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D5559-95 – Standard Test Method Technical Guide
ASTM D5559-95 (Reapproved 2023) specifies the standard titrimetric test method for measuring the acidity, expressed either as free fatty acids (FFA) or as the acid number, in sulfonated and sulfated oils. This method is specifically formulated for quality assurance contexts where ammonium or triethanolamine soaps are absent from the sample matrix.
📐 Scope and Significance
This test method is applicable to the determination of acidity in sulfonated or sulfated oils. It is designed to provide a reliable measure of free fatty acids present in the sample. The method relies on dissolving the sample in an alcohol-ether mixture and titrating with a standard base.
⚠️ Critical Limitation: This standard is not applicable in the presence of ammonium or triethanolamine soaps, salts, or other compounds that do not react neutral to phenolphthalein when dissolved in alcohol. The presence of these substances will invalidate the acidity determination.
The values stated in SI units are to be regarded as standard. Users must establish appropriate safety, health, and environmental practices prior to use.
⚙️ Test Procedure and Reagent Preparation
The procedure requires a precise titration. Accurately weigh 10 g of the sample and dissolve it in an alcohol-ether mixture (50 mL neutral alcohol + 25 mL ether). Add five drops of phenolphthalein indicator solution (10 g/L in 95% ethanol). Titrate the solution with 0.5 N sodium hydroxide (NaOH) or potassium hydroxide (KOH) standard solution until a pink color persists after vigorous shaking.
| 🧪 Reagent | 🟦 Specification / Preparation |
|---|---|
| Alcohol-Ether Mixture | 50 mL of neutral alcohol mixed with 25 mL of ether |
| Phenolphthalein Indicator | 10 g/L (dissolve 1 g in 100 mL of 95 % ethanol) |
| Standard Base Solution | 0.5 N NaOH or KOH (standardized) |
💡 Procedural Note: The endpoint requires a persistent pink color after vigorous shaking. This ensures the complete neutralization of fatty acids within the alcohol-ether solvent system.
📊 Calculation and Key Measured Properties
The acidity is expressed in two ways: the acid number (K) in mg KOH per gram of sample, and the percentage of free fatty acids. The calculations are performed as follows:
Acid Number: K = (C × E) / W
Free Fatty Acids: % FFA = 0.503 × K
| 📐 Variable | 🎯 Definition | ⚡ Unit |
|---|---|---|
| K | Acid Number (mg of KOH per g of sample) | mg KOH/g |
| C | Volume of NaOH or KOH solution required for titration | mL |
| E | Strength of NaOH or KOH solution (mg of KOH equivalent per mL) | mg KOH/mL |
| W | Weight of the sample taken for analysis | g |
| % FFA | Percentage of free fatty acids (0.503 × K) | % |
Precision and bias data for this test method are not currently available. The original interlaboratory test data from the American Leather Chemists Association (ALCA) is no longer accessible. Users are cautioned to verify that the reproducibility of this test method is adequate for their specific intended use, ideally by employing reference materials or conducting in-house validation studies.
⚠️ Important Notice on Reproducibility: As stated in Section 6, the interlaboratory precision data is no longer available. Validation of the method’s suitability for the user’s specific materials and conditions is strongly recommended.
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D5559?
The primary purpose is to determine the acidity, as free fatty acids or acid number, in sulfonated and sulfated oils for quality assurance, specifically in the absence of ammonium or triethanolamine soaps.
💡 Why is the indicator solution prepared at 10 g/L?
The 10 g/L concentration of phenolphthalein in 95% ethanol provides a sufficiently strong color change to detect the endpoint in the mixed alcohol-ether solvent, ensuring the persistent pink color is observable against the sample matrix.
⚡ What titrant strength is specified in the procedure?
The standard specifies a 0.5 N sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution for the titration of the 10 g sample.
📌 How is the free fatty acid percentage calculated?
First, the acid number K is calculated as (C × E) / W. The percent free fatty acids is then calculated by multiplying the acid number by the constant factor 0.503.