Silver halide electrodes

The electrochemical photovoltaic effect was discovered in 1839 by A. E. Becquerelt when a silver/silver halide electrode was irradiated in a solution of diluted HN03. Becquerel also first described the photogalvanic effect in a cell consisting of two Pt electrodes, one immersed in aqueous and the other in ethanolic solution of Fe(C104)3. This discovery was made about the same time as the observation of the photovoltaic effect at the Ag/AgX electrodes. The term Becquerel effect often appears in the old literature, even for denoting the vacuum photoelectric effect which was discovered almost 50 years later. The electrochemical photovoltaic effect was elucidated in 1955 by W. H. Brattain and G. G. B. Garrett the theory was further developed... 

Silver halide electrodes (with properties similar to electrodes of the second kind) are made of AgCl, AgBr and Agl. These electrodes, containing also Ag2S, are used for the determination of Cl-, Br, I and CN ions in various inorganic and biological materials. 

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

Work with silver halide electrodes is complicated by two specific problems ... 

The other group comprises silver-silver halide electrodes, mercury pools, metal-metal-ion electrodes, and others normally prepared In the solvent used for the compound being studied (and often, indeed, employed as internal "reference" electrodes). For such an electrode, the abbreviation alone signifies that the solvent was the same throughout the cell, while the symbol "(w)" for ("water") following the abbreviation signifies that the reference electrode was prepared with water and used as an external reference electrode. 

S) Bishop, E., and R. G. Dhaneshwar Differential Electrolytic Potentiometry. VIII. The Behaviour and Energetics of Current-Carrying Silver and Silver Halide Electrodes in the Semi-Micro Scale Titration of Nanogram Amounts of Halides at Extreme Dilution. Analyst 87, 845 (1962). 

Silver halide microcrystals are wide band gap semiconductors which exhibit weak photoconductivity. Early experiments demonstrated that dyes that sensitized silver halide photographic action also sensitized silver halide photoconductivity [6c]. Since the observation of photoconductivity necessitates the movement of free charge within the crystals, dye sensitization processes must inject charge into the silver halide lattice in some way. Initial theories of sensitization were based on the semiconductor view of silver halides, especially as espoused by Gurney and Mott [10]. Current ideas are based on thorough studies of the absorption spectroscopy and luminescence of silver halide emulsions and of adsorbed, sensitizing dyes, and the oxidation-reduction properties of the dyes at silver/silver halide electrodes [11]. 

The so-called primary titration technique is attempted only with electrodes of silver metal, silver-silver halide, or mercury amalgams, which are the source of the electrogenerated species. The substance to be determined reacts directly at the electrode or with a reactant electrogenerated from the working electrode. This class of titrations is limited generally to non-diffusible reactants such as mercury amalgams, silver ions generated by anodization of silver metal, and halides liberated by reduction of the appropriate silver-silver halide electrode. 

The Standard Potentials of the Silver-Silver Halide Electrodes. 

The book edited by Ives and Janz [1 ] and more recently that by Bard, Parsons, and Jordan [2] contain both theoretical and practical aspects related to reference electrodes. Preparation, application and limitations of various types of reference electrodes such as the hydrogen electrode, the calomel and other mercury-mercurous salt electrodes, the silver-silver halide electrodes, and sulfide and sulfate electrodes are covered and general reference to these excellent critical reviews is recommended. 

The glass electrode has found extensive use in non-aqueous solvents and seems to function quite well. ° On the other hand silver halide electrodes appear to be soluble in most aprotic solvents but behave reasonably well in protic organic solvents such as the alcohols. Butler has tabulated a fairly complete listing of solubility and stability constants for the various polynuclear silver halide species in many solvents. 

Since we know the values for the silver-silver halide electrodes versus the standard hydrogen electrode (see above), we can easily set up a series of standard potentials vs. H2 assuming (H+ H2) = 0.0 V. Recently Scrosati et made measurements on double amalgam cells in pure methanol. Their results for the alkali metal chlorides and bromides are in good agreement with others but the iodide systems appear to involve some error since a value of --0.2993 V is obtained for of cell (Vc). Table 2.7.2 lists the standard potentials of the alkali-metal electrodes. 

Deleterious Effect of Quaternary Amines. Anion exchange membranes are all amine based and amines, even quaternary, may not be compatible with silver halide electrodes because they accelerate silver deposition on the cathode. 

Meanwhile, the photoelectric effect was a focus of scientific interest, with a history synchronized with that of photography. The concept of dye enhancement of the photoeffect was carried over from photography to the sensitization of an electrode already in 1887 by Moser [5] (Fig. 2) using the dye ery-throsin on silver halide electrodes and confirmed by Rigollot in 1893 [6]. In the retrospectively quaint report, written substantially before the Einstein theory of the photoelectric effect, Moser records his observations on dye-induced enhancement as an increased photopotential [V] rather than the more fundamental current (A), despite the title - Strengthened Photoelectric Current Through Optical Sensitization. ... 

K. H. Khoo, J. Chem. Soc., 2932 (1971). The standard potentials of silver-silver halide electrodes and ion solvation in dimethylsulfoxide-water mixtures at 25°C. 

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