Description
Basic chromium sulfate (BCS) is an inorganic chemical compound with the formula Cr(OH)SO₄. It’s an odorless, green solid, typically sold as a powder or flakes, and is highly soluble in water.It’s most notably used in the leather tanning industry, where it’s the main agent in the “chrome tanning” process. This process transforms raw animal hides into durable, flexible, and water-resistant leather by stabilizing collagen fibers.
Chemical Properties
| Property | Description |
|---|---|
| Chemical name | Basic Chromium Sulfate |
| Common formula (approx.) | Cr(OH)<sub>x</sub>(SO₄)<sub>y</sub>·nH₂O |
| CAS number | 12336-95-7 (commonly cited) |
| Chromium oxidation state | +3 (Cr³⁺) |
| Molecular weight | Variable (depends on basicity and hydration) |
| Basicity | Typically 30–45% (OH⁻/Cr ratio) |
| Appearance | Green to bluish-green powder or flakes |
| Odor | Odorless |
| Solubility in water | Soluble (forms green solution) |
| Solution pH (10% w/v) | ~2.5–4.0 |
| Hydration | Highly hydrated compound |
| Density | ~1.7–1.9 g/cm³ (bulk, varies) |
| Thermal stability | Stable at room temperature; decomposes on strong heating |
| Decomposition products | Chromium(III) oxide (Cr₂O₃), sulfur oxides |
| Chemical nature | Basic chromium(III) salt |
| Reactivity | Reacts with strong acids and strong bases |
| Redox behavior | Stable Cr(III); does not oxidize easily |
| Coordination behavior | Forms coordination complexes with collagen |
Uses and Properties
Basic chromium sulfate is primarily valued for its ability to cross-link collagen fibers in animal hides, which is the key to its effectiveness in tanning. The result is leather with desirable qualities like:
Durability and flexibility: The treated fibers are stabilized, making the leather resistant to decomposition and wear.
Water and heat resistance: The cross-linking process imparts a high degree of resistance to water and heat.
Softness and drape: It helps achieve a soft texture, making the leather suitable for a wide range of products like footwear, apparel, and upholstery.
Textile dyeing: It is used as a mordant to help dyes bond with fabrics, improving colorfastness.
Pigments: It is used in the creation of green pigments for paints, inks, and ceramic glazes.
Basicity
Basicity is a critical parameter that refers to the degree of hydrolysis of the chromium sulfate. It indicates the ratio of hydroxyl groups (OH−) to sulfate groups (SO42−) in the compound. Basicity is expressed as a percentage and can be adjusted during the production process by adding a base like sodium carbonate.
The basicity of BCS directly affects its tanning performance:
Higher basicity (e.g., 40-50%): A higher basicity means the chromium complex is more hydrolyzed and has a greater affinity for collagen fibers. This leads to a faster and more efficient tanning process. However, if the basicity is too high, it can result in premature surface tanning (“case hardening”) and poor penetration of the tanning agent into the hide.
Lower basicity (e.g., 30-33%): A lower basicity allows for better penetration of the tanning agent into the leather hide before the actual cross-linking begins. This results in a more uniform tan and a softer, more pliable final product.
The most common basicity for general chrome tanning is around 33%. Producers may also offer higher basicity grades (e.g., 38-42%) which may be used in combination with other chemicals for specific leather properties.
Grades of BCS:
Water insolubles: This measures the amount of insoluble impurities. A lower percentage (typically <0.1%) indicates a higher-purity product.
Iron (Fe) content: The presence of iron can discolor the leather, so a low iron content is desirable.
Hexavalent chromium (Cr6+) content: This is a crucial safety parameter. Hexavalent chromium is highly toxic and a known carcinogen. All commercial grades of BCS must have non-detectable or extremely low levels of Cr6+ to comply with environmental and safety regulations. The production process is specifically designed to reduce all Cr6+ to the safer Cr3+ state.
pH of the solution: The pH of a BCS solution (typically a 2% or 10% solution) is another indicator of its properties and can be a specification.
These grades and specifications are essential for tanners to select the appropriate product for their specific processes and desired leather characteristics.
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