- Cadmium oxalate is an inorganic salt with the formula CdC₂O₄, composed of divalent cadmium cations (Cd²⁺) and oxalate anions (C₂O₄²⁻). The oxalate ion is a dicarboxylate species containing two carbon atoms joined by a single bond, each bonded to two oxygen atoms, forming a planar, bidentate ligand that can chelate metal ions.
- Cadmium oxalate typically occurs as the dihydrate CdC₂O₄·2H₂O in laboratory preparations. The anhydrous form has a molar mass of 168.43 g/mol, while the dihydrate has a molar mass of 204.46 g/mol.
- Cadmium oxalate is a white crystalline solid that is sparingly soluble in water but readily soluble in mineral acids, where it forms soluble cadmium salts. It is insoluble in most organic solvents. The dihydrate form is the most common, often crystallizing in the monoclinic crystal system, with cadmium ions coordinated by oxygen atoms from oxalate ligands and water molecules. Its density is about 2.6–2.8 g/cm³. It is stable under ambient conditions but decomposes upon heating.
- Cadmium oxalate is typically prepared by precipitation, where a soluble cadmium salt (such as cadmium chloride, cadmium nitrate, or cadmium acetate) is reacted with a soluble oxalate salt (such as sodium oxalate or ammonium oxalate):
- Cd(NO₃)₂ + Na₂C₂O₄ → CdC₂O₄↓ + 2 NaNO₃
- The precipitate is filtered, washed with water to remove excess soluble salts, and dried under mild conditions to preserve the hydrated form.
- Cadmium oxalate is stable at room temperature but undergoes thermal decomposition when heated above ~300 °C. The decomposition occurs in multiple stages:
- Loss of water of crystallization (for hydrates) upon gentle heating.
- Decomposition of the oxalate anion to produce cadmium oxide (CdO), carbon monoxide (CO), and carbon dioxide (CO₂): CdC₂O₄ → CdO + CO + CO₂
- The gaseous products (CO and CO₂) are toxic in confined spaces, and CdO is itself hazardous.
- Cadmium oxalate has limited direct industrial applications but is important as a precursor to cadmium oxide of high purity. CdO produced from its decomposition is used in pigments, ceramics, glass coloring, and as a semiconductor material. In analytical chemistry, cadmium oxalate has been used in gravimetric methods for cadmium determination, where the insoluble salt can be quantitatively precipitated, dried, and weighed.
- Cadmium oxalate is highly toxic due to the cadmium content. Inhalation of dust or decomposition fumes (containing CdO and CO) can cause acute respiratory irritation, pulmonary edema, and long-term lung damage. Ingestion can lead to gastrointestinal distress, kidney injury, and systemic cadmium poisoning. Chronic exposure can result in kidney dysfunction, bone demineralization, and increased cancer risk (lung and prostate). While oxalate toxicity (due to calcium oxalate formation in tissues) is a concern for some oxalate salts, in cadmium oxalate, cadmium toxicity is the dominant hazard.
- Cadmium oxalate is toxic to aquatic life and persistent in the environment. If released, it can deposit in soils and sediments and gradually dissolve under acidic conditions, releasing bioavailable cadmium ions. This can lead to bioaccumulation in plants, fish, and other organisms. Cadmium oxalate is regulated under hazardous substance laws in most countries, including the EU’s REACH and the U.S. EPA’s RCRA regulations. Disposal must be carried out as hazardous waste.
- For transport, cadmium oxalate is generally included under UN 2570 – Cadmium compounds, n.o.s., hazard class 6.1 (toxic substances). Occupational exposure limits are typically ≤0.005 mg/m³ (as Cd) for an 8-hour time-weighted average. Handling requires engineering controls (such as fume hoods), gloves, goggles, and in some cases respirators.
