Platinum electrodes in aqueous solution

In this paper we extend our SHG studies to silver electrodes in acetonitrile solutions and to platinum electrodes in aqueous solutions. Three different examples are chosen to demonstrate how SHG can be used both qualitatively and quantitatively to study the adsorption of chemical species onto... 

Electrodes may be classified into the following two categories as shown in Fig. 4-3 one is the electronic electrode at which the transfer of electrons takes place, and the other is the ionic electrode at which the transfer of ions takes place. The electronic electrode corresponds, for instance, to the case in which the transfer of redox electrons in reduction-oxidation reactions, such as Fe = Fe + e,occurs and the ionic electrode corresponds to the case in which the transfer of ions, such as Fe , , = Fe, occiirs across the electrode interface. Usually, the former is found with insoluble electrodes such as platinum electrodes in aqueous solution containing redox particles and the latter is found with soluble metal electrodes such as iron and nickel. In practice, both electron transfer and ion transfer can take place simultaneously across the electrode interface. 

Fig. 5-32. Nonnalized density profile of oxygen atom and hydrogen atom on an interface (111) plane of platinum electrode in aqueous solution p = atomic density Pbuu, = mean atomic density in aqueous solution x= distance normal to the interface. [From Heinzinger, 1993.]...

Figure 9.1, Varialion of peak potential with pH for the oxidation on,3-dimethy -5-ethylbarbituric acid 2 at a platinum electrode in aqueous solution, sweep rate 5 mV s . Data from ref. [17],...

Finally, two papers have been concerned with a study of adsorption on platinum electrodes in aqueous solution. One was concerned with the adsorption of CO (55) and the other with I2 (56), and although again very weak signals were detected [v(PtC0) at 2096 and 2081 cm-l, and v(I-I) at 174 cm"l], the experiments do illustrate one of the major advantages of the Raman technique, viz, the ability to study adsorption from aqueous media. 

Most of the earlier studies were carried out on polycrystalline platinum electrodes in aqueous solutions. During the last decade extensive studies have been undertaken with platinum single-crystal surfaces in order to establish the influence of the surface structure on the reaction of the oxidation of methanol. 

Platinum electrode in aqueous solution containing sulphuric acid (1 mol L ) ... 

Aminodiphenylamine (PAD) Allendoerfer and coworker [114] studied the oxidation of PAD at platinum electrodes in aqueous solution, in the pH range 1.2-4.8. Cyclic voltammetry in buffered solution produces a single peak, with (=0-5( p +... 

In addition, electrooxidation of cystine and cysteine at platinum and gold electrodes has been described [158-160]. All a-amino acids have been found oxidizable at solid metal electrodes at approximately the same potentials [161, 162]. This oxidation leads to the formation of an imine intermediate, which is further oxidized to nornitril. At a silver electrode slow hydrolysis of this intermediate to noraldehyde also takes place. The electrochemical oxidation reactions of a- and jS-alanine at a platinum electrode in aqueous solutions produce free radicals accompanied by a second reaction involving loss of CO2 [163]. In the electrooxidation of a-alanine, the adsorbed intermediate species is either hydrolyzed anodically to acetaldehyde and ammonia, or is oxidized to a carbonium ion which is subsequently hydrolyzed to acetaldehyde and ammonia in solution, analoguous to the behaviour of glycine [164]. The mechanism for jS-alanine is similar except carbonium ion formation is accompanied by a hybrid transfer to form acetaldehyde. 

The thermodynamic standard potential of the methanol electrode has a value of + 0.02 V (RHE) that is, it is quite close to the hydrogen electrode potential. The steady-state potential of a platinum electrode in aqueous methanol solutions is about + 0.3 V (RHE). 

Examples of these processes are the oxidation of p-aminophenol at platinum electrodes in aqueous acidic solution, the reduction of dopamine at glassy carbon electrodes or that of the cation 2,6-diphenylpyrylium in acetonitrile [9]. Another interesting example arises from the facilitated ion transfer of amines from aqueous to organic media in the presence of crown ethers like the dibenzo-18-crown-6 [52, 53] (see Fig. 3.22). 

In our investigation we have used BiOHal electrodes obtained by the exhaustive anodic oxidation of a bismuth layer with a thickness of about 200 nm on a platinum substrate in aqueous solutions of potassium halides (KHal) using the method previously reported in full details [93]. The resulted BiOHal films exhibited porous structure (according to the BET data, the surface area was 47, 25, and 6 m2/g for BiOCl, BiOBr, and... 

An example of this scheme is the oxidation of p-aminophenol (PAP) at a platinum electrode in aqueous acidic solutions ... 

Chlorinated hydrocarbons can also be decomposed by cathodic reduction at a copper electrode in aqueous solutions at pH = 4 to 7. The reaction was carried out in a fixed-bed, flow--through electrolyser filled with copper balls, 0.2 - 0.6 mm in diameter, supported on a platinum gauze [43]. The best results were obtained with hexachlorocyclohexane, which was completely and rapidly dechlorinated less reactive compounds were tetra-chloroethylene, trichloroethane, and chlorobenzene unsatisfactory results were obtained with a polychlorinated biphenyl. 

For platinum electrodes in aqueous NaCl solution, we have at the anode - = 0 1 +H"... 

K.D. SneU, A.G. Keenan, Chloride inhibition of ethanol electrooxidation at a platinum electrode in aqueous add solution, Electrochim. Acta 26 (1981) 1339-1344. 

This also follows from the results obtained by Koehl [98]. In the oxidation of monoesters of methylpropylmalonic and methyl-isopropylmalonic acids Eberson [93] obtained a mixture of the diastereoisomers of Q, Q -dimethyl-Q, o -dipropylsuccinic and dimethyl-Q, Q -diisopropylsuccinid acids. During electrolytic oxidation of meso- and racemic 2,3-diphenylsuccinic acids in a mixture of pyridine and water (9 1) Corey and Casanova [94] obtained only trans-stilbene, which is also formed in the oxidation of these acids by lead tetraacetate. It is pointed out that these results exclude the possibility of cis-elimination. In the case of anodic oxidation of butylboric acid Humphrey and Williams [96] found that a mixture of 1-butene, cis-2-butene, and trans-2-butene in the proportions 3.3 1 1 was formed at a platinum electrode in aqueous alkaline solutions at low current densities. 

Overlap of the oxide and hydrogen regions of platinum electrodes in aqueous acid solution, L. D. Burke and A. J. Ahem, J. Solid State Electrochem., 2001, 5, 553. 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

General discussion. The limiting reactions in aqueous solution at platinum electrodes are ... 

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