- Cadmium phosphate is an inorganic salt with the general formula Cd₃(PO₄)₂, composed of divalent cadmium cations (Cd²⁺) and phosphate anions (PO₄³⁻).
- In its pure form, it has a molar mass of 439.18 g/mol. The phosphate anion consists of a central phosphorus atom in the +5 oxidation state, tetrahedrally coordinated by four oxygen atoms.
- Cadmium phosphate can occur in various hydrated forms, such as Cd₃(PO₄)₂·xH₂O, depending on synthesis conditions and storage environment.
- Cadmium phosphate is typically a white, odorless, crystalline or amorphous solid. It is practically insoluble in water but soluble in mineral acids, where it reacts to release cadmium ions into solution. The mineral form is known as otavite phosphate (distinct from cadmium carbonate “otavite”) and crystallizes in the monoclinic or orthorhombic systems, depending on hydration. Its density is approximately 5.2 g/cm³, and it is stable under ambient conditions, not hygroscopic, and resistant to light degradation.
- Cadmium phosphate can be prepared by precipitation from aqueous solutions. The most common method involves reacting a soluble cadmium salt (e.g., cadmium nitrate, cadmium chloride, or cadmium acetate) with a soluble phosphate salt (e.g., disodium hydrogen phosphate or ammonium phosphate):
- 3 Cd(NO₃)₂ + 2 Na₃PO₄ → Cd₃(PO₄)₂↓ + 6 NaNO₃
- The reaction produces an insoluble white precipitate, which is filtered, washed, and dried. It can also be obtained through hydrothermal synthesis, producing highly crystalline forms for research applications.
- Cadmium phosphate is stable and chemically inert under neutral and basic conditions due to the low solubility of phosphate salts. In acidic environments, it dissolves to release free cadmium ions and phosphoric acid:
- Cd₃(PO₄)₂ + 6 H⁺ → 3 Cd²⁺ + 2 H₃PO₄
- When strongly heated, it decomposes to cadmium oxide (CdO) and phosphorus oxides. The insolubility of cadmium phosphate makes it a candidate for certain immobilization applications, such as fixing cadmium in hazardous waste to reduce leaching.
- Cadmium phosphate has limited industrial use today due to cadmium’s toxicity, but historically, it has been used in ceramics and as a pigment component to create pastel shades in glass and enamels. In specialized materials science, it has been studied for potential application in luminescent materials, corrosion-resistant coatings, and in the synthesis of cadmium-containing catalysts. In nuclear waste immobilization research, it has been investigated as a stable host phase for cadmium containment.
- Like all cadmium compounds, cadmium phosphate is highly toxic and carcinogenic. Although its low solubility reduces immediate acute toxicity compared to more soluble cadmium salts, it still poses a serious hazard through inhalation of dust or ingestion. Once inside the body, acidic gastric conditions can solubilize it, releasing cadmium ions that accumulate in the kidneys, liver, and bones. Chronic exposure can lead to kidney damage, bone demineralization, and cancer of the lungs and prostate. The compound has a long biological half-life, often exceeding decades, and must be handled with strict protective measures.
- Cadmium phosphate is highly toxic to aquatic life and can persist in soils and sediments. While its low solubility limits mobility, environmental acidification can release cadmium into water systems. It is classified as a hazardous substance under regulations such as the EU’s REACH, the U.S. EPA Resource Conservation and Recovery Act (RCRA), and similar frameworks worldwide. Waste containing cadmium phosphate must be managed as hazardous waste, with no direct disposal to municipal systems.
- For transport, cadmium phosphate generally falls under UN 2570 – Cadmium compounds, n.o.s., hazard class 6.1 (toxic substances). Occupational exposure limits are the same as for other cadmium compounds—typically ≤0.005 mg/m³ (as Cd) for an 8-hour time-weighted average. Handling should be done in enclosed systems or fume hoods, with full personal protective equipment including respirators, gloves, and protective clothing.
