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D5555-95 – Standard Test Method Technical Guide
📐 Scope, Significance, and Apparatus
ASTM D5555-95 (Reapproved 2023) provides a standardized test method for determining the free fatty acid (FFA) content of animal, marine, and vegetable fats and oils intended for use in fat liquors and stuffing compounds. As stated in Section 1, the values in SI units are the standard, and the method fulfills a critical role in quality assurance for the leather industry. The procedure relies on simple volumetric titration and requires access to specific reagents.
| 🟦 Reagent | 📏 Specification per ASTM D5555 |
|---|---|
| Ethyl Alcohol | 95% by volume; must be neutralized to a faint, permanent pink endpoint with alkali just before use. |
| Isopropanol | 99% concentration; permitted as an alternate solvent for crude and refined vegetable oils. |
| Indicator Solution | Phenolphthalein, prepared as a 1% solution in 95% alcohol. |
| Titrant | Sodium hydroxide (NaOH) solution, accurately standardized to a specific normality. |
The apparatus required includes 115 mL or 230 mL oil-sample bottles or standard 250 mL Erlenmeyer flasks (Section 3.1). Accurate weighing equipment is also essential for preparing the sample mass according to the procedure’s sample size table.
⚙️ Test Procedure and Sample Preparation
The sample must be liquid and thoroughly mixed prior to weighing. The exact sample weight, volume of hot neutralized alcohol, and alkali normality are determined by the expected free fatty acid range of the oil, as specified in the standard’s Table 1. The weighed sample is dissolved in the specified volume of hot, neutralized alcohol along with 2 mL of phenolphthalein indicator. The mixture is titrated with vigorous shaking until the first permanent pink color appears, which must persist for 30 seconds (Section 4.3).
| 🎯 Expected FFA Range (%) | ⚖️ Sample Weight (g) | 🧪 Alcohol Volume (mL) | 💧 Alkali Normality (N) |
|---|---|---|---|
| 0.0 to 0.2 | 20.00 ± 0.05 | 50 | 0.1 |
| 0.2 to 1.0 | 10.00 ± 0.02 | 50 | 0.1 |
| 1.0 to 5.0 | 5.00 ± 0.02 | 75 | 0.25 |
| 5.0 to 10.0 | 2.50 ± 0.02 | 75 | 0.25 |
| 10.0 to 50.0 | 1.00 ± 0.01 | 75 | 0.50 |
| 50.0 to 100.0 | 0.50 ± 0.01 | 75 | 1.0 |
💡 Key Procedural Note: The alcohol must be hot during the titration to ensure complete dissolution of the sample and the sharpest possible endpoint. The endpoint color intensity should match the neutralized alcohol blank. Titration must be performed with vigorous shaking.
📊 Calculation, Reporting, and Conversions
Free fatty acid content is calculated using the formula specified in Section 5.1. The result is expressed as a percentage by weight, calculated as % FFA = (mL of alkali × N × 28.2) / (Sample Weight in g). The factor 28.2 corresponds to the equivalent weight of oleic acid. The test report must reference this specific test method (D5555).
The standard also provides a recognized conversion for expressing results. Free fatty acids are frequently reported as Acid Value (AV). Since the % FFA calculated here is expressed as oleic acid, it can be converted: Acid Value = % FFA (as oleic) × 1.99.
⚠️ Precision and Bias Consideration: Section 6 of the standard states that original interlaboratory study data is no longer available to establish formal precision and bias statements. Users of this test method are strongly advised to verify its reproducibility for their specific matrices using appropriate reference materials.
❓ Frequently Asked Questions
🔍 What is the scope of this test method?
This method is specifically designed for determining the free fatty acid content of animal, marine, and vegetable fats and oils used in the formulation of fat liquors and stuffing compounds for the leather industry (Section 1.1).
© 2026 TNLab — This article is a technical interpretation for reference only. The original standard as published by ASTM International takes precedence.