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D2807-17 – Standard Test Method Technical Guide
🔬 Scope and Significance
This standard test method, designated D2807-17a, covers the determination of chromic oxide (Cr₂O₃) in leathers that have been partially or completely tanned with chromium compounds. Samples analyzed by this procedure typically contain between 1 and 5 % chromium, calculated as chromic oxide. The method is highly specific for chromium in leather; vanadium is the only common interfering element and it is rarely present in sufficient quantity to affect results.
The chromium content of leather is a direct indicator of the degree of tannage achieved, making this test method crucial for quality control and specification in the purchasing of leather. Values are stated in SI units per standard practice.
⚙️ Test Procedure and Apparatus
The perchloric acid method is applied directly to the ash residue obtained from Test Method D2617. In this acid digestion, any remaining organic matter is completely destroyed and the chromium is oxidized to the hexavalent state (Cr⁶⁺). On dilution, the chromium content is determined by volumetric titration with thiosulfate or a ferrous salt. While this method requires less manipulation than procedures based on fusion of the ash, extreme caution is required due to the potential hazards of perchloric acid (see Section 7 of the standard for specific safety hazards).
Essential Apparatus for Potentiometric Titration
| 🟦 Component | 📐 Specification / Role |
|---|---|
| Stirrer | Agitates the solution during titration to ensure uniformity. |
| Electrodes | Calomel (reference) and Platinum (indicator) electrodes. |
| Potentiometer | Null-point device (cathode-ray tube or galvanometer) to signal the abrupt potential change at the endpoint. |
⚠️ Safety Hazard: The use of perchloric acid presents significant safety risks. Users must establish appropriate safety, health, and environmental practices before use. Digestion should always be performed in a fume hood specifically designed for perchloric acid work.
📊 Key Measured Properties and Referenced Standards
The primary property measured by this standard is the chromic oxide (Cr₂O₃) content in leather. The precision and accuracy of this method are generally as good as the sampling of the leather itself. The test method relies on several other ASTM standards for sample preparation and complementary analysis.
| 📏 Referenced Standard | 🎯 Purpose in D2807 |
|---|---|
| D2617 | Provides the ash sample for the perchloric acid digestion step. |
| D2813 | Standard practice for sampling leather for physical and chemical tests. |
| D2868 | Determination of Nitrogen and Hide Substance content. |
| D3495 | Test method for hexane extraction of leather. |
| D3790 | Test method for volatile matter (moisture) of leather by oven drying. |
💡 Technical Note: The accuracy of the Cr₂O₃ determination is highly dependent on the ashing procedure from D2617. Strictly adhere to the specified muffle furnace temperatures and ashing times to prevent loss of chromium or incomplete combustion of organic matter.
❓ Frequently Asked Questions
🔍 What is the typical concentration range for chromium in leather samples analyzed by this method?
The standard covers samples containing between 1 and 5 % chromium, calculated as chromic oxide (Cr₂O₃).