Hydrogenation potentiometric reduction

Other methods have been described which depend on reduction of the tellurium compound by means of titanous chloride, but these generally are not trustworthy owing to the formation of hydrogen telluride. Potentiometric titration with titanous chloride in the presence of hydrochloric acid has been recommended.1... 

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

Beck and Gerischer (34) used also the potentiometric method to study the kinetics of reduction of various simple-chain and cyclic olefins. Hydrogenation on a vibrating platinized platinum electrode was zero order in alkene and independent of pH in the region 2-8. In the presence of halide ions, specific catalyst poisoning caused decline of the reduction rate. 

Reduction of acetylene as well as ethylene, propylene, and cyclopropane, at positive potential was first reported by Langer and co-workers (25, 26, 33). Mass spectrometric and coulometric analyses showed quantitative hydrogenation of the unsaturates. Reductions were fastest in acidic electrolytes in agreement with the potentiometric results discussed earlier. The relative rates of the reactants on Pt black appeared to follow the strength of adsorption, with acetylene the most difficult to hydrogenate and with cyclopropane reducing readily. 

The reaction continues and current passes until all the iodide is used up. At this point some means of endpoint detection is needed. Two methods are commonly adopted. The first uses an amperometric circuit with a small imposed voltage that is insufficient to electrolyze any of the solutes. When the mercury ion concentration suddenly increases, the current will rise because of the increase in the concentration of the conducting species. The second method involves using a suitable indicator electrode. An indicator electrode may be a metal electrode in contact with its own ions or an inert electrode in contact with a redox couple in solution. The signal recorded is potentiometric (a cell voltage vs. a stable reference electrode). For mercury or silver we may use the elemental electrodes, because they are at positive standard reduction potentials to the hydrogen/hydrogen ion couple. 

In addition to the amperometric feedback mode described above, other amperometric operation modes are also possible. For example, in the substrate generation/tip collection (SG/TC) mode, ip is used to monitor the flux of electroactive species from the substrate and vice versa for the tip generation/substrate collection (TG/SC) mode. These operation modes will be described in Section 12.3.1.3 and are useful in studies of homogeneous reactions that occur in the tip-substrate gap (see Section 12.4.2) and also in the evaluation of catalytic activities of different materials for useful reactions, e.g., oxygen reduction and hydrogen oxidation (see Section 12.4.3). In addition to the amperometric methods, other techniques, e.g., potentiometric method is also applicable for SECM and will be discussed in Section 12.3.2. We will also update the techniques suitable for the preparation of SECM amperometric tips in Section 12.3.1.1 and potentiometric probes in Section 12.3.2.2. 

Standard oxidation potentials referred to the normal hydrogen electrode (E°) for the divalent and trivalent lanthanide aquo ions are given in table 24.8 based primarily on the selected experimental results compiled by Chariot et al. (1971) and the systematic correlations summarized by Nugent (1975). Only Eu and Yb can persist for times of the order of minutes to hours in dilute acid solution, and can be readily produced by reduction of the trivalent ionic species (Laitinen and Taebel, 1941 Laitinen, 1942 Christensen et al., 1973). The value of E° = -0.43 V for the Eu(II-III) couple quoted in many previous compilations was obtained using both potentiometric and polarographic techniques. However, in a reevaluation of the solution thermodynamic properties of europium, Morss and Haug (1973) recommended the value E° = —0.35 V. 

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