Rhodium(III) sulfate
[CAS# 10489-46-0]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Metal sulfides and sulfates
• Name: Rhodium(III) sulfate
• Synonyms: Dirhodium trisulphate
• Molecular Formula: Rh₂.(SO₄)₃
• Molecular Weight: 494.00
• CAS Registry Number: 10489-46-0
• EC Number: 234-014-5
• SMILES: [O-]S(=O)(=O)[O-].[O-]S(=O)(=O)[O-].[O-]S(=O)(=O)[O-].[Rh+3].[Rh+3]

Properties

• Density: 1.217 g/mL (25 °C) (Expl.)

Safety Data

• Hazard Symbols: GHS05;GHS07;GHS08;GHS09 Danger
• Risk Statements: H290-H314-H315-H318-H319-H335-H341-H400-H410
• Safety Statements: P203-P234-P260-P261-P264-P264+P265-P271-P273-P280-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P351+P338-P305+P354+P338-P316-P317-P318-P319-P321-P332+P317-P337+P317-P362+P364-P363-P390-P391-P403+P233-P405-P406-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Skin irritation	Skin Irrit.	2	H315
Serious eye damage	Eye Dam.	1	H318
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Germ cell mutagenicity	Muta.	2	H341
Skin corrosion	Skin Corr.	1B	H314
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Skin corrosion	Skin Corr.	1A	H314
Oxidising solids	Ox. Sol.	1	H271
Acute toxicity	Acute Tox.	4	H302
Acute toxicity	Acute Tox.	4	H312
Acute toxicity	Acute Tox.	4	H332

Discovery and Applications

Rhodium(III) sulfate is an inorganic compound composed of rhodium in the +3 oxidation state and sulfate anions. It is typically encountered as the hydrated form Rh₂(SO₄)₃·xH₂O, where the exact hydration state depends on preparation and storage conditions. Rhodium(III) sulfate is water-soluble and forms red to reddish-brown solutions, which are used in coordination chemistry, catalyst preparation, and electroplating applications.

The compound is usually prepared by dissolving rhodium(III) oxide (Rh₂O₃) or rhodium metal in concentrated sulfuric acid, often under elevated temperature or with the aid of oxidizing agents to facilitate dissolution. This process yields a solution containing the rhodium(III) sulfate complex, which may be used directly or evaporated to isolate the solid hydrate. Because rhodium is a noble metal, its dissolution is generally slow and requires harsh acidic or oxidative conditions.

In aqueous solution, rhodium(III) sulfate forms complex ions where the Rh³⁺ center is surrounded by water and possibly sulfate ligands. The rhodium ion adopts an octahedral geometry, commonly coordinating with six ligands such as water molecules or monodentate sulfate groups. The actual composition of the complex species depends on the concentration of sulfuric acid and the pH of the solution. Sulfate may act as either a counterion or a coordinated ligand, depending on these conditions.

One of the main applications of rhodium(III) sulfate is as a precursor for rhodium electroplating baths. Rhodium coatings produced from sulfate-based baths are highly valued for their exceptional hardness, corrosion resistance, and bright white finish. These coatings are used extensively in jewelry, electrical contacts, optical instruments, and decorative finishes. Compared to chloride-based baths, sulfate-based systems tend to offer more stable pH and longer bath life.

Rhodium(III) sulfate is also used in the synthesis of rhodium coordination compounds. Its ability to react with various ligands, including phosphines, amines, and carboxylates, makes it useful for preparing complexes with catalytic and electronic properties. These complexes may be used in homogeneous catalysis for hydrogenation, hydroformylation, and carbon-carbon coupling reactions.

In materials chemistry, rhodium(III) sulfate serves as a source of rhodium in the preparation of catalysts, ceramic components, and functional coatings. Thermal decomposition of the hydrated salt or its solutions can produce rhodium oxide or elemental rhodium under appropriate conditions. This feature is used in the fabrication of rhodium-containing films and nanoparticle catalysts.

The handling of rhodium(III) sulfate requires standard precautions for transition metal salts. Although rhodium compounds are generally considered to have low toxicity, ingestion or prolonged exposure should be avoided. As with all sulfate-containing compounds, care should be taken to avoid inhalation of dust or aerosols. Storage in tightly sealed containers away from incompatible substances such as strong reducing agents is recommended.

In summary, rhodium(III) sulfate is a versatile rhodium salt with applications in electroplating, catalysis, and materials synthesis. It is valued for its solubility, stability in aqueous sulfuric acid, and ability to form a range of rhodium(III) complexes. Its use in high-performance coatings and advanced materials highlights its importance in both industrial and scientific contexts.

References

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