reduction and ion-combination reactions especially for dilute solutions. 

Knowledge of oxidation-reduction titrations is not required for the MCAT. However, it is possible that there will he 8 passage wlv-rli explains them. They are included here just so you won t H. shocker if you see one In a lassagy... 

Because most redox indicators respond to changes in electrode potential, the vertical axis in oxidation/reduction titration curves is generally an electrode potential instead of the logarithmic p-functions that were used for complex formation and... 

We can observe end points for many oxidation/reduction titrations by making the solution of the analyte part of the cell... 

How is an oxidation/reduction titration curve generated through the use of standard electrode potentials for the analyte species and the volumetric titrant ... 

How does calculation of the electrode potential of the system at the equivalence point differ from that for any other point of an oxidation/reduction titration ... 

Under what circumstance is the curve for an oxidation/reduction titration asymmetric about the equivalence point ... 

The analyte in an oxidation/reduction titration must be in a single oxidation state at the outset. Often, however, the steps that precede the titration, such as dissolving the sample and separating interferences, convert the analyte to a mixture of oxidation states. For example, when a sample containing iron is dissolved, the resulting solution usually contains a mixture of iron(II) and iron(III) ions. If we choose to use a standard oxidant to determine iron, we must first treat the sample solution with an auxiliary reducing agent to convert all of the iron to iron(II). If we plan to... 

A further advantage of the coulometric procedure is that a single constant-current source provides reagents for precipitation, complex formation, neutralization, or oxidation/reduction titrations. Finally, coulometric titrations are more readily automated, since it is easier to control electrical current than liquid flow. 

Oxidation/Reduction Titrations Coulometric titrations have been developed for many, but not all, redox titrations. Table 22-4 reveals that a variety of redox... 

Titrations requiring the use of colour-change indicators often suffer from the lack of an indicator reacting in the equivalence point zone required. There is no lack of indicators to chose from for neutralisation titrations there is often, however, a lack of indicator choices to meet the requirements of precipitation, complexation and oxidation-reduction titrations. 

In precipitation titrations, the value of K would be governed by the Ksp for the precipitated substance. In complexometric titrations K would be governed by the instability constant for the complex formed (this is the reciprocal of the stabUity constant). Where oxidation-reduction titrations are involved the value of Kcq for the titration reaction would govern the situation. 

The potentiometric technique involves the use of glass, ISE and platinum electrodes, the latter used in connection with nearly all oxidation-reduction titrations. These electrodes use external or internal reference electrodes. In the main, the reference is an Ag/AgCl (3M KCl) unit with an outer compartment capable of being filled with an electrolyte of choice and changeable. For chloride titrations, for example, the indicator electrode is often a silver billet coated with AgCl, with a Ag/AgCl reference 3M KNO3 filled. 

An equation for the potential Et observed during an oxidation-reduction titration, which includes a consideration of the formation of semiquinones may be obtained as follows.23... 

Oxidation-Reduction Titrations. The theory and an outline of the practice of oxidation-reduction titrations for organic substances have already been described in Chapter 16. A full discussion of the possible analytical methods depending upon polentiometric titration is not possible here. For detailed accounts of specific methods the reader is referred to more extended treatises oil the subject. 

A. J. Bard and S. H. Simpsonsen, The General Equation for the Equivalence Point Potential in Oxidation-Reduction Titrations, J. Chem. Educ., 37 (1960) 364. 

Activity coefficient data for ions in solutions of the types commonly used in oxidation-reduction titrations and electrochemical work arc somewhat limited. As a result, we must use molar concentrations rather than activities in many calculations.. sing molar concentrations may cause appreciable errors. Such calculations... 

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