The Differential Thermal Analysis of Potassium Oxalate

From the differential thermal and thermogravimetric analyses of potassium oxalate together with the X-ray diffraction studies, three crystallographic modifications of the anhydrous salt were found. When potassium oxalate monohydrate crystals were dehydrated at, e.g., 140°C, phase II, which is stable below 381°C, appears. The crystal in phase II has an orthorhombic unit cell (a=10.9, b=6.11, and c=3.44 Å at 17°C) and transforms at 381°C into phase I, which is stable above 381°C up to the temperature where the decomposition begins. The crystals in phase I belong to the tetragonal system (a=7.01 and c=7.53 Å at 404°C). The transition between phases II and I is not reversible in an exact sense, and when cooled phase I endures down to 215°C, where it transforms(reversibly) into phase III. Phase III can exist at room temperature. When heated, phase III transforms into phase I at 215°C (often accompanied by a partial transformation to phase II). The resulting phase then transforms into phase II. At all temperatures below 381°C phases I and III are metastable with respect to phase II, and they transform into phase II. The decomposition of potassium oxalate in an oxygen-free atmosphere seems to proceed in two stages.