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D4653-87 – Standard Test Method Technical Guide
ASTM D4653-87 (Reapproved 2020) specifies the standard test method for determining total chlorides in leather, particularly mineral tanned leather. The procedure employs a titration technique with silver nitrate to quantify chloride content, essential for quality control and material specification.
🧪 Scope and Significance
This test method applies to mineral tanned leather and measures soluble chlorides. The results are reported in SI units, and the method does not address all safety concerns; users must establish appropriate safety practices.
⚙️ Apparatus and Reagents
The principal apparatus is a 250 mL volumetric flask. The required reagents and their specifications are listed in the table below.
| 🟦 Reagent | 📐 Concentration | 🎯 Function |
|---|---|---|
| Acetic Acid | 5% or 10% by volume | Neutralization |
| Methyl Orange | 0.02% in 50% alcohol | pH indicator |
| Ammonium Hydroxide | 0.1 N | Extraction solvent |
| Silver Nitrate | 0.1 N standard solution | Titrant |
| Potassium Chromate | 5% solution | Endpoint indicator |
📋 Procedure and Calculation
Test specimens of 1 g each are prepared from the composite sample per Practice D2813. Two specimens are tested for accuracy. The detailed procedure is as follows:
- Weigh the specimen to the nearest milligram and record as W.
- Place in a 250 mL volumetric flask with 200 mL of 0.1 N ammonium hydroxide.
- Heat in a boiling water bath for 2 hours, swirling occasionally to ensure thorough wetting.
- Cool, dilute to volume with distilled water, and filter through folded filter paper.
- Discard the first 20–25 mL of filtrate. Pipette 200 mL of the remaining filtrate into a 600 mL beaker.
- Add methyl orange indicator and neutralize with 5% acetic acid to an orange color.
- Add 2–3 mL of 5% potassium chromate solution and titrate with 0.1 N silver nitrate until the color changes. Record the volume as A.
- Perform a blank titration using 200 mL of distilled water with the same indicators. Record the volume as B.
Calculate the percentage of total chlorides using the formula:
Total Chlorides (%) = ((A – B) × N × 0.0355 × (250/200) × 100) / W
Key variables for the calculation are described in the table below.
| 🟦 Symbol | 📏 Description | ⚡ Unit |
|---|---|---|
| A | Volume of standard AgNO₃ for sample titration | mL |
| B | Volume of standard AgNO₃ for blank titration | mL |
| N | Normality of the standard AgNO₃ solution | N |
| W | Weight of the leather specimen | g |
The total chlorides content is reported as the average of the two test specimens.
💡 Experimental Tip: During the boiling water bath immersion, ensure the flask is immersed up to the neck to maintain consistent temperature and complete extraction of chlorides.
⚠️ Safety Precaution: Handle silver nitrate with care as it is corrosive and a strong oxidizer. Use gloves and eye protection. Dispose of waste according to regulatory guidelines.
❓ Frequently Asked Questions
🔍 What types of leather are suitable for this test method?
This test method is specifically intended for mineral tanned leather, as stated in the scope of the standard.
💡 Why is the first portion of filtrate discarded?
Discarding the initial 20–25 mL removes any impurities or fine particles that might have passed through the filter initially, ensuring accurate titration results.
⚡ How is the blank titration used in the calculation?
The blank titration accounts for any chlorides present in the distilled water and reagents, so subtracting its volume (B) from the sample volume (A) isolates the chlorides from the leather.
📌 What does the constant 0.0355 represent in the formula?
This constant is the milliequivalent weight of chloride, used to convert the volume of silver nitrate titrant into grams of chloride for the percentage calculation.