Oxidation-reduction titration oxidants

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 ... 

A. Internal oxidation-reduction indicators. As discussed in Sections 10.10-10.16, acid-base indicators are employed to mark the sudden change in pH during acid-base titrations. Similarly an oxidation-reduction indicator should mark the sudden change in the oxidation potential in the neighbourhood of the equivalence point in an oxidation-reduction titration. The ideal oxidation-reduction indicator will be one with an oxidation potential intermediate between... 

Oxidation-reduction titrations revealed the existence of two other oxidation-reduction states, EPR silent, designated as Ni-Si (Ni-Si-lent) and Ni-R (Ni-Reduced) (178, 180). A detailed study of the oxidation-reduction pattern involved enabled the following sequence of events (in the oxidation direction) to be written ... 

Note the exchange of E and E° compared to Equation 13.12.) There are also quite a number of systems that can occur in three subsequent oxidation states, usually with the intermediate state being half-integer spin, such as Coin/Con/Co, or Movi/Mov/Moiv. In a reductive titration, the EPR intensity initially increases, and subsequently decreases again according to... 

Experiment 8 Determination of Concentration by Oxidation-Reduction Titration and an Actual Student Lab Write-Up ... 

The amount of hypochlorite ion present in bleach can be determined by an oxidation-reduction titration. In this experiment, an iodine-thiosulfate titration will be utilized. The iodide ion is oxidized to form iodine, I2. This iodine is then titrated with a solution of sodium thiosulfate of known concentration. Three steps are involved ... 

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. 

Ceric ammonium nitrate is used as a volumetric oxidizing reagent in many oxidation-reduction titrations. Cerium(IV) ion is a strong oxidant simdar to permanganate ion. It is the most widely-used primary standard among all Ce(IV) compounds. Other apphcations of this compound are in organic oxidation reactions and as a catalyst in polymerization of olefins. 

Oxidation-Reduction Titrations. The extent of reduction resulting from reaction of niobium (V) chloride and bromide with pyridine was determined by indirect titration of crude reaction mixtures with standard ammonium tetrasulfato-cerate(IV) solution. Samples were stirred overnight in a stoppered flask with an excess of iron (III) ammonium sulfate. Any iron (II) formed by reaction with the niobium complex mixture was then titrated with the standard tetrasulfato-cerate(IV) solution using ferroin as indicator. Results of these determinations are given in Table III. 

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 ... 

Similarly, it was concluded that in the reduction reaction of niobium(V) bromide with pyridine the major products were tetrabromodi (pyridine )niobium-(IV), pyridinium bromide, and l-(4-pyridyl) pyridinium bromide. Oxidation-reduction titrations indicated approximately 90% reduction. The remaining 10% of niobium was assumed to be present as pentabromo(pyridine)niobium(V). The over-all reaction was indicated to be ... 

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 prototypical high-temperature superconductor is yttrium barium copper oxide, YBa2Cu307, in which two-thirds of the copper is in the +2 oxidation state and one-third is in the unusual +3 state. Another example is BijSrjlCaQgYoyjCujOgjys, in which the average oxidation state of copper is +2.105 and the average oxidation state of bismuth is +3.090 (which is formally a mixture of Bi3+ and Bi5+). The most reliable means to unravel these complex formulas is through wet oxidation-reduction titrations, described in this chapter. 

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 ... 

As ascribed, the EPR spectrum with g = 2.10 can be low-spin Fec(III). When the isolated enzyme is reductively titrated this signal disappears at a potential Emj -0.3 V [65]. This would seem to indicate that the putative Fec(III) form is not relevant, at least not to hydrogen-production activity. The cubane is a one-electron acceptor as it can shuttle between the 2+ and 1 + oxidation states. Therefore, if the active center were to take up a total of two electrons, then the oxidation state of the Fec would, as least formally, shuttle between II and I. Recently, a redox transition in Fe hydrogenase with an Em below the H2/H+ potential has been observed in direct electrochemistry [89]. This superreduced state has not been studied by spectroscopy. It might well correspond to the formal Fec(I) state. For NiFe hydrogenases Fec(I) has recently been proposed as a key intermediate in the catalytic cycle [90] (cf. Chapter 9). 

Potentiometric reductive titration, using both fresh thylakoids and PS II particles previously oxidized with ferricyanide, has revealed that the LP couple exhibits a constant midpoint redox potential ( o- +° 12 v ) above pH 7.6, but becomes pH-dependent below this pH, with a slope of about -60 mV pH pH unit, whereas the HP couple is pH-independent in the pH range between 6.5 an 8.5 ( o-+0 36 V) in general, cytochrome b-559 exhibits potential values about 40 mV lower in thylakoids than in PS II particles. After mild heating of the fresh preparations or treatment with the detergent Triton X-100, the HP couple is converted into the LP couple, which preserves its characteristic pH-dependence. In contrast, in the presence of the uncoupler CCCP, the HP couple is also converted into the LP couple, but the pH-dependence proper to the latter is now lost. 

---