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D4655-95 – Standard Test Method Technical Guide
ASTM D4655-95 (Reapproved 2023) provides standardized test methods for determining total, neutral, and combined acid sulfates in mineral-tanned leather. These methods are critical for evaluating leather basicity when used in conjunction with Test Method D4654. The gravimetric analysis involves precise reagent preparation and controlled digestion to ensure accuracy and repeatability.
📜 Test Scope and Significance
These test methods are intended for use in determining the total, neutral, and combined acid sulfate in mineral-tanned leather. They are essential for the leather industry to assess basicity, which influences leather properties such as durability and flexibility. The procedures follow SI units and emphasize reliability through testing two specimens per determination.
⚙️ Procedure for Total Sulfates
Weigh a 1 g specimen to the nearest milligram and record as W1. Transfer to a 250 mL volumetric flask and add 200 mL of 0.1 N ammonium hydroxide or an alternative phosphate solution. Heat the flask in a boiling water bath for 2 hours, then cool to room temperature. Dilute to volume with distilled water, shake, and immediately filter through folded filter paper. Discard the first 20-25 mL of filtrate, then pipette 200 mL into a 600 mL beaker. Add 20 mL of 1.5 N hydrochloric acid, heat to boiling, and add 20 mL of 1% barium chloride dropwise with stirring. Cover and keep in a warm place for at least 2 hours, preferably overnight. Filter the precipitate through ashless filter paper, wash with hot water until chloride-free, and ignite in a weighed crucible.
🧪 Essential Reagents and Apparatus
The table below summarizes key reagents. Use reagent-grade chemicals and distilled water for all solutions.
| 🟦 Reagent | 📏 Concentration | 📐 Preparation |
|---|---|---|
| Ammonium Hydroxide | 0.1 N | 7 mL/L concentrate NH4OH |
| Hydrochloric Acid | 1.5 N | 125 mL/L concentrate HCl |
| Barium Chloride | 1% | BaCl2·2H2O |
| Sodium Hydroxide | 0.01 N | 0.4 g/L |
| Mixed Indicator | — | 60 mL 0.1% methyl red + 40 mL 0.1% methylene blue in 95% alcohol |
Apparatus includes a 250 mL volumetric flask, ashless fine-grained filter paper, porcelain crucible, and optionally a Gooch crucible. Proper filtration and digestion setup are crucial.
💡 Tip: Digest the barium sulfate precipitate overnight for complete recovery, minimizing solubility losses and ensuring accuracy.
⚠️ Warning: Hydrochloric acid and ammonium hydroxide are corrosive. Use in a fume hood with appropriate PPE.
| 🟦 Step | ⚡ Action | 🎯 Specification |
|---|---|---|
| 1 | Weigh specimen | 1 g, record as W1 |
| 2 | Extract with NH4OH | 200 mL, heat 2 h at boiling |
| 3 | Filter and discard | First 20-25 mL |
| 4 | Precipitate sulfate | Add 20 mL 1.5 N HCl, then 20 mL 1% BaCl2 |
| 5 | Digest precipitate | ≥2 h or overnight |
| 6 | Filter and wash | Until chloride-free |
❓ Frequently Asked Questions
🔍 What is the primary purpose of these test methods?
To determine total, neutral, and combined acid sulfate in mineral-tanned leather, primarily for basicity calculation per Test Method D4654.
💡 How many specimens are required per determination?
Two specimens from the composite sample are tested, each weighing 1 g, following Practice D2813 for sampling.
⚡ Why is the first portion of filtrate discarded?
The initial 20-25 mL may contain impurities from the filter paper, so discarding it ensures a clean sample for accurate analysis.
📌 What alternative extraction solutions can be used?
Instead of 0.1 N NH4OH, use 0.1 M KH2PO4 (13.6 g/L) or 0.1 M NaH2PO4·H2O (13.8 g/L).