The solubilities of KHC2O4 and KHC2O4·H2C2O4were measured as 5.8 g and 3.0 g per 100 mL of water at 20 °C, respectively. The NH4HC2O4 and NaHC2O4 were synthesized for measuring the solubilities in water at 20 °C. The details on the synthesis of NH4HC2O4 and NaHC2O4 can be found in the Supporting Information. The solubilities of common oxalate compounds are compared in Table 1. The potassium-based oxalate compounds are unique because of the low solubility of KHC2O4 and KHC2O4·H2C2O4and the high solubility of K2C2O4. The high solubility of K2C2O4 is critical for separating pure metals (e.g. Al and Fe) during the hydrolysis step without any impurity.
Table 1. Solubilities of common oxalate compounds relevant to this work.