Redox electrode, platinum

Organic acids, 26 Oxide OH groups, 169 Platinum redox electrode, 235, 253 Point of zero charge (PZC), 146 Clay dispersion, 146 Oxides, 131,146 Soils, 158-159... 

Orban et al. (1982-1) showed that the reaction between chlorite and thiosulphate in a CSTR exhibits oscillations. The oscillations in the potential of platinum redox electrode were recorded. These periodic oscillations showed different patterns as the input concentrations of chlorite and thiosulfate as well as pH (between 2 and 5) and flow rate varied. 

The complex and generally non-nernstian behavior of redox electrodes in natural systems has been discussed by many authors (8-lT). Problems include mixed potentials (12-15), poisoning of platinum redox electrodes (16), lack of internal redox equilibrium (8,15,16), and lack of electrochemical equilibrium (L7). Several reviews of the use of redox electrodes in geochemical studies have been published (18-20). 

Figure 1. Oscillatory behavior following an initial induction period in the potentials of a platinum redox electrode and a bromide sensitive electrode in a BZ reaction mixture containing an aqueous solution of bromate, malonic acid, a ceric salt, and sulfuric acid. Reproduced from Field et al. (6). Copyright 1972 American Chemical Society,...

The gross complexity constant of the cobalt(IIl)-ammine system was determined by redox electrode measurements. JB found that for a solution containing Co and Co under conditions at which the hexaamminecobalt(III) would be the completely dominant Co species at equilibrium, his platinum redox electrode had a potential that followed the expression ... 

Figure 8.6 Bursting oscillations in the BZ reaction in a CSTR. Signal is the potential of a platinum redox electrode. (Adapted from Sorensen, 1974.)...

If two redox electrodes both use an inert electrode material such as platinum, tlie cell EMF can be written down iimnediately. Thus, for the hydrogen/chlorine fiiel cell, which we represent by the cell Fl2(g) Pt FICl(m) Pt Cl2(g) and for which it is clear that the cathodic reaction is the reduction of CI2 as considered in section... 

The Kad Fischer jack on the back of most pH meters, used to monitor Kad Fischer titrations, suppHes a constant regulated current to the cell, which can consist of two identical (platinum) working electrodes. The voltammograms shown in Figure 9 illustrate the essential features of this technique. The initial potential difference, AH, is small because both redox forms of the sample coexist to depolarize the electrodes. The sample corresponds to the wave on the right-hand (cathodic) side of each figure and is therefore easily oxidized. The titrant is represented by the wave on the left-hand (anodic) side and is therefore easily reduced. Halfway to the end point the potential difference,, remains small, but at the end point the potential difference,... 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

Two identical stationary micro-electrodes (usually platinum) across which a potential of 0.01-0.1 V is applied can be used in place of either the DME or the rotating platinum micro-electrode. The equivalence point is marked by a sudden rise in current from zero, a decrease to zero, or a minimum at or near zero (Figures 6.16(a), (b) and (c)). The shape of the curve depends on the reversibility of the redox reactions involved. The two platinum electrodes assume the roles of anode and cathode, and in all cases a current flows in the cell only if there is a significant concentration of both the oxidized and reduced forms of one of the reactants. In general, two types of system can be envisaged ... 

However, most of the electroanalyst s experiments require solid electrodes, which naturally fall within two categories, namely either inert electrodes such as platinum, gold and glassy carbon, or redox electrodes such as copper, lead or magnesium. We will also consider optically transparent electrodes in this discussion. 

The difference between the two reactions of Scheme 2.9 may also be considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion-radical and metallocomplex cation-radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of increased concentration of reactive anion-radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, and therefore, there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind. In special experiments, only 2% of the anion-radical of a-nitrostilbene were prepared after interruption of controlled-potential electrolysis at a platinum gauze electrode. The kept potential was just past the cathodic peak. The electrolysis was performed in the well-stirred solution of trani -a-nitrostilbene in AN. Both processes developed in this case, namely, trans-to-cis conversion and dimerization (Kraiya et al. 2004). The partial electrolysis of a-nitrostilbene resulted in redox-catalyzed equilibration of the neutral isomers. 

RedOx electrode potentials are the result of an exchange of electrons between metal and electrolyte. In Section 5.4 we have shown that the metal/metal-ion electrode potentials are the result of an exchange of metal ions between metal and electrolyte. In the RedOx system the electrode must be made of an inert metal, usually platinum, for which there is no exchange of metal ions between metal and electrolyte. The electrode acts as a source or sink for electrons. The electrolyte in the RedOx system contains two substances electron donors (electron-donating species) and electron acceptors (electron-accepting species). One example of a RedOx system is shown in Figure 5.4. In this case the electron donor is Fe ", the electron acceptor is Fe , the electrode is Pt, and the electrode process is... 

In carrying out redox titrations, standard Ag/AgCl or Hg/Hg2Cl2 electrodes are used as a reference in conjunction with an inert redox electrode, e.g platinum, which takes its potential from the particular redox pair in the solution in which it is immersed. 

The redox properties of a solution are expressed quantitatively by the redox potential, which is measured as the potential of an inert redox electrode (e.g. platinum electrode) immersed in the solution under study. Thus, for an aqueous solu-... 

Redox Electrodes If a platinum electrode is immersed in a solution containing the oxidized and reduced forms (Ox, Red) of a redox reaction Ox+ne <=> Red, its potential is given by the Nernst equation (Section 5.2.1) ... 

The electrochemistry of cytochrome c is attracting much attention,690 in particular the development of electrodes for cytochrome c, which either have a redox mediator bound to the surface, or utilize a solution mediator. Examples are a platinum gauze electrode functionalized with 2,3,4,5-tetramethyl-l-(dichlorosilyl)methyl[2]ferrocenophane,691 and 4,4 -bipyridyl, which serves as a bridge between cytochrome c and the electrode.692... 

Fig. 3. Schematic diagram of a CSTR. In the configuration shown, up to three different solutions can be pumped (by the peristaltic pump, PP) into the reactor, R. The detectors shown in the diagram are , light absorption (A/, monochromator PM, photomultiplier), platinum (redox), and iodide (or bromide) selective electrodes. The reference electrode is the Hg/Hg2S04 couple, in place of the usual calomel electrode, to avoid adventitious introduction of chloride into the reactor. In addition to these detectors, a thermocouple, or thermistor, and a pH electrode can be inserted into the reactor from above. The recordings of periodic behavior were taken from studies on the chlorite-iodide reaction...

Scheme 5. Scheme of electrochemical ion transport with redox-active crown ether. W W2 mini-grid platinum working electrodes. C C2 platinum plate counter electrodes. R1, R2 saturated calomel reference electrodes. 

The first redox electrode materials to be used were the noble metals, namely gold and platinum, and also mercury. At present this designation includes many types of material such as glassy carbon, different types of graphite, and semiconductor oxides, so long as a zone of potential is employed where surface reactions involving the electrode material do not occur. 

Quinhydrone electrode — introduced by Biilmann in 1921 [i-vi] is a redox electrode for pH measurements [vii, viii]. Quinhydrone is a sparingly soluble charge-transfer complex consisting of quinone and hy-droquinone in 1 1 ratio. At an inert electron conductor (e.g., - platinum wire or modified -> graphite) the following electrochemical equilibrium is established ... 

The -> potential of this -> redox electrode (platinum, quinhydrone solution) depends on the ratio quinone to hydroquinone and on the pH of the solution ... 

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