The separation of the heavier alkali metals from the lighter ones by precipitation with hexafluorophosphate (PF6−) and hexafluoroarsenate (AsF6−) has been attempted. It was not possible to obtain a satisfactory separation of potassium from the two heavier alkali metals. The alkali metal salts of hexafluorophosphoric acid have been prepared and characterised, and their solubilities have been determined under comparable conditions, by means of atomic-absorption spectrophotometry. Although the values of these solubilities gave some hope for useful analytical purposes, it was not possible to obtain a satisfactory separation of potassium from rubidium and caesium because of solubility and co-precipitation effe