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D5558-95 – Standard Test Method Technical Guide
🔬 Scope and Significance
This test method, ASTM D5558-95 (Reapproved 2023), covers the determination of the saponification value of fats and oils. It is specifically intended for quality assurance in the manufacture of fat liquors for leather processing. The saponification value represents the milligrams of potassium hydroxide required to saponify one gram of the sample, providing an index of the average molecular weight of the constituent triglycerides. All values in this standard are stated in SI units.
🧪 Required Apparatus and Reagents
The standard calls for specific equipment to ensure reliable results. Alkali-resistant Erlenmeyer flasks (250 mL or 300 mL) and air condensers with a minimum length of 650 mm are required. A water bath or hot plate with variable heat control serves as the heat source.
The key reagents are accurately standardized 0.5 N hydrochloric acid and freshly prepared alcoholic potassium hydroxide. The alcoholic KOH is prepared by distilling 95% ethyl alcohol and dissolving 40 g of low-carbonate KOH per liter, keeping the temperature strictly below 15.5 °C (60 °F).
| 🟦 Component | 📏 Specification |
|---|---|
| Reaction Flask | 250 mL or 300 mL, alkali-resistant (Corning, Kimble, or equivalent) |
| Condenser | Air condenser, minimum 650 mm length |
| Heat Source | Water bath or hot plate with variable heat control |
| Acid Titrant | Hydrochloric acid, 0.5 N, accurately standardized |
| Base Reagent | Alcoholic KOH (40 g/L KOH in distilled, 95% ethyl alcohol) |
💡 Reagent Preparation Tip: To ensure a clear, stable solution, the alcoholic KOH must be prepared by distilling the alcohol with a few grams of KOH prior to the final dissolution. The temperature must be kept below 15.5 °C (60 °F) to prevent darkening and the formation of aldehydes.
⚙️ Test Procedure and Key Calculations
The sample must first be melted, filtered, and thoroughly dried. Accurately weigh 4 g to 5 g of the sample, adjusting the size so the back titration is 45 % to 55 % of the blank. A blank determination is conducted simultaneously. Exactly 50 mL of the alcoholic KOH is added using a pipette with a consistent drain time.
Attach the air condenser and boil the solution gently but steadily for approximately 1 hour. Ensure the vapor ring does not rise to the top of the condenser to prevent solvent loss. After cooling slightly, wash down the condenser with distilled water. Add approximately 1 mL of indicator and titrate with 0.5 N HCl until the pink color just disappears.
| ⚡ Parameter | 🎯 Requirement |
|---|---|
| Sample Weight | 4 g to 5 g (targeting 45–55% back titration against blank) |
| Reagent Volume | 50 mL alcoholic KOH |
| Reaction Time | ~1 hour (may require longer for difficult samples) |
| End Point | Pink color of the indicator just disappears © 2026 TNLab — This article is a technical interpretation for reference only. The original standard as published by ASTM International takes precedence. 📥 Standard Documents Download |