Calcium Chromate
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Product Description
Calcium chromate (CaCrO₄) is a yellowish crystalline inorganic compound composed of calcium cations (Ca²⁺) and chromate anions (CrO₄²⁻). It is slightly soluble in water and is primarily recognized for its vivid yellow color. Historically, calcium chromate has been used as a corrosion inhibitor, a depolarizer in batteries, and as a pigment known as Gelbin or Yellow Ultramarine. However, due to its severe health risks, including ulcerations, respiratory issues, and carcinogenicity, its use is now heavily restricted. Calcium chromate is a known human carcinogen, associated with an increased risk of lung cancer and cancer of the sinonasal cavity. Recent studies have also investigated its potential tumorigenic effects in animal models, further highlighting its hazardous nature. Despite these risks, calcium chromate continues to be a subject of research, particularly in the context of environmental remediation and the synthesis of nanomaterials with unique optical properties.
Thermodynamic Properties
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Heat Capacities:
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Cp(CaCrO4)=127.92+35.73×10−3T−2.257×10−6/T2 (298-1123 K)
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Cp(β−CaCr2O4)=166.69+18.33×10−3T−2.92×10−6/T (298-1203 K)
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Standard Free Energy of Formation:
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Determined using solid/gas equilibrium and thermogravimetric analysis.
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Comparison with Other Chromates
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Sodium Chromate (Na₂CrO₄) and Potassium Chromate (K₂CrO₄):
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More soluble in water than Calcium Chromate.
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Often used in laboratories as a source of chromate ions.
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Lead Chromate (PbCrO₄):
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Also a yellow pigment (Chrome Yellow), but it has the dual toxicity of lead and hexavalent chromium.
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Strontium Chromate (SrCrO₄):
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Has very low solubility and was historically a preferred pigment and corrosion inhibitor for its slower release of chromate ions, though it is still highly toxic.
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Synthesis Methods
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Reaction of Sodium Chromate with Calcium Chloride:
Na2CrO4+CaCl2→CaCrO4↓+2NaCl -
Oxidation of Chromium(III) Oxide with Calcium Carbonate in Air at High Temperature:
2Cr2O3+4CaCO3+3O2→4CaCrO4+4CO2
Uses
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Corrosion Inhibition: Used in pigments and primers for metals, especially in aerospace and marine environments.
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Pigment: Used as a yellow pigment in paints, dyes, ceramics, glass, plastics, and cement.
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Oxidizing Agent: Used in chemical synthesis and pyrotechnic compositions.
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Photographic Processing and Metal Surface Treatment: Used in photographic processing and for treating metal surfaces.
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Battery Applications: Used as a positive electrode material for calcium-based thermal batteries.
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Laboratory Chemical: Used in various laboratory experiments.
Basic Information
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Chemical Name: Calcium Chromate
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Other Names: Gelbin, Yellow Ultramarine, Calcium Monochromate, C.I. Pigment Yellow 33
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Appearance: Bright yellow crystalline solid or powder
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Odor: Odorless
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Solubility: Slightly soluble in water; insoluble in alcohol, ethanol, or acetone
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Crystal Structure: Orthorhombic
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Oxidation States: Ca²⁺ and Cr⁶⁺ (chromium in +6 oxidation state)
Chemical and Physical Properties
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Color: Bright lemon-yellow
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Melting Point: Decomposes before melting (around 200–250 °C)
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Density: ~2.89 g/cm³ to 3.1 g/cm³
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pH (aqueous solution): Slightly basic
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Reactivity:
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Strong oxidizing agent
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Reacts with organic matter and reducing agents, potentially causing combustion or explosion
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Can decompose to chromium(III) oxide and calcium oxide upon heating
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Toxicity and Safety
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Toxicity:
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Highly toxic due to its hexavalent chromium (Cr⁶⁺) content.
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Carcinogenic (IARC Group 1 — proven human carcinogen).
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Causes severe irritation to skin, eyes, and respiratory tract; may cause lung cancer upon prolonged exposure.
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Toxic to aquatic life due to Cr⁶⁺ ions.
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Health Risks:
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Inhalation: Strongly linked to lung cancer, asthma, nasal septum perforation, and chronic bronchitis.
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Skin and Eyes: Causes severe irritation, allergic reactions (chromate sensitivity), and ulcers ("chrome sores").
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Ingestion: Highly toxic if swallowed, causing severe gastrointestinal irritation and systemic poisoning.
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Safety Precautions:
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Use in a well-ventilated area, preferably a fume hood.
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Wear appropriate Personal Protective Equipment (PPE) including gloves, safety goggles, and a respirator.
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Avoid any action that creates dust.
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Strictly follow local regulations for the handling and disposal of hazardous materials.
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Environmental and Regulatory Notes
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Regulations:
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Restricted or banned in many countries under RoHS and REACH regulations.
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Often replaced with safer alternatives like zinc phosphate or organic corrosion inhibitors.
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