Oxidation-reduction titrants

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

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... 

Common chemical titrations include acid-base, oxidation-reduction, precipitation, and complexometric analysis. The basic concepts underlying all titration are illustrated by classic acid-base titrations. A known amount of acid is placed in a flask and an indicator added. The indicator is a compound whose color depends on the pH of its environment. A solution of base of precisely known concentration (referred to as the titrant) is then added to the acid until all of the acid has just been reacted, causing the pH of the solution to increase and the color of the indicator to change. The volume of the base required to get to this point in the titration is known as the end point of the titration. The concentration of the acid present in the original solution can be calculated from the volume of base needed to reach the end point and the known concentration of the base. 

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. 

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. 

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... 

Electrochemical endpoint detection methods provide a number of advantages over classical visual indicators. These methods can be used when visual methods of endpoint detection cannot be employed because of the presence of colored or clouded solutions and in the case of detection of several components in the same solution. They are more precise and accurate. In particular, such methods provide increased sensitivity and are often amenable to automation. Electrochemical methods of endpoint detection are applicable to most oxidation-reduction, acid-base, and precipitation titrations, and to many complex-ation titrations. The only necessary condition is that either the titrant or the species being titrated must give some type of electrochemical response that is indicative of the concentration of the species. 

Coulometric titration procedures have been developed for a great number of oxidation-reduction, acid-base, precipitation, and complexation reactions. The sample systems as well as the electrochemical intemediates used for them are summarized in Table 4.1, and indicate the diversity and range of application for the method. An additional specialized form of coulometric titration involves the use of a spent Karl Fischer solution as the electrochemical intermediate for the determination of water at extremely low levels. For such a system the anode reaction regenerates iodine, which is the crucial component of the Karl Fischer titrant. This then reacts with the water in the sample system according to the... 

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]. 

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. 

In secondary coulometric titrations, an oxidation-reduction buffer serves as the titrant precursor. An active intermediate from the titrant precursor must first be generated with 100% efficiency by the electrode process. The intermediate must react rapidly and completely with the substance being analyzed. 

Titrations are widely used in analytical chemistry to determine acids, bases, oxidants, reductants, metal ions, proteins, and many other species. Titrations are based on a reaction between the analyte and a standard reagent known as the titrant. The reaction is of known and reproducible stoichiometry. The volume, or the mass, of the titrant needed to react essentially completely with the analyte is determined and used to obtain the quantity of analyte. A volume-based titration is shown in this figure, in which the standard solution is added from a buret, and the reaction occurs in the Erlenmeyer flask. In some titrations, known as coulometric titrations, the quantity of charge needed to completely consume the analyte is obtained. In any titration, the point of chemical equivalence, experimentally called the end point, is signaled by an indicator color change or a change in an instrumental response. 

General oxidation/reduction indicators are substances that change color on being oxidized or reduced. In contrast to specific indicators, the color changes of true redox indicators are largely independent of the chemical nature of the analyte and titrant and depend instead on the changes in the electrode potential of the system that occur as the titration progresses. 

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

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. 

Equivalence-point potential The electrode potential of the system in an oxidation/reduction titration when the amount of titrant that has been added is chemically equivalent to the amount of analyte in the sample. 

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. 

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. 

Selected points along the titration curve are calculated in a manner that closely resembles those we used in the last two chapters. With oxidation-reduction titrations, the method is even simpler when we recognize that on either side of the equivalence point there is an excess of one of the two redox couples, allowing us to calculate the concentration ratio of oxidized to reduced forms of the substance being titrated or of the titrant. Knowing... 

A titration is a procedure in which increments of a known reagent—the titrant— are added to analyte until the reaction between analyte and titrant is complete. Titrant is usually delivered as a solution from a buret (Figure 6-1). Each increment of titrant should be completely and quickly consumed by reaction with analyte until analyte is used up. Common titrations are based on acid-base, oxidation-reduction, complex formation, or precipitation reactions. 

In analytical chemistry, a redox titration is based on an oxidation-reduction reaction between analyte and titrant. Common analytical oxidants include iodine (I2), permanganate (MnOJ), cerium(IV), and dichromate (Cr207 ). Titrations with reducing agents such as Fe " (ferrous ion) and Sn " (stannous ion) are less common because solutions of most reducing agents need protection from air to prevent reaction with O2. 

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... 

The two strongest oxidizing titrants are Mn04 and Ce +, for which the reduction half-reactions are... 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

The most important method for reduction of compounds to an oxidation state suitable for titration with one of the common oxidising titrants is based upon the use of metal amalgams, but there are various other methods which can be used, and these will be discussed in the following sections. 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

In the first group the titrant is generated either directly from a participating or active electrode, or indirectly from an inert or passive electrode, in which case it is necessary to add previously an auxiliary substance that generates the titrant by either cathodic reduction or anodic oxidation the end-point detection is usually potentiometric or amperometric. The following selected examples are illustrative of the first group in non-aqueous media ... 

As stated previously, the iodine titrant is generated electrochemically in the coulometric method. Electrochemical generation refers to the fact that a needed chemical is a product of either the oxidation halfreaction at an anode or the reduction half-reaction at a cathode. In the Karl Fischer coulometric method, iodine is generated at an anode via the oxidation of the iodide ion ... 

This chapter introduces the theory of redox titrations and discusses some common reagents. A few of the oxidants and reductants in Table 16-1 can be used as titrants.2 Most reductants react with 02 and require protection from air to be used as titrants. 

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