Liquid junction characteristics

Despite the potential impact of novel photosynthetic routes based on these developments, the most ambitious application remains in the conversion of solar energy into electricity. Dvorak et al. showed that photocurrent as well as photopotential response can be developed across liquid-liquid junctions during photoinduced ET reactions [157,158]. The first analysis of the output power of a porphyrin-sensitized water-DCE interface has been recently reported [87]. Characteristic photocurrent-photovoltage curves for this junction connected in series to an external load are displayed in Fig. 22. It should be mentioned that negligible photoresponses are observed when only the platinum counterelectrodes are illuminated. Considering irradiation AM 1, solar energy conversions from 0.01 to 0.1% have been estimated, with fill factors around 0.4. The low conversion... 

When the electrolytes on either side of a liquid junction are different, the mathematical analysis of the interfacial potential becomes complex. In nearly all these cases the potential is a function of the geometrical characteristics of the boundary itself. In one general case, however, i.e., for the junction between two uni-univalent electrolytes at the same concentration and having a common ion (e.g., the pair KC1, NaCl), the liquid junction potential is independent of the structure of the boundary and is provided by following equation ... 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

Potentiometric titration experiments of Zr chloride, nitrate, sulphate and perchlorate solutions were conducted at (25.00 + 0.05)°C until the onset of precipitation. Initial solutions (0.038, 0.019, 0.0095 and 0.0047 M in Zr) contained < 0.4% Hf and had an excess of 2 M of the acid of the anion studied. Titrations were performed with carbonate free 0.101 N NaOH. Glass electrodes where calibrated regularly but no correction for differences between liquid junction potential of reference and measured solutions was performed. The pH convention used was not reported and it is assumed that a NBS type convention was used. The pH at the onset of precipitation and coagulation of an uncharacterised and presumably amorphous solid were determined optically. The pH of coagulation was the pH at whieh the precipitate coagulated and the supernatant solution was clear. Reproducibility of these characteristic pH values was within 0.05 to 0.07 pH units. 

Major problems could be encountered due to errors associated with the liquid junction. It is recommended that either a free diffusion junction is used or it is verified that the junction is working correctly using dilute solutions as follows. For commercial electrodes calibrated with lUPAC aqueous RVS or PS standards, the pH(X) of dilute solutions should be within 0.02 of those given in Table 1. The difference in determined pH(X) between a stirred and unstirred dilute solution should be < 0.02. The characteristics of glass electrodes are such that below pH 5 the readings should be stable within 2 min, but for pH 5 to 8.8 or so minutes may be necessary to attain stability. Interpretation of pH(X) measured in this way in terms of activity of hydrogen ion, is subject to an uncertainty of 0.02 in pH. 

Fig. 5. Output power characteristics of a liquid-junction solar cell with n-CuInSe2 photoanode and KI-KI3-CUI-HI electrolyte under 100 mW/cm solar simulated irradiation [7].

J. F. McCann, S. Hinckley, and D. Haneman, An analysis of the current-voltage characteristics of thin-film front wall illuminated and back wall illuminated liquid junction and Schottky barrier solar cells, J. Electrochem. Chem. 137 (1982) 17-37. 

TABLE 3.3. Characteristic Liquid Junction Potentials of Salt Solutions [6]... 

Semiconductor-Liquid Junction From Fundamentals to Solar Fuel Generating Structures, Fig. 7 Overview of selected output power characteristics of photoelectrochemical solar cells operating in the photovoltaic mode note that here, also 2-electron transfCT redox couples have been used, for which the Marcus-Gerischer theory does not apply the output power characteristics have been normalized and the respective efficiencies are given at each characteristic the conditions (illumination intensity and source) are as follows n-GaAs 95mWcm (sunlight) [15, 17] p-InP ... 

This chapter is concerned with electrodes. Section 3.1 provides information about the structure and characteristics of glass electrodes along with possible sources of error, such as sodium ion interference. This should assist in optimum selection and use of electrodes. In Section 3.2, the structure and functioning of reference electrodes are discussed. The liquid junction between the reference electrode electrolyte filling solution and the sample can introduce a junction potential, which is a major source of error in pH measurement. This subject is discussed in detail. The combination electrode is compared with an electrode pair and its advantages in small-volume samples or flat-surface measurements are discussed in Section 3.3. 

In most practical cases reference electrodes are in contact with the sample solutimi via a diaphragm, which is typically a porous ceramic plug. The diaphragm is filled with the usually used KCl soluticai as reference electrolyte to form a liquid junction at its outer surface. Details and characteristics of liquid junctions are described elsewhere. Especially for the iodine-iodide electrode it should be mentioned that... 

By using a third solution, which must not react, and in which the cation and anion have similar mobilities and diffusion characteristics, to connect the two electrode solutions the liquid junction potential may be minimized. Concentrated potassium chloride, about 4 M, is often used, since K and Cl" ions have almost the same mobility (see Topic CIO). Potassium nitrate or ammonium nitrate is also suitable if chloride ions react. Use of a salt bridge reduces the effective El to about 1 mV. 

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