Properties of fused polysulphides—III. EMF measurements on the sodium-sulphur cell, and sulphur activities and constitution in fused sodium polysulphides

Abstract

Expressions are derived for the emf of the sodium-sulphur cell in the fully charged state ( E 2) and in a partly discharged state ( E 1). It is shown that ( E 2  E 1) is related to the variation of sulphur activity with the composition of the sodium polysulphide melt produced in the cell reaction, and that the sulphur activity can be calculated if E 1 is known over a range of compositions. The sulphur activity in sodium polysulphide melts was also measured by a transpiration method. The two sets of results are in good agreement. The emf of a sodium polysulphide concentration cell with transference ( E 3) was also measured. The results show that all the sulphur in the melt is contained in polysulphide anions; no free sulphur is present.