Chromium chromate (41261-95-4) Physical and Chemical Properties
Chromium chromate
Inorganic chromium chromate salt commonly employed in industrial metal surface treatment and corrosion‑inhibiting formulations supplied to metal finishing and coatings manufacturers.
| CAS Number | 41261-95-4 |
| Family | Chromates (inorganic salts) |
| Typical Form | Dark granular solid |
| Common Grades | BP, EP, USP |
Chromium chromate is an inorganic chromium salt in which transition‑metal cations are associated with oxyanions of chromium. Structurally this material is best described as a mixed chromium salt comprising an oxido/oxo chromium oxyanion and a reduced chromium cation; the simplest molecular formula reported is \(\ce{Cr2O4}\). Electronic structure is dominated by chromium–oxygen multiple bonding in the oxyanion (formally high oxidation state, oxyanion character) and a reduced chromium cation with localized d‑electrons. The oxyanion component behaves as a typical chromate/dichromate system, while the cationic chromium center contributes to complex solid‑state coordination and mixed‑valence chemistry.
Chemically the substance combines the oxidizing character of hexavalent chromium oxyanions with the reductive potential of lower‑valent chromium cations in mixed phases or salts. In aqueous media the oxyanion part participates in chromate ⇄ dichromate equilibria that are strongly pH dependent (chromate favored at alkaline pH, dichromate at acidic pH), whereas trivalent chromium species display amphoterism and form stable coordination complexes. Chromate species are polar, water‑soluble oxyanions; overall materials in this class are typically hydrophilic and not lipophilic. Industrial relevance arises from the strong corrosion‑inhibiting and conversion‑coating behavior of chromate materials in metal finishing, passivation and protective coatings.
Common commercial grades reported for this substance include: BP, EP, USP.
Basic Physical Properties
Density
Relative density: 2.269 \(\mathrm{g}\,\mathrm{cm}^{-3}\) at \(\,20\,^\circ\mathrm{C}\).
This value reflects the dense, inorganic oxide/oxyanion lattice typical of chromium oxide/chromate solids; packing is influenced by mixed‑valence coordination and the amorphous or granular solid morphology often reported.
Melting or Decomposition Point
Melting/thermal behaviour: >300 \(\,^\circ\mathrm{C}\) (capillary method according to EU Method A1).
The material is thermally robust relative to many organics and decomposes or melts above 300 \(\,^\circ\mathrm{C}\). High‑temperature behavior is consistent with strong Cr–O bonding and the tendency of chromates to undergo redox/condensation reactions on heating.
Solubility in Water
Reported experimental solubility: 96.6 \(\mathrm{g}\,\mathrm{L}^{-1}\) at \(\,20\,^\circ\mathrm{C}\) (EU Method A.6 (Water Solubility)).
Additional experimental description: "Soluble in water" and earlier literature summaries describe the solid as "Dark purple to black solid; Sparingly soluble in water."
Measured solubility on the order of 10^1 \(\mathrm{g}\,\mathrm{L}^{-1}\) is indicative of significant aqueous dissolution to give chromate oxyanions and associated chromium cations; qualitative descriptors may vary with particle morphology, degree of hydration and sample composition (mixed oxidation states).
Solution pH (Qualitative Behavior)
No single numeric solution pH is reported for a standardized aqueous solution of this specific solid. Qualitatively: chromate/dichromate speciation is strongly pH dependent (chromate \(\ce{CrO4^2-}\) at alkaline pH, dichromate \(\ce{Cr2O7^2-}\) at acidic pH), while trivalent chromium species are amphoteric and form cationic or hydroxo complexes depending on \(\mathrm{pH}\). Mixed chromium salts may therefore produce acidic, neutral or mildly alkaline solutions depending on composition and degree of hydrolysis.
Chemical Properties
Acid–Base Behavior
Chromate oxyanions are basic oxyanions that undergo protonation equilibria to form dichromate under acidic conditions; the canonical equilibria are class‑level behaviors of the \(\ce{CrO4^2-}/\ce{Cr2O7^2-}\) system. Trivalent chromium components (when present) are amphoteric and form hydroxo complexes and coordination species; they are less easily oxidized in acidic media and require strong oxidants for conversion to higher oxidation states. Overall acid–base chemistry is therefore dictated by the relative proportions of hexavalent oxyanion and any lower‑valent chromium present.
Reactivity and Stability
- Oxidizing behavior: Chromate (hexavalent chromium) components are strong oxidizers and will oxidize many organic and readily oxidizable materials; the substance class is described as a "very powerful oxidizer".
- Incompatibilities: Reactive or combustible organics, hydrazine (decomposed explosively by chromates), sulfur, aluminum and many organics are incompatible; chromates are corrosive to some metals.
- Redox transformations: Hexavalent chromium readily enters cells (as chromate) and is reduced intracellularly to Cr(V) and Cr(IV) intermediates and finally to Cr(III). In environmental and waste treatment contexts, Cr(VI) can be reduced to lower‑toxicity Cr(III) by reductants; Cr(VI) may be removed from sludge by reduction (e.g., carbon at high temperatures reported).
- Stability: In the absence of strong reducing agents the solid is chemically stable; exposure to strong reducing conditions leads to redox transformations and potential release of lower‑valent chromium species.
Molecular and Ionic Parameters
Formula and Molecular Weight
Molecular formula: \(\ce{Cr2O4}\)
Molecular weight: 167.99 \(\mathrm{g}\,\mathrm{mol}^{-1}\)
Computed small‑molecule descriptors: exact mass 167.860668 \(\mathrm{u}\) (monoisotopic mass 167.860668 \(\mathrm{u}\)); topological polar surface area (TPSA) 80.3; formal charge 0.
Constituent Ions
Reported/interpreted constituent ions for the stoichiometry \(\ce{Cr2O4}\): cationic reduced chromium and chromate oxyanion, e.g., \(\ce{Cr^2+}\) (reduced chromium cation) paired with \(\ce{CrO4^2-}\) (chromate anion). Some literature descriptions also characterize related materials as mixed chromium(III)/chromium(VI) salts produced by partial reduction of chromic acid; the precise ionic composition may vary with sample preparation and oxidation state distribution.
Additional computed/structural descriptors: hydrogen bond donor count 0; hydrogen bond acceptor count 4; rotatable bond count 0; heavy atom count 6; complexity 62.2.
SMILES: [O-][Cr](=O)(=O)[O-].[Cr+2]
InChI: InChI=1S/2Cr.4O/q;+2;;;2*-1
InChIKey: ZTPKOWRHFDMHPS-UHFFFAOYSA-N
Identifiers and Synonyms
Registry Numbers and Codes
- CAS: 41261-95-4
- Additional CAS reported in the literature: 24613-89-6
- DSSTox Substance ID: DTXSID20194177
(Use the CAS 41261-95-4 exactly as given for this substance.)
Synonyms and Common Names
Synonyms and depositor‑supplied names appearing in source material include:
CHROMIUM CHROMATE; Chromium chromate (H2CrO4); Chromic acid, chromium salt; Chromium chromic acid; Chromium (III) chromate; Dichromium tris(chromate) (as a related composition).
Industrial and Commercial Applications
Functional Roles and Use Sectors
Chromium chromate materials and closely related chromate salts are used primarily for metal surface treatment and corrosion control. Functional roles include conversion coatings, passivation layers and formulation components in chromate treating baths for zinc, cadmium, magnesium and aluminum alloys. The material class is used where durable, adherent conversion coatings with good salt‑spray resistance are required.
Typical Application Examples
Reported application examples include: chromate coatings and corrosion inhibitors for zinc, cadmium and magnesium; metal surface treatment for aircraft components, aluminum and zinc castings; galvanized steel passivation and pretreatment prior to organic coating. Dichromium tris(chromate) and related mixed chromium salts are reported as components in formulations for hot‑dip galvanizing post‑treatment and passivation of galvanized sheets. In practice the choice of a chromate material for a given application is governed by corrosion performance, adhesion, bath stability and regulatory constraints.
Safety and Handling Overview
Health and Environmental Hazards
Chromium chromate and related chromate/hexavalent chromium compounds present significant health hazards associated with hexavalent chromium chemistry:
- Carcinogenicity: Hexavalent chromium compounds are classified as human carcinogens by inhalation routes; occupational inhalation exposures to Cr(VI) compounds have been associated with increased lung cancer risk.
- Local and systemic toxicity: Inhalation or contact with chromic acid/ chromate dusts or mists may cause severe respiratory and mucosal irritation, nasal ulcers, asthma and bronchitis. Dermal exposure can produce severe skin irritation, ulceration and sensitization; ingestion may cause gastrointestinal ulceration, hemolysis and renal toxicity.
- Systemic fate: Cr(VI) species can cross cell membranes via sulfate transporters and are reduced intracellularly to lower oxidation states, generating reactive intermediates that damage DNA and other biomolecules.
- Regulatory and waste considerations: Waste containing chromium may be subject to hazardous‑waste characterization and regulation; the EPA hazardous waste number D007 is associated with chromium‑containing wastes under certain conditions.
Occupational exposure limits and guidance quoted in source materials (use only as operational context): example recommended exposure levels and limit values for chromates and chromium species have been stated by occupational health organizations; respirable exposure controls and medical surveillance are typical controls where Cr(VI) exposure may occur.
Storage and Handling Considerations
- Engineering controls: Use local exhaust ventilation or containment to minimize airborne dust and mists where handling or processing generates particulates or aerosols.
- Personal protective equipment (PPE): Impervious gloves, protective clothing, eye protection and respiratory protection appropriate to the measured airborne concentrations should be used when handling chromates. Contaminated clothing should be segregated and laundered before reuse.
- Spill and emergency measures: Isolate the area, restrict access, ventilate, collect spilled material into sealed containers for reclamation or disposal. Absorb liquids with vermiculite, dry sand or earth. Persons not wearing protective equipment should be excluded until cleanup is complete. Avoid generating dusts and do not allow releases to sewerage without appropriate treatment.
- Incompatibilities and fire response: Chromates are strong oxidizers and can accelerate combustion of organic materials; they are incompatible with hydrazine (explosive decomposition reported) and readily oxidizable substrates. When firefighting, use self‑contained breathing apparatus and full protective clothing; avoid using agents that may react with oxidizing chromates.
- Disposal: Manage wastes in accordance with local, state and national regulations. Wastewater and sludge containing Cr(VI) should be evaluated and, if required, pretreated (e.g., reduction to Cr(III) and precipitation) prior to discharge. Generators of chromium‑containing wastes should consult applicable hazardous‑waste regulations for storage, transport and disposal requirements.
For detailed hazard, transport and regulatory information, users should refer to the product‑specific Safety Data Sheet (SDS) and local legislation.
