- Cadmium chromate is an inorganic compound with the formula CdCrO₄, consisting of divalent cadmium cations (Cd²⁺) and chromate anions (CrO₄²⁻).
- The chromate ion contains a central chromium atom in the +6 oxidation state, tetrahedrally coordinated by four oxygen atoms.
- The compound has a molar mass of 228.40 g/mol and is typically encountered as a bright yellow to orange crystalline solid or powder. It is sparingly soluble in water but can dissolve in acids with decomposition.
- Cadmium chromate crystallizes in an orthorhombic crystal system similar to other alkaline earth chromates. The cadmium ion is surrounded by oxygen atoms from the chromate group, forming a three-dimensional lattice structure. The mineral form is known as croftite, which is rare in nature. The vivid yellow color of cadmium chromate arises from electronic transitions in the chromate anion, and this property has historically made it a valuable pigment. It has a density of about 4.34 g/cm³ and decomposes upon heating before reaching a distinct melting point.
- Cadmium chromate can be synthesized by the precipitation method, where a soluble cadmium salt (such as cadmium nitrate or cadmium acetate) is reacted with a soluble chromate salt (such as potassium chromate or sodium chromate):
- Cd(NO₃)₂ + K₂CrO₄ → CdCrO₄↓ + 2 KNO₃
- The yellow precipitate is filtered, washed, and dried. Alternatively, it can be formed via controlled oxidation of cadmium(II) compounds in the presence of chromium(VI) sources.
- Cadmium chromate is generally stable under normal conditions but should be kept away from strong reducing agents, which can convert the toxic Cr(VI) to Cr(III) while producing potentially hazardous cadmium-containing byproducts. It decomposes at high temperatures to yield cadmium oxide (CdO) and chromium(III) oxide (Cr₂O₃), both of which are still toxic. In acidic environments, cadmium chromate dissolves, releasing highly toxic cadmium ions and carcinogenic hexavalent chromium ions.
- Historically, cadmium chromate was widely used as a corrosion inhibitor and protective coating pigment in primers for metals, particularly in aerospace and military applications, due to its ability to passivate metal surfaces and prevent oxidation. It was also valued as a pigment in paints, known as “cadmium yellow” or as part of mixed pigment systems, due to its brilliant color and excellent lightfastness. However, because of its extreme toxicity and carcinogenicity, its use in consumer products and paints has been almost entirely phased out in favor of safer alternatives such as organic pigments and zinc chromate.
- Cadmium chromate is extremely hazardous because it combines the dangers of both cadmium and hexavalent chromium compounds. Inhalation of dust or aerosolized particles can cause severe lung damage, irritation of mucous membranes, and respiratory tract cancer. Skin contact can cause ulcerations and dermatitis, and ingestion can lead to acute systemic poisoning affecting the kidneys and liver. Chronic exposure can cause irreversible kidney damage, bone demineralization, and multiple cancers, including lung, nasal, and gastrointestinal cancers. Both Cd²⁺ and Cr(VI) ions are classified as Group 1 carcinogens by the International Agency for Research on Cancer (IARC).
- Cadmium chromate is highly toxic to aquatic life and persistent in soil and sediment. It can leach into groundwater from industrial waste, leading to bioaccumulation in plants and animals. Many countries have banned or heavily restricted its use under chemical safety regulations such as the EU’s REACH Regulation, the U.S. EPA hazardous waste rules, and the OSHA standards for cadmium and hexavalent chromium. Disposal of cadmium chromate-containing waste requires specialized hazardous waste management to prevent environmental contamination.
- Cadmium chromate is classified under UN 2570 – Cadmium compounds, n.o.s., hazard class 6.1 (toxic substances), with additional labeling for carcinogenicity and environmental hazards. Strict occupational exposure limits are enforced, often ≤0.005 mg/m³ (as Cd) and ≤0.005 mg/m³ (as Cr(VI)) for an 8-hour time-weighted average. Its handling requires full PPE, including respirators, gloves, and protective clothing, and all operations should be conducted in fume hoods or enclosed systems.
