Molten salts reference electrodes

Fig. 7.10 Tungsten tip junction for molten salt reference electrodes...

Many of the electroanalytical techniques that are routinely employed in conventional solvents, such as, chronoamperometry, chronocoulometry, chronopotentiometry, coulometry, cyclic (stationary electrode) voltammetry, rotating electrode voltammetry, and pulse voltammetry, have also been applied to molten salts. Some of these techniques are discussed next with special attention to their employment in molten salts. References to noteworthy examples appearing in the literature are included. Background information about these techniques is available elsewhere in this book. 

The experiment may also be repeated using a platinum (indicator) electrode and a tungsten wire reference electrode. If the tungsten electrode has been left idle for more than a few days, the surface must be cleaned by dipping into just molten sodium nitrate (CARE ). The salt should be only just at the melting point or the tungsten will be rapidly attacked it should remain in the melt for a few seconds only and is then thoroughly washed with distilled water. 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

Several investigations have been made of the reduction of cobalt(II) to cobalt(O) in molten salt media. Eor a eutectic melt of LiCl-KCl at 450°C[10], a 1 1 NaCl-KCl melt at 450°C[11], and a MgCh-NaCl-KCl (50 30 20 mol%) mixture at 475 °C [12], the apparent standard potentials for the cobalt(II)-cobalt(0) couple have been deduced to be —1.207 V, — 1.277 V, and—1.046 V, respectively, each with respect to a chlorine-chloride ion reference electrode. 

A cell with a capacity of 1 L was made of mild steel. An amorphous carbon rod (diameter 25 mm length 15 cm) was used as anode, the inside wall of the cell as cathode and a platinum wire was used as reference electrode. The anode compartment of the cell was separated from the cathode compartment by a skirt of steel welded to the cell cover. The anode gas was passed through a tube filled with tablets of NaF to absorb anhyd HF gas and then led to a gas sampler. Fluorine was detected with K.I soln. After the starting material was added into the molten KIIF2/HF salt, the electrolyte was pre-electrolyzed at a low current density until NF2 was detected, and then current efficiency of each product and polarization curves by galvanostatic or potential sweep method were determined (Table 1). At optimum conditions the current efficiency of NF3 was 55%. 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

Molten salt Liquidus temperature (°C) Potential window (V)/ working electrode Reference electrode Ref... 

Reference Electrodes For Use in Fused-Salt Systems. The kinds of investigations that require reliable reference electrodes in molten salts duplicate those in aqueous electrolytes. Specifically, these include (1) thermodynamic... 

Figure 5.18 Design for silver-silver ion reference electrode for use in molten-salt systems.

In spite of large experimental difficulties in molten salts, many reference electrodes are used for the determination of thermodynamic activity and for kinetic measurements [88-90],... 

There is no fundamental difference between the two half-cells or electrodes in a cell for measuring emf (electromotive force), especially in molten salts. However, it is usual to designate one of the electrodes as reference electrode if it is used for the measurement of an emf series. In many cases a diaphragm is used to separate the two half-cells. 

Halogen electrodes in molten salts are being used especially for the study of molten halides. In 1930 Hildebrand and Salstrom [98-100] were the first who applied a chlorine reference electrode in molten salts. Up to the present, other authors used and improved this electrode [101-105], The reversible bromine/ bromide electrode was set up for the first time by Hildebrand and Salstrom [98] and improved by Murgulescu et al. [106,107], In the following an example is shown of how to obtain and use a reversible gas electrode in molten salts for the determination of thermodynamic activity. 

Carbon activities in alkali metals are also estimated by electrochemical meters. These are based on the activity differences between two carbon bearing electrodes separated by a carbon ions conducting electrolyte. The electrolyte is a molten salt mixture, consisting of the eutectic of lithium and sodium carbonate, melting at approximately 500 °C. The molten salt mixture has to be kept free from any impurities or humidity. The mixture, acting as liquid electrolyte is kept in an iron cup. The iron wall is in contact with both the liquid electrolyte and the liquid metal. Thus, it exchanges carbon with both up to the equilibrium. Iron, with the same carbon potential as the liquid metal, acts as one electrode. The reference electrode of graphite or any other material with a well defined and stable carbon activity is immersed in the molten electrolyte. The Nernst equation defines the potential of the electrochemical chain ... 

In addition to aprotic solvents, films have been prepared in strong acid media. These preparations are carried out in polyethylene or polypropylene cells, using a platinum, gold or glassy carbon working electrode and a polymer-coated Ag/AgCI or Pd/H reference electrode [118,119]. Thus polypyrrole films have been prepared in aluminium chloride/ammonium chloride molten salts mixtures at room temperature [120,121]. 

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