Polarography in Molten Salts

TYPES OF MELTS From the practical viewpoint, melts may be classified according to melting points which may limit the available techniques. The lowest melting, with working temperatures up to 250°, are typified by the LiNO3—NaNO3— KNO3 eutectic, NaNO3—KNO3 eutectic, LiClO4, and ammonium formate. With these solvents it is possible to use the dropping mercury electrode, or hanging mercury drop electrode, or various solid microelectrodes such as are commonly used in aqueous or non-aqueous solvents at room temperature. At temperatures above 250°, the vapour pressure of mercury becomes large enough to prohibit the convenient use of mercury electrodes. A relatively large range of working temperatures is accessible to solid microelectrodes insulated by glass or quartz. For temperatures up to 550° we have used platinum sealed in a special glass having the thermal properties of lead glass but lacking the reducible lead ion5. Using fine gauge platinum wire sealed in Jena Supremax glass, Stromatt6 was able to work in NaC1—KC1 melts at 716°. Tungsten or molybdenum wires sealed into quartz permit work up to about 1000°. A limitation to the use of these metals, however, is imposed by their being attacked by moderately strong oxidizing species, such as U022+. A special situation is encountered when an insulation of glass or quartz is prohibited by the corrosive nature of the melt, as with carbonates, hydroxides, or fluorides. Manning7 has used boron nitride as an insulator, but it is difficult to avoid creepage of melt between electrode and insulator. Saito, Suzuki and Goto8 have used a graphite sheath around the "dipping" type of molybdenum electrode, supporting the graphite out of electrical contact with the electrode. Other electroanalytical techniques, notably potentiometry, coulometry and chronopotentiometry, have been carried out without using an insulating seal by bringing the electrode directly through the solution—gas interface9. Such techniques should also be valuable in extending the useful temperature range of electroanalytical measurements in less corrosive melts. Chemically, a more fundamental classification of melts can be based upon the acid—base characteristics of the melt, using the Lewis definitions of acids