Tetraalkylammonium electrode

Creager and colleagues designed a salicylate ion-selective electrode using a PVC membrane impregnated with tetraalkylammonium salicylate. To determine the ion-selective electrode s selectivity coefficient for benzoate,... 

When applied versus a glass indicator electrode the potassium salt can better be replaced by a tetraalkylammonium salt like (C2H5)4NC104. 

Among cations, potassium, acetylcholine, some cationic surfactants (where the ion-exchanger ion is the / -chlorotetraphenylborate or tetra-phenylborate), calcium (long-chain alkyl esters of phosphoric acid as ion-exchanger ions), among anions, nitrate, perchlorate and tetrafluoro-borate (long-chain tetraalkylammonium cations in the membrane), etc., are determined with this type of ion-selective electrodes. 

Ito et al.40 examined the electrochemical reduction of C02 in dimethylsulfoxide (DMSO) with tetraalkylammonium salts at Pb, In, Zn, and Sn under high C02 pressures. At a Pb electrode, the main product was oxalic acid with additional products such as tartaric, malonic, glycolic, propionic, and n-butyric acids, while at In, Zn, and Sn electrodes, the yields of these products were very low (Table 3), and carbon monoxide was verified to be the main product even at a Pt electrode, CO was mainly produced in nonaqueous solvents such as acetonitrile and DMF.41 Also, the products in propylene carbonate42 were oxalic acid at Pb, CO at Sn and In, and substantial amounts of oxalic acid, glyoxylic acid, and CO at Zn, indicating again that the reduction products of C02 depend on the electrode materials used. 

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

Interesting results have been obtained from polarographic studies in various donor solvents. Measurements have been made of various metal perchlorates in solutions of donor solvents containing tetraalkylammonium perchlorate as supporting electrolyte against an aqueous saturated calomel electrode 113. In order to eliminate differences in liquid-liquid junction potentials bisbiphenylchromium (I) has been used as a reference ion 114 118). 

A fundamental improvement in the facilities for studying electrode processes of reactive intermediates was the purification technique of Parker and Hammerich [8, 9]. They used neutral, highly activated alumina suspended in the solvent-electrolyte system as a scavenger of spurious impurities. Thus, it was possible to generate a large number of dianions of aromatic hydrocarbons in common electrolytic solvents containing tetraalkylammonium ions. It was the first time that such dianions were stable in the timescale of slow-sweep voltammetry. As the presence of alumina in the solvent-electrolyte systems may produce adsorption effects at the electrode, or in some cases chemisorption and decomposition of the electroactive species, Kiesele constructed a new electrochemical cell with an integrated alumina column [29]. 

The most widely studied examples are cyclooctatetraene (COT, 1) and its derivatives. In such conventional aprotic solvents as DMF, dimethyl sulfoxid (DMSO), or acetonitrile containing tetraalkylammonium salts, two distinct one-electron reduction waves are observed at approximately —1.64 V and —1.80 V vs. saturated calomel electrode (SCE), with separations... 

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

Table 4.3. They vary with the electrode material and with the tetraalkylammonium cation used. Early workers used mercury electrodes but mercury may be involved in the overall reaction. Glassy carbon is generally favoured as the electrode material. Reproducibility of data depends critically on methods used for cleaning the glassy carbon surface [33].

Procedures 3 and 4 are for acids of the HA type, but can be applied with minor modifications to acids of the BH+ type, for which homoconjugation is often negligible. From these studies, we find an appropriate pH buffer that is to be used to calibrate the glass electrode in routine pH measurements. Mixtures (1 1) of picric acid/tetraalkylammonium picrate and diphenylguanidine/diphenylguanidinium perchlorate are examples of the candidates for such a pH buffer. 

If a tetraalkylammonium salt is used as supporting electrolyte, this process is either reversible or quasi-reversible and occurs at around -0.8 V vs aqueous SCE in various aprotic solvents and with various electrode materials (Hg, Pt, GC). If a Bmisted acid is added to the solution, the first step is converted to a two-electron process 0 produced in the first step is protonated to form 02H, which is more reducible than 02. Thus, 02H is further reduced to 02H at the potential of the first step. According to detailed polarographic studies in H20-DMS0 mixtures, about 30% v/v water is needed to convert the one-electron process to the two-electron process [41]. A metal ion, M+, interacts with 02 to fonn an ion-pair M+-02 (often insoluble) and shifts the half-wave potential of the first wave in a positive direction [42]. Electrogenerated superoxide 02 can act either as a nucleophile or as an electron donor and has been used in organic syntheses [43],... 

Although tetraalkylammonium salts are most frequently used as supporting electrolyte in aprotic solvents, it should be noted that even tetraalkylammonium ions give significant influences on electrode reactions. An appropriate R4N+ should be selected for each measurement. 

Ion exchange between heparin and Cl associated with tetraalkylammonium ions in the membrane of the ion-selective electrode. Ion-selective electrodes described in this chapter reach an equilibrium electric potential... 

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... 

Using supporting electrolytes such as tetraalkylammonium salts, one may apply potentials as negative as -2.6 V vs. SCE in aqueous solutions, while in some nonaqueous systems even -3.0 V vs. SCE (aqueous) is accessible. Unfortunately, mercury electrodes have serious limitations in applications at positive potentials (with the exception of passivated mercury electrodes, which are described in Section VI), and this has led to extensive research in the development of solid metal and carbon electrodes. Oxidation of mercury occurs at approximately +0.4 V vs. SCE in solutions of perchlorates or nitrates, since these anions do not form insoluble salts or stable complexes with mercury cations. In all solutions containing anions that form such compounds, oxidation of the mercury proceeds at potentials less than +0.4 V vs. SCE. For example, in 0.1 M KC1 this occurs at +0.1 V, in 1.0 M KI at -0.3 V, and so on. 

Solvent effects in electrochemistry are relevant to those solvents that permit at least some ionic dissociation of electrolytes, hence conductivities and electrode reactions. Certain electrolytes, such as tetraalkylammonium salts with large hydrophobic anions, can be dissolved in non-polar solvents, but they are hardly dissociated to ions in the solution. In solvents with relative permittivities (see Table 3.5) s < 10 little ionic dissociation takes place and ions tend to pair to neutral species, whereas in solvents with 8 > 30 little ion pairing occurs, and electrolytes, at least those with univalent cations and anions, are dissociated to a large or full extent. The Bjerrum theory of ion association, that considers the solvent surrounding an ion as a continuum characterized by its relative permittivity, can be invoked for this purpose. It considers ions to be paired and not contributing to conductivity and to effects of charges on thermodynamic properties even when separated by one or several solvent molecules, provided that the mutual electrostatic interaction energy is < 2 kBT. For ions with a diameter of a nm, the parameter b is of prime importance ... 

Electrochemical studies are usually performed with compounds which are reactive at potentials within the potential window of the chosen medium i.e. a system is selected so that the compound can be reduced at potentials where the electrolyte, solvent and electrode are inert. The reactions described here are distinctive in that they occur at very negative potentials at the limit of the cathodic potential window . We have focused here on preparative reductions at mercury cathodes in media containing tetraalkylammonium (TAA+) electrolytes. Using these conditions the cathodic reduction of functional groups which are electroinactive within the accessible potential window has been achieved and several simple, but selective organic syntheses were performed. Quite a number of functional groups are reduced at this limit of the cathodic potential window . They include a variety of benzenoid aromatic compounds, heteroaromatics, alkynes, 1,3-dienes, certain alkyl halides, and aliphatic ketones. It seems likely that the list will be increased to include examples of other aliphatic functional groups. 

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