CaC4O4H2O

CaC4O4H2O, commonly known as calcium oxalate monohydrate, is a crystalline compound composed of calcium, oxalate ions, and a water molecule. Its stoichiometric formula indicates a molar ratio of one calcium ion to one oxalate anion, with one additional water of crystallisation. The compound crystallises in the orthorhombic system and exhibits a tabular habit, often forming needle‑like aggregates. In the solid state, calcium ions are coordinated by four oxygen atoms from two oxalate ligands, creating a layered structure that stabilises the lattice.

In nature calcium oxalate monohydrate occurs as a biomineral in many plant species, where it functions in calcium sequestration and defence against herbivores. The compound frequently appears as the principal constituent of kidney stones in humans and animals, and it is often identified by infrared and X‑ray diffraction techniques in clinical diagnostics. Industrially, calcium oxalate is produced as a by‑product of the manufacturing of oxalic acid and is employed as a neutralising agent for acidic metals in wastewater treatment. The monohydrate form is also used in the textile and paper industries as a bleaching and decolourising aid, owing to its strong interaction with metal cations.

CaC4O4H2O is chemically stable under ambient conditions but dehydrates to calcium oxalate dihydrate (CaC2O4·2H2O) at temperatures above 125 °C. The loss of water leads to a loss of crystallinity and a change in solubility behaviour. The compound’s low solubility in water (approximately 0.2 g L⁻¹ at 20 °C) contributes to its propensity to precipitate as crystalline deposits in biological and industrial settings.