Reference Electrodes for Use in Fused-Salt Systems

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 

The literature on fused-salt systems is too vast to survey here, but good access to the literature is provided in two reviews.55,56 Both contain useful information on solvent systems and suitable reference electrodes. 

Alkali metal chloride and nitrate eutectic mixtures that melt in the range 200-500°C are widely used as fused-salt solvents, because they have a large liquid range from their melting to their boiling points, and because they can be used at temperatures below the softening point of glass. 

The silver-silver Ion electrode. Of the reversible metal electrodes, silver has been most often employed. There is only one stable oxidation state of silver above 300°C there is no danger of oxide formation because Ag20 is unstable.57 The metal has no observable tendency to dissolve in molten silver salts and is highly reversible in mixed chloride and nitrate eutectics. The Ag(I) ion can be introduced into the melt by either adding silver nitrate to a nitrate melt (AgCl to a chloride melt) or by anodizing a silver electrode. The potentials of silver nitrate concentration cells show ideal thermodynamic behavior up to 0.5 mol % in (Na,K)N03 eutectic and in NaN03.58 

Silver metal usually is employed in the form of wires or foils. In contact with a melt that contains Ag(I), silver metal continuously reciystallizes, so that a wire of small diameter may eventually be converted to a fragile string of loosely joined crystals. The rate of the process depends on temperature, and also appears to proceed more rapidly at a kinked or otherwise strained point in the wire. 

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