Titration, potentiometric oxidation-reduction

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... 

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) ... 

The first electrochemical studies of Mb were reported for the horse heart protein in 1942 (94) and subsequently for sperm whale Mb (e.g., 95) through use of potentiometric titrations employing a mediator to achieve efficient equilibriation of the protein with the electrode (96). More recently, spectroelectrochemical measurements have also been employed (97, 98). The alternative methods of direct electrochemistry (99-102) that are used widely for other heme proteins (e.g., cytochrome c, cytochrome bs) have not been as readily applied to the study of myoglobin because coupling the oxidation-reduction eqiulibrium of this protein to a modified working electrode surface has been more difficult to achieve. As a result, most published electrochemical studies of wild-type and variant myoglobins have involved measurements at eqiulibrium rather than dynamic techniques. 

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... 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

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]. 

The oxidation-reduction potential (E6) of adrenodoxin is 164 mV at pH 7.4 and 26° C by the potentiometric titration method with dithionite in a nitrogen atmosphere (29). However, Estabrook and his colleagues obtain a value of —196 mV in the determination of the potential anaerobically by titration with NADPH plus adrenodoxin reductase using dyes with electromotive activity. The difference in value between the two... 

The excess oxidant is removed by warming for a few minutes. Ag(II) undergoes reduction by the solvent at a rate proportional to the square of the Ag(Il) concentration, and inversely proportional to the Ag(I) concentration. The decomposition is believed to proceed in a series of reactions involving Ag(III), H2O2, HO2, and a Ag " -H02 complex. Alternatively, the solution can be titrated potentiometrically with Fe(II) two inflections are observed, the first due to reduction of excess Ag" " and the second to the reduction of permanganate, dichromate, or Ce(IV). 

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. 

Coulometric titrations, like their volumetric counterparts, require a means for determining when the reaction between analyte and reagent is complete. Generally, the end points described in the chapters on volumetric methods are applicable to coulometric titrations as well. Thus, for the titration of iron(II) just described, an oxidation/reduction indicator, such as 1,10-phenanthroline, can be used as an alternative, the end point can be determined potentiometrically. Potentioinetric or... 

Part IV is devoted to electrochemical methods. After an introduction to electrochemistry in Chapter 18, Chapter 19 describes the many uses of electrode potentials. Oxidation/reduction titrations are the subject of Chapter 20, while Chapter 21 presents the use of potentiometric methods to obtain concentrations of molecular and ionic species. Chapter 22 considers the bulk electrolytic methods of electrogravimetry and coulometry, while Chapter 23 discusses voltammetric methods including linear sweep and cyclic voltammetry, anodic stripping voltammetry, and polarography. 

Where potentiometric titrations, other than those involving pH measurement are concerned, such as precipitation, complexation, oxidation-reduction, nonaqueous media, etc., the data obtained will be in the form of E versus V. All of the titration theory, and that of titration curves, will apply to such titrations, as will the general methods of endpoint location. 

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. 

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. 

Since potentiometric titrations are an old and well-known technique, particularly in regard to acid-base and oxidation-reduction titrations, only a few selected examples will be presented here. For more detailed treatments, the student is urged to consult the bibliography at the end of the chapter. It will be assumed that the student is already familiar with titration curves and their calculation from ionic equilibria and other pertinent data. 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

Titrations that use the human eye as the primary detector are based on color change. Other types of titrations that rely on color change are oxidation-reduction, precipitation, and complexometric titrations. Other detectors indicate voltage or other types of changes such as potentiometric titrations, conductometric titrations and amperometric titrations—all of which require additional instrumentation—and may be quite colorless. 

Figure R.2 Experimental arrangement for measuring the electrode potential of an oxidation-reduction electrode during potentiometric titration...

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