Oxidative titration

Conductimetric measurements can also be used to ascertain the end-point in many titrations, but such use is limited to comparatively simple systems in which there are no excessive amounts of reagents present. Thus, many oxidation titrations which require the presence of relatively large amounts of acid are not suited to conductimetric titration. Conductimetric titrations have been largely superseded by potentiometric procedures (see Chapter 15), but there are occasions when the conductimetric method can be advantageous.14... 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Figure 4. Oxidative titration of activated aconitase with potassium fenicyanide. Aliquots of activated aconitase and a measured amount of K3Fe(CN)6 were added to individual EPR tubes, assayed for enzymatic activity ( ), and frozen. After deteimination of the number of spins by EPR at g = 2.01 (x), the samples were thawed and assayed again (o). (Reproduced with permission from Ref. 47. Copyright 1983 American Society Biological Chemists.)...

The oxidative titration of chlorpromazine with ceric sulfate or KBrOj-KBr in acid solution has been described, with the end point being determined by a dead-stop end point technique [55]. A similar method involving visual or potentiometric detection of the end point was also reported [56]. 

When current is applied to a Y203-doped Zr02 oxygen electrode, the electrode becomes a coulometric generator of 02 and has been used for oxidative titrations at 600°C. [K. Hirakata, W. E. Rhine, and M. J. Cima. 

Several analytical procedures are baaed on the hydration rather than hydrohalogenation of epoxide . Tlie resultittg 1,2-diols are assayed by oxidative titration with periodic acid in aqueous sulfurio acid or perchloric acid.4711 Alternatively, carbonyl compounds formed on periodic acid oxidation of 1,2-diola may be determined color imetricajlv wkh phenylbydrwine or other suitable reagents.247 374... 

Figure 3. Values of H0 for alkaline earth metal oxides titrated with benzoic acid in benzene, using nitroaniline indicators. Key a, MgO b, CaO and c, SiO. (Reproduced, with permission, from Ref. 11.

Roberts LM, Lindahl PA (1994) Analysis of oxidative titrations of Desulfovibrio gigas hydrogenase implications for the catalytic mechanism. Biochemistry 33 14339—14350... 

Reaction of [FePc] with thionyl chloride produces [FePcCl2], as in equation (84), which is considered to contain iron(III) and the oxidized macrocycle Pc, with chloride ions in the axial positions.956,957 The overall oxidation level is confirmed by oxidative titration and the observed magnetic moment (3.1 BM) indicates a low-spin S — % state. Considerable care is needed to exclude... 

The electrochemical potentials of different states of the [FeNij-hydrogenase enzyme from D. gigas have been probed using oxidative titrations at pH 8.0 [49, 50j. In addition to the redox events of the iron-sulfur clusters the data are best fit to a model involving four different active site redox states between 0 and —400 mV. The potentials of these four redox states are collected in Table 3. 

Hpoxidation of olefins. Reif and House worked out the following procedures for I ho preparation of rra/ij-stilbene oxide. Titration of the commercial 40% peracetic acid showed it to contain 0.497 g. of peracid per ml. For reaction with 0.3 mole of t/wi.v-stilbene, 65 ml. (0.425 mole) of reagent was used after addition of 5 g. of Na()Ac-3 H 0 to neturalize the sulfuric acid present. A solution of the hydrocarbon... 

Fig. 6. (A) Redox titration of the primary electron acceptor in Rb. sphaeroides chromatophores at pH 11. The amplitude of absorption changes Induced by short flashes is plotted as a function of the redox potential solid dots and empty circles represent reductive and oxidative titrations, respectively. The solid line is the theoretical Nernst curve. (B) Equilibrium midpoint potentials of the primary acceptor as determined in (A) plotted as a function of pH. Figure source Prince and Dutton (1978) Protonation and the reducing potential of the primary electron acceptor. In RK Clayton and WR Sistrom (eds) The Photosynthetic Bacteria, p 443, 444. Plenum.

Fig. 5. Redox titration of Chl-a fluorescence yield in isolated chloroplasts containing redox mediators that excludes (top) and includes (bottom) neutral red. Round and square dots are for reductive and oxidative titrations, respectively. Dashed line represents Nernst plots for n=l at both -247 and -45 mV (top) and for n=1 at -85 and -40 mV and n=2 at -375 mV (bottom). See text for discussion. Figure source Horton and Croze (1979) Characterization of two quenchers of chlorophyit fiuorescence with different midpoint oxidation-reduction potentiais in chloroplasts. Biochim Biophys Acta 545 191,192.

Fig. 6 (C) shows the redox titration curve consisting of data points from triplet signals produced in samples poised at various potentials at pH 11. Again, the solid curve is a fit for the Nernst equation based on a one-electron change. The empty-circled data points are taken from the reductive titrations, and several data points (solid-dots) are shown for the reverse oxidative titration. All points coincide reasonably well with the theoretical curve, confirming that the redox reaction is reversible.The redox potential of estimated from the titration curve is -604 mV, very close to the value derived from the attenuated absorbance-change measurements by Klimov etal.. ...

Fig. 6. (A) Plot of amplitude of light-induced pheophytin-reduction signal vs. ambient potential of the medium. (B) Effed of ambient redox potential on the extent of light-induced PS-II reaction-center triplet signal in pea chloroplast particles. (C) Plot of the extent of the light-induced triplet EPR signal in (B) vs. redox potential. Open and closed circles are for reductive and oxidative titrations, respedively. The solid curve is a computer fit of the Nernst equation with n=1 and E , was estimated to be -604 mV. Figure source (A) Klimov. Allakhverdiev. Demeter and Krasnovsky (1979) Photoreduction ofpheophytin in photosystem 2 ofchloroplasts with respect to the redox potential of the medium. DokI Akad NaukSSSR 249 229 (B and C) Rutherford, Mullet and Crofts (1981) Measurement of the midpoint potential of the pheophytin acceptor of photosystem II. FEBS Lett 123 236,237...

Neubauer-Loewenthal = permanganate method Direct or indirect (after precipitation of tannins) oxidative titration using permanganate. poor reproducibility 58,105,106... 

One of the earliest separations in gas liquid chromatography was that of James et al. who used a mixture of hendecanol and liquid paraffin on celite using ammonia and the methyl amines as eluents in the order of their melting points. Other stationary phases used for this and for other similar separations include triethanolamine, a mixture of w-octadecane and n-hendecanol, and polyethylene oxide. Titration cell, the first detector designed specifically for gas chromatography, was used in these early studies of the separation of ammonia and ethylamines. More recently thermal conductivity cells have been used for the detection of these compounds. 

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