Aqueous saturated calomel electrode

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). 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

The quoted potentials are rarely expressed versus standard hydrogen electrode (SHE) but mainly versus an aqueous saturated calomel electrode (SCE) or versus Ag/Ag couple in ACN or in a solvent used in particular experiments. However, in some cases the Mg/Mg + couple in THE or other solvents is used as the reference electrode. 

In many cases, the electrolytes on the two sides are different. Even in such cases, the LJP is stable and reproducible if the electrolyte concentration on one side is more than ten times that on the other side.13 Here the behavior of the LJP is determined by the electrolyte of higher concentration. As described in Section 6.1.2, aqueous saturated calomel electrodes and silver-silver chloride electrodes are often used as reference electrodes, their tips being inserted in non-aqueous solutions. This is justified to some extent if the composition of the non-aqueous solution is fixed, the LJP is fairly reproducible and stable, unless the junction dogs with the solidified electrolyte. The big problem, however, is that the LJP is large in magnitude and varies drastically with the solvent used. 

Table 1 gives a short list of approximate potential ranges available in various aqueous solutions and for different electrodes. The values of the potentials quoted are, for practical convenience, relative to an aqueous saturated calomel electrode (SCE) as this is one of the most common and convenient reference electrodes used in aqueous electrochemical studies. 

Aqueous saturated calomel electrode (SCE) as reference in water media. 

The aqueous saturated calomel electrode (SCE) is generally employed as the reference electrode though in a few cases as the normal calomel electrode (NCE), connected to the solution in the non-aqueous solvent by means of a salt bridge involving the latter solvent, in order to avoid contamination of the solution to be studied with water. Otherwise, an Ag/Ag+ or an Hg/Hg22+ electrode or... 

The mercury-mercurous chloride (calomel) electrode. The calomel electrode was used extensively as a chloride electrode, but it has been all but abandoned for this purpose in favor of the silver chloride electrode. The fixed-potential saturated or 3.5 M KC1 calomel electrode always has been popular for use with glass electrodes in pH measurements and in polarographic work most of the vast compilations of aqueous polarographic half-wave potentials were referred to the aqueous saturated calomel electrode (SCE). 

Bis[dimethylpropylsilylmethyl] Ditellurium5 An H-shapcd electrolytic cell with a ealholyte volume of 100 ml is fitted with a tellurium cathode (99.99% purity, 15-20 cm2 surface area) and a platinum-net anode. To the cathode compartment, separated from the anode by a ceramic diaphragm of 1.6 gm pore size, are added 100 ml of a 1 molar solution of dry sodium perchlorate in dimethylformamide and the catholyte is deaerated with argon. At a cathode potential of 1.4 V with respect to an aqueous saturated calomel electrode, 2500... 

Multimetallic sandwiches, including polymetallocenes, bis(fulvalene)-dimetal compounds, multi-decker sandwiches, and compounds in which two metals are bonded to a common unsaturated cyclic hydrocarbon are discussed in Section II. Sections III and IV are subdivided according to the metal group, and describe ligand-bridged bimetallics and metal clusters, respectively. Where appropriate, redox potentials have been included. Unless otherwise stated they are referenced versus the aqueous saturated calomel electrode (see). 

For 50% methanol-water mixtures three series of buffers involving acetate, succinate, and phosphate have been established in the temperature range from 10 to 40°C by emf data obtained without liquid junction. With 50% methanol solutions an aqueous saturated calomel electrode is used as reference electrode, since liquid-junction potentials are adequately reproducible for operational pH values. 

The normal hydrogen electrode (NHE) is the primary reference electrode and is used to define the accepted scale of standard potentials in aqueous media. It is also one of the most reproducible electrodes that are available. The hydrogen electrode has been successfully employed in dipolar aprotic solvents however, it is not frequently used. The aqueous saturated calomel electrode (SCE), connected to the electrolyte under study by a non-aqueous salt bridge, has become the reference electrode of choice for most investigators. Whether it is the SCE that is used, or any other suitable reference electrode for a given solvent, junction potentials will exist between the reference electrode and the electrolyte under study. These junction potentials will affect electrode potential measurements and will vary from one solvent/electrolyte system to another. In addition, the instability of the SCE in non-aqueous solvents has been noted. ... 

Table 11. Half-wave Potentials of Alkali and Alkaline Earth Metal Ions in Different Solvents in Volts versus Aqueous Saturated Calomel Electrode and versus the Half-wave Potential of Rb+...

Electrochemistp. The electrochemical experiments were performed with a Princeton Applied Research 173 potentiostat, a 175 PAR universal programmer equipped with a model 179 digital coulometer. Signals were recorded on an X-Y-Y Kipp and Zonen recorder. The water jacketed cell used was thermostatted at 21.4 C. The cell had an aqueous saturated calomel electrode as reference (SCE) and all... 

Electrosynthesis of the polythiopene was realized on an indium-tin-oxide (ITO) electrode (glass blade covered with an indium-doped tin-oxide film). Before conducting the experiment, each electrode was cleaned by ultrasonication for 10 min in different solvents (acetone, dichloromethane, ether). Electrochemical experiments were performed in a three-compartment cell. The working electrode was the ITO electrode, the counter electrode was a Pt wke, and the reference electrode was an aqueous-saturated calomel electrode (E°/SCE = E°/NHE — 0.2412 V) with a salt bridge containing the supporting electrode. The SCE electrode was checked against the ferrocene/ferricinium couple (E = -1-0.405 V/SCE) before and after each experiment. 

---