Distribution of anion chain lengths in molten sodium polysulphides

Abstract

The available activity data for sulphur in fused sodium polysulphides are reanalysed. Non-linear least squares are used to fit the data to models which impose no upper bound to the length of the polysulphide chain. It is found that fits can be found with significantly fewer variable parameters than the needed for models which artificially impose a maximum chain length. The best and most self-consistent fits are found for models in which the dominant term in the free energy change of the reaction ▪ is coulombic, arising from the increased separation of the negative charges at the ends of the polysulphide chain. The fits are used to predict the emf of the sodium-sulphur cell as a function of polysulphide composition, and also to predict the upper temperature limit to the two-liquid region of the Na 2SS phase of nominal composition Na 2S 5.2, contains significant amounts of all polysulphide species up to and including S 2− 8.