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D4900-99 – Standard Test Method Technical Guide
📌 Scope and Significance
This test method, under the designation D4900-99 (Reapproved 2020), is intended for detecting the presence of lignosulfonates (sulfite cellulose) in extracts of tanning materials. It is useful to detect possible adulteration of tanning material extracts with cheap and inferior lignosulfonates. The values stated in SI units are to be regarded as standard.
⚙️ Specimen and Reagents
The sample shall be drawn as described in Practice D6404. The specimen consists of 5 mL of a solution of analytical strength (4.0 g tannin per L), prepared according to Practices D4901, D4905, and D6405. The reagents required include aniline and concentrated hydrochloric acid. A comparison solution is prepared containing lignosulfonate in the proportion of 1 g total solids per 2000 mL of solution, with tanning material similar to that being tested but known to be free from lignosulfonate, adjusted to analytical strength.
| 🟦 Reagent | 🟦 Quantity |
|---|---|
| Specimen (analytical strength, 4.0 g/L) | 5 mL |
| Aniline | 0.5 mL |
| Concentrated Hydrochloric Acid | 2 mL |
| Comparison Solution (lignosulfonate) | 1 g per 2000 mL |
🔬 Procedure and Results
Place the specimen in a test tube. Add 0.5 mL of aniline and shake well. Add 2 mL of concentrated hydrochloric acid and shake the mixture again. After 15 minutes, compare the precipitate with that obtained from the comparison solution. If at least as much precipitate remains as in the comparison, lignosulfonates are present in the tanning extract.
⚠️ Safety: Aniline and concentrated hydrochloric acid are corrosive and toxic. Conduct all steps in a fume hood with proper personal protective equipment.
💡 Tip: For consistent results, ensure the comparison solution is freshly prepared and the reaction time is strictly observed at 15 minutes.
❓ Frequently Asked Questions
🔍 What is the principle behind this test method?
The method relies on the reaction between aniline and lignosulfonates in the presence of concentrated hydrochloric acid, producing a characteristic precipitate that indicates adulteration.
💡 How is the comparison solution prepared?
The comparison solution is prepared by mixing lignosulfonate at 1 g total solids per 2000 mL with a tanning material free from lignosulfonate, adjusting the mixture to analytical strength (4.0 g tannin per L).
⚡ What constitutes a positive result?
A positive result is indicated when, after 15 minutes, the specimen retains at least as much precipitate as the comparison solution under identical treatment conditions.
📌 Why is this test important for tanning extracts?
This test detects adulteration with cheap lignosulfonates, ensuring the quality and integrity of tanning extracts used in leather production.