Experiment 8 Oxidation—Reduction Titration

The concentration of either an oxidizing or a reducing agent may be determined by titrating a solution of an unknown concentration versus a solution of a known concentration or containing a known mass of solute. (See the Stoichiometry chapter.) 

The volume of titrant added is calculated by the difference between measurements 2 and 3. 

The pipeted volume is converted to moles by multiplying the liters of solution by its molarity. The moles of titrant are determined from the mole ratio in the balanced chemical equation for the reaction. The molarity of the solution is calculated by dividing the moles of titrant by the liters of titrant used. 

Common oxidants are potassium permanganate and potassium dichromate. 

The initial masses of various reactants may be determined and then converted to moles. A similar calculation may be done for the products. (See the Stoichiometry chapter.) 

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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 ... 

Reflect and Apply You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation-reduction titration involving iodine. You get a starch indicator from the stock-room. Why do you need it ... 

The standard potential of the indicator system is not known exactly, but experiments have shown that in not too strongly acid solutions the sharp color change from colorless to violet, with green as a possible intermediate, occurs at a potential of about — 0.75 volt. The standard potential of the ferrous-ferric system is 0.78 whereas that of the di-chromate-chromic ion system in an acid medium is approximately — 1.2 volt hence a suitable oxidation-reduction indicator might be expected to have a standard potential of about — 0.95 volt. It would thus appear that diphenylamine would not be satisfactory for the titration of ferrous ions by acid dichromate, and this is actually true if a simple ferrous salt is employed. In actual practice, for titration purposes, phosphoric acid or a fluoride is added to the solution these substances form complex ions with the ferric ions with the result that the effective standard potential of the ferrous-ferric system is lowered (numerically) to about — 0.5 volt. The change of potential at the end-point of the titration is thus from about — 0.6 to — 1.1 volt, and hence diphenylamine, changing color in the vicinity of — 0.75 volt, is a satisfactory indicator. 

In contrast to the other MoFe proteins, epr-monitored oxidation-reduction experiments with isolated Cvl yielded two 1-electron redox processes with midpoint potentials at -260 and 60 mV. Albrecht and Evans (1973) suggested that both centers were present in each molecule of Cvl however, O Donnell and Smith, to bring this observation in line with evidence from the redox titrations of the other MoFe proteins, argued that the two centers could arise from two different forms of the enzyme. 

If existence of a persulphide or other potentially electron accepting sulphur group is confirmed, this might explain why redox titration experiments have shown the number of electron equivalents which the xanthine oxidase molecule can accept to be greater than is required for reduction of the three non-protein components (58, 91). Certainly, this interpretation seems more probable than the original suggestion (58, 91) that the molybdenum can be reduced to lower oxidation states than Mo(IV) by some substrates. 

Perhaps the most important application of redox chemicals in the modern laboratory is in oxidation or reduction reactions that are required as part of a preparation scheme. Such preoxidation or prereduction is also frequently required for certain instrumental procedures for which a specific oxidation state is essential in order to measure whatever property is measured by the instrument. An example in this textbook can be found in Experiment 19 (the hydroxylamine hydrochloride keeps the iron in the +2 state). Also in wastewater treatment plants, it is important to measure dissolved oxygen (DO). In this procedure, Mn(OH)2 reacts with the oxygen in basic solution to form Mn(OH)3. When acidified and in the presence of KI, iodine is liberated and titrated. This method is called the Winkler method. 

The nitric oxide was determined as usual—by absorption in water and titration. Control experiments on the combustion of the gas at low temperature and of analysis according to Kjeldahl (reduction of nitric and nitrous acids to NH3 by the action of MgAl-alloy in alkaline solution, distillation and absorption of NH3 by acid) convinced us that the gas contained no noticeable traces of sulphur and that the acidity as ordinarily determined depends on nitric and nitrous acids. 

Several findings in the above results are not consistent with earlier reports (Yoshikawa et al., 1995 Van Gelder, 1966 Tiesjema et al., 1973 Schroedl and Hartzell, 1977 Babcock et al., 1978 Blair et al., 1986 Steffens et al., 1993). It has been widely accepted that four electron equivalents are sufficient for complete reduction of the fuUy oxidized enzyme as prepared. However, most of the previous titrations were performed in the presence of electron transfer mediators. In the presence of electron transfer mediators, such as phenazine methosulfate (PMS) under anaerobic conditions, the bovine heart enzyme purified with crystallization also showed a four-electron reduction without the initial lag phase as observed in Fig. 9. A catalytic amount of PMS induced a small spectral change corresponding to the initial lag phase. These results suggest that electron transfer mediators in other titration experiments also induce autoreductions to provide the enzyme form that receives four electrons for the complete reduction. 

Surface oxide formation undoubtedly is involved in the Fe(II)-dichromate titration curves, which Smith and Brandt found to be different when the direction of titration was reversed (Figure 15-2, right). Kolthoff and Tanaka found that the rate of oxidation with dichromate was slow, whereas the rate of reduction with Fe(II) was fast. Ross and Shain found the same sort of behavior and noted also that the rates of oxidation and reduction decreased in more dilute solutions. The oxidized surface in a dichromate solution may be largely covered with adsorbed dichromate, as chromium surfaces have been shown to be in some experiments with radio-chromium, so that it is relatively ineffective as an electron-transfer surface for the Fe(III)-Fe(II) system. 

In a more recent series of experiments, George (97) has shown that the secondary hydrogen peroxide complexes of horseradish peroxidase and cytochrome-c peroxidase can be titrated with ferroevanide or ferrous ions and also appear to take part in a one oxidizing equivalent reduction to the ferric form of the enzyme. In this important chemical property they thus resemble the metmyoglobin complex in spite of marked spectroscopic differences in the visible region of the spectrum (Keilin and Hartree, 48). If the peroxide molecule is not a component part of the structure it... 

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