Platinum, standard reduction potentials

Of the Group 10 elements, nickel, palladium and platinum, only the +2 states of Ni and Pd are well characterized in aqueous acid solutions. Their + 2/0 standard reduction potentials in acid solution are given in the Latimer diagram ... 

The third largest class of enzymes is the oxidoreductases, which transfer electrons. Oxidoreductase reactions are different from other reactions in that they can be divided into two or more half reactions. Usually there are only two half reactions, but the methane monooxygenase reaction can be divided into three "half reactions." Each chemical half reaction makes an independent contribution to the equilibrium constant E for a chemical redox reaction. For chemical reactions the standard reduction potentials ° can be determined for half reactions by using electrochemical cells, and these measurements have provided most of the information on standard chemical thermodynamic properties of ions. This research has been restricted to rather simple reactions for which electrode reactions are reversible on platinized platinum or other metal electrodes. 

By convention, standard reduction potentials are defined with reference to the half-reaction of the standard hydrogen electrode at pH 0, 25 °C, and 1 atmosphere of hydrogen gas which is in contact with a platinum electrode (6) ... 

Platinum—The high standard reduction potential for platinum makes it an ideal anode material (although Pt corrosion does occur during some oxidations reactions, such as the Kolbe oxidative coupling reaction [29, 58, 59]). For anode potentials greater than about 0.50 V (on the hydrogen scale), an oxide film covers the Pt surface, so the electrode material is often platinum oxide. Large anodes are often titanium coated with platinum in order to reduce costs. 

Tabulated values of standard reduction potentials, E°, refer to single electrodes. For example, for the half-cell reaction 7.6, the value of °cT+/cu = +0.34 V. However, it is impossible to measure the potential of an individual electrode and the universal practice is to express all such potentials relative to that of the standard hydrogen electrode. The latter consists of a platinum wire immersed in a solution of ions at a concentration of Imoldm (strictly, unit activity) in equilibrium with H2 at 1 bar pressure (equation 7.12). This electrode is taken to have a standard reduction potential E° = QY at all temperatures. 

Standard hydrogen electrode (SHE) (13.3) Electrode used to define the scale of standard reduction potentials, consisting of a platinum electrode at which 1 M H30 ions are reduced to H2 gas at 1 atm. 

Most of the transition elements do not react with strong acids, such as HCl and H2SO4. Some do have negative standard reduction potentials for the reaction M"+ -I- ne - M, and liberate hydrogen from hydrochloric acid. These include Mn, Cr, and the iron triad. Silver, gold, the palladium triad, and the platinum triad, the so-called noble metals, are especially inert to acids, both to the nonoxidizing species, such as hydrochloric and hydrofluoric acids, and to the oxidizing acids, such as nitric acid. 

Standard hydrogen electrode a platinum conductor in contact with 1 Af ions and bathed by hydrogen gas at one atmosphere. (17.2) Standard reduction potential the potential of a half-reaction under standard state conditions, as measured against the potential of the standard hydrogen electrode. (17.2)... 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

Chloride ions will be discharged at a platinum, graphite or magnetite anode from saturated neutral solutions of sodium or potassium chloride rather than hydroxyl ions although at equilibrium conditions it should be the very opposite, as the reversible deposition potential (reduction potential) of oxygen in neutral solution is much lower ) (7Eoh- i o2. Pt = 0.815 V at 25 °C) than the standard... 

In contrast to standard borohydride reductive nanoparticle synthesis, we have developed an alternative strategy to amino acid encapsulated nanoparticles by utilizing a metal nanoparticle (M°-(Ligand))/metal ion (M"+) precursor redox pair with matched oxidation/reduction potentials. Simply, a metal nanoparticle such as Pt°-(Cys) acts as the principal reductant to a complimentary selected metal ion of Au + resulting in a new stabilized metal nanoparticle of Au°-(Cys) and the oxidation product of the original nanoparticle Pt"+. Malow et al. have reported a metathesis/transmetallation type reaction between a platinum colloid and a Au cyanide compound. Similarly, we employed a Pt°-(Cys)/AuCl4 pair and 0.5-2.0 equivalents of Au to Pt -(Cys). XRD analysis of the nanoparticle products revealed differences in crystallinity... 

Determination of Standard Oxidation-Reduction Potentials.—In principle, the determination of the standard potential of an oxidation-reduction system involves setting up electrodes containing the oxidized and reduced states at known activities and measuring the potential B by combination with a suitable reference electrode insertion of the value of B in the appropriate form of equation (3) then permits B to be calculated. The inert metal employed in the oxidation-reduction electrode is frequently of smooth platinum, clthough platinized platinum, mercury and particularly gold are often used. 

A standard hydrogen electrode consists of a platinum electrode with hydrogen gas at 1 atm pressure bubbling into an acidic solution that is 1/W in hydrogen ions. The reduction potential for this configuration is 0 V. 

Determining Ehaif-ceii The Standard Hydrogen Electrode What portion of ceii for the zinc-copper reaction is contributed by the anode half-cell (oxidation of Zn) and what portion by the cathode half-cell (reduction of Cu ) That is, how can we know half-cell potentials if we can only measure the potential of the complete cell Half-cell potentials, such as Ezine and °opper. are not absolute quantities, but rather are values relative to that of a standard. This standard reference halfcell has its standard electrode potential defined as zero (E fereiice — 0.00 V). The standard reference half-cell is a standard hydrogen electrode, which consists of a specially prepared platinum electrode immersed in a 1 M aqueous solution of a strong acid, H (fl ) [or H30 (a )], through which H2 gas at 1 atm is bubbled. Thus, the reference half-reaction is... 

The oxidation-reduction potentials for half reactions such as Fe" —> Fe + e are measured by putting a piece of platinum or other inert metal into a solution containing ferrous and ferric ions in standard concentrations, and combining this half cell with the standard hydrogen half cell. Again, the platinum serves to conduct electrons and to catalyze the equilibrium between ferrous and ferric ions. The electromotive force of the cell... 

Redox potential (Eh). The potential that is generated between an oxidation or reduction halfreaction and the standard hydrogen electrode (SHE) (0.0 V at pH = 0). In soils, it is the potential created by oxidation-reduction reactions that take place on the surface of a platinum electrode measured against a reference electrode minus the Eh of the reference electrode. This is a measure of oxidation-reduction potential of redox active components in the soil (see Chapter 4). 

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