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D6402-99 – Standard Test Method Technical Guide
ASTM D6402 −99 (Reapproved 2020), formally titled the Standard Test Method for Determining Soluble Solids and Insolubles in Extracts of Vegetable Tanning Materials, provides a standardized gravimetric framework for evaluating the composition of tannin extracts and tanning liquors. This method is essential for quality control in the leather industry, allowing analysts to quantify the proportion of solid material that is fully dissolved or dispersed versus material that remains particulate or insoluble. The values stated in SI units are regarded as standard.
📐 Scope and Key Terminology
This test method is intended for determining the quantity of soluble solids and insolubles in solutions of tannin extracts, water extracts of vegetable tanning materials, or tanning liquors. It applies to the analysis of liquid, solid, pasty, and powdered tannin extracts, as well as the water extracts of raw or spent materials.
The standard defines two critical operational parameters based on the filtration process:
| 🟦 Term | 📐 Operational Definition |
|---|---|
| Soluble Solids | Non-volatile materials present in the solution that are dissolved or suspended in water and pass through the specified filtering process. |
| Insolubles | Non-volatile materials present in the solution that are dissolved or suspended in water and do not pass through the filtering process. |
⚙️ Test Method Procedure and Analytical Summary
The analysis follows a comparative gravimetric procedure. Analytical solutions must be prepared according to specific ASTM practices, depending on the sample form (Practices D4901 or D4905 for liquid, solid, pasty, and powdered extracts, or Practice D6405 for raw and spent materials).
- Total Solids Determination: A measured aliquot of the analytical solution is dried overnight in a forced-air oven in accordance with Test Method D4902. The weight of the resulting solid residue is used to calculate the Total Solids for that sample per Test Method D4903.
- Soluble Solids Determination: A second aliquot of the exact same solution is passed through the specified filtering medium. The clear filtrate is then evaporated and dried using the identical process described in D4902. The weight of the solid residue from this filtrate defines the Soluble Solids.
- Insolubles Calculation: The Insolubles content is mathematically determined as the difference between the Total Solids and the Soluble Solids.
💡 Important Measurement Principle: The results are entirely operational and depend strictly on the filtering and drying procedures specified. The calculation for a given sample is: Insolubles = Total Solids − Soluble Solids.
📊 Significance and Referenced Standards
This test method is vital for determining the purity and yield of tannin extracts. It provides a clear quantitative distinction between the active, tannin-bearing soluble fraction and the inert insoluble burden. A high proportion of insolubles can indicate inefficient extraction or the presence of inert contaminants. This standard was developed under the jurisdiction of ASTM Committee D31 on Leather and has been adapted from ALCA Method A21.
The test method integrates seamlessly with the following ASTM standards:
| 🟦 Designation | 📏 Specific Function in D6402 |
|---|---|
| D4901 / D4905 | Preparation of analytical solutions from liquid, solid, pasty, or powdered tannin extracts. |
| D4902 | Specifies the evaporation and forced-air oven drying procedures for the analytical solutions. |
| D4903 | Provides the calculation method for Total Solids content from the dried residue. |
| D6405 | Defines the water extraction method for obtaining analytical solutions from raw and spent vegetable tanning materials. |
⚠️ Health and Safety Advisory: Per Section 1.3, this standard does not purport to address all safety concerns associated with its use. It is the responsibility of the user to establish appropriate safety, health, and environmental practices and to determine the applicability of regulatory limitations prior to use.
❓ Frequently Asked Questions
🔍 What types of materials can be analyzed using ASTM D6402?
The method is applicable to liquid, solid, pasty, and powdered vegetable tannin extracts, as well as water extracts from raw and spent vegetable tanning materials.
💡 How is the insolubles value specifically derived in this test?
The insolubles content is not weighed directly. It is calculated by subtracting the weight percentage of the Soluble Solids (obtained from the filtrate residue) from the Total Solids content of the original analytical solution.
⚡ What drying method is specified for the analytical solutions?
The standard mandates drying the analytical solutions overnight in a forced-air oven, strictly following the detailed procedures outlined in ASTM Test Method D4902.
📌 What is the relationship between D6402 and ALCA Method A21?
ASTM D6402 has been adapted from and serves as the direct replacement for Method A21 of the Official Methods of the American Leather Chemists Association (ALCA).