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.