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D2868-24 – Standard Test Method Technical Guide
🔬 Scope and Significance
This test method covers the determination of nitrogen content in all types of leather, wet blue, and wet white. The nitrogen content is used to calculate the hide substance (protein fiber) content. The original test method was a composite of Federal Test Method Standard No. 311 and American Leather Chemists Association Method B5. Bonded leather containing melamine may yield artificially high protein fiber values (see Note 2).
The values stated in SI units are standard. This standard was developed in accordance with internationally recognized principles on standardization established by the WTO TBT Committee.
According to Section 4, the nitrogen content is related to the amount of hide substance present. A factor of 5.62 is normally used to calculate hide substance from nitrogen content. This factor is an average result from many analyses of animal hides and varies between hide types and thicknesses.
⚙️ Test Method Summary
The specimen, prepared according to Practice D2813 for leather or Practice D6659 for wet blue and wet white, is digested with acid in the presence of a catalyst to convert nitrogen to ammonium ion. The ammonium ion is nonvolatile under highly acid conditions.
The acid mixture is then made alkaline, and the liberated ammonia is distilled into a boric acid solution or a sulfuric acid solution. If boric acid is used, the ammonia is determined by back titration with standardized acid using a green to purple color change indicator. If sulfuric acid is used, the ammonia is determined by back titration with standardized base using a purple to green-blue color change indicator.
📊 Key Parameters and Calculations
Nitrogen content is expressed as a percentage of the sample mass. The hide substance content is calculated by multiplying the nitrogen content by 5.62, which is the average factor from hide analyses.
| 🟦 Parameter | 📏 Value | 🎯 Notes |
|---|---|---|
| Hide Substance Factor | 5.62 | Average from animal hide analyses |
| Indicator (Boric Acid Method) | Green to Purple | End point for titration using standardized acid |
| Indicator (Sulfuric Acid Method) | Purple to Green-Blue | End point for titration using standardized base |
⚠️ Warning: Melamine in bonded leather can cause artificially high protein fiber content values, leading to inaccurate hide substance calculations.
💡 Tip: The 5.62 factor is not precise; it varies somewhat from hide to hide of the same type, across hide types, and with final leather thickness (split vs. original thickness).
❓ Frequently Asked Questions
🔍 What is the primary purpose of this test method?
To determine the nitrogen content and calculate the hide substance (protein fiber) content in leather, wet blue, and wet white samples.
💡 How is the nitrogen content determined?
Through acid digestion to convert nitrogen to ammonium ion, alkalinization to liberate ammonia, distillation into an absorbing solution, and titration with a standardized solution. A distinct color change marks the endpoint.
⚡ Why is the factor 5.62 used for hide substance calculation?
It represents the average result from many analyses of animal hides, providing a standard conversion from nitrogen content to hide substance content in leather testing.
📌 What sampling practices are recommended?
For leather, use Practice D2813 (Sampling Leather for Physical and Chemical Tests). For wet blue and wet white, use Practice D6659 (Sampling and Preparation of Wet Blue and Wet White for Physical and Chemical Tests).