Electrochemical Techniques II

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

Two volumes of the electrochemistry series in Topics in Current Chemistry are dedicated to new applications of electrochemical techniques. The contributions in Volume II cover two fields ... 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

The peptide fragments of metalloth-ioneins Lys-Cys-Thr-Cys-Cys-Ala [56-61] (FT) were studied by different electrochemical techniques. The cyclic voltam-metric behavior of the peptide fragment in the presence of Cd(II) indicated two reversible electrochemical processes due to the oxidation of the mercury electrode in the presence of CdFT and reduction of CdFT complex, both from the dissolved and adsorbed state [105]. The influence of the experimental conditions on electroreduction of Cd-metallothioneins... 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

In Part II, we deal with various electrochemical techniques and show how they are applicable in non-aqueous solutions. In this chapter, we give an overview of electrochemical techniques, from the principles of basic techniques to some recent developments. It will help readers from non-electrochemical fields to understand the latter chapters of Part II. Many books are available to readers who want to know more about electrochemical techniques [1], In particular, the excellent book by Bard and Faulkner [la] provides the latest information on all important aspects of electroanalytical chemistry. 

Electrochemical techniques are the most widely used methods to obtain nickel(III) complexes. Generally the oxidation of the nickel(II) complexes is performed in acetonitrile solutions under an inert atmosphere using a platinum electrode.3052 A tetraalkylammonium salt, usually the perchlorate, is employed as supporting electrolyte (ca. 0.1 M). The complete procedure is often carried out in the dark at ca. 5°C to prevent possible photoreduction reactions.3053-3055... 

Adsorption phenomena have been studied by means of virtually every electrochemical technique, including recently developed spectroelectrochemical methods. Electrocapillary methods and measurements of double-layer capacitance have played a central role in the understanding of adsorption. AC studies have also been very useful and are very sensitive to adsorption effects. More recently, chronocoulometry (Chap. 3, Sec. II.C) has been applied effectively to the measurement of quantities of adsorbed electroactive species. The interested reader is referred to the sections that deal with these techniques for more detailed information. 

D. Gingell and J. A. Pomes, "Demonstration of intermolecular forces in cell adhesion using a new electrochemical technique," Nature (London), 256, 210-11 (1975) D. Gingell and I. Todd, "Red blood cell adhesion. II. Interferometric examination of the Interaction with hydrocarbon oil and glass," J. Cell Sci., 41, 135-49 (1980). 

The characterization of pure platinum catalysts and of Pt catalysts modified by lead was achieved in situ by linear potential sweep cyclic voltammetry. This technique allowed to measure the active platinum surface area in the absence and in the presence of deposited lead and to determine the surface fraction covered by lead adatoms (9-12). The adsorption stoichiometry of lead on platinum was also evaluated by electrochemical techniques and found to be equal to two (one lead atom covers two platinum atoms on the surface) (II). 

Refs. [i] Kelly G, Scully JR, Shoesmith DW, Buchheit RG (2003) Electrochemical techniques in corrosion science and engineering. Marcel Dekker, New York, p 36 [ii] Nagy Z (ed) (2005) Online electrochemistry dictionary, http //electrochem.cwru.edu/ed/dict.htm... 

Minerals, electrochemistry of — Many minerals, esp. the ore minerals (e.g., metal sulfides, oxides, selenides, arsenides) are either metallic conductors or semiconductors. Because of this they are prone to undergo electrochemical reactions at solid solution interfaces, and many industrially important processes, e.g., mineral leaching and flotation involve electrochemical steps [i-ii]. Electrochemical techniques can be also used in quantitative mineral analysis and phase identification [iii]. Generally, the surface of minerals (and also of glasses) when in contact with solutions can be charged due to ion-transfer processes. Thus mineral surfaces also have a specific point of zero charge depending on their sur-... 

The reaction of [OH] and other oxidizing radicals with Ni(II)-macrocyclic complexes generates the corresponding Ni(III) species characterized by esr, spectroscopic and electrochemical techniques ". Information available includes pK, and re-... 

---