Solubility of potassium ferrate in 12 M alkaline solutions between 20°C and 60°C

The solubility of potassium ferrate (K₂FeO₄) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As₂O₃. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO₄^?. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K⁺, and the ferrate anion, KFeO₄^?, was found to be 0·044 ± 0·006 mol² dm^?6 for 20°C, 0·093 ± 0·004 mol² dm^?6 for 40°C and 0·15 ± 0·09 mol² dm^?6 for 60°C. From these results the heat of dissolution of K₂FeO₄ was calculated as ?14·3 kJ mol^?1. At 60°C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20°C or 40°C.