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D1386-15 – Standard Test Method Technical Guide
📐 Scope and Significance of the Acid Number Test
Standard D1386-15 (Reapproved 2022) defines an empirical test method for determining the acid number of synthetic and natural waxes. The acid number, as defined in Section 3.1.1 of the standard, represents the milligrams of potassium hydroxide (KOH) necessary to neutralize 1 g of the sample, providing a direct measure of the free acid content. According to Section 4.1, this property is critical for determining wax utility and serves as a significant quality control parameter.
The method, utilizing an ethanol-xylene mixture, is applicable to all natural waxes including carnauba, as well as oxidized microcrystalline waxes, oxidized Fischer-Tropsch waxes, oxidized polyethylene, and montan esters (Section 1.2). All values are to be regarded as standard in SI units.
💡 Key Definition
Acid Number (or Acid Value): The number of milligrams of potassium hydroxide required to neutralize 1 g of the sample (Section 3.1.1).
⚙️ Test Procedure and Reagent Preparation
Section 7 of the standard details the procedure. Transfer 1 g to 2 g of the sample, weighed to the nearest 0.001 g, into a 250 mL acid-value flask. Add 40 mL of xylene and heat the mixture on a hot plate or water bath until the sample is completely dissolved, using occasional swirling as needed.
Following dissolution, 3 to 5 drops of phenolphthalein indicator solution (10 g/L) are added. The hot solution is immediately titrated with standardized ethanolic potassium hydroxide solution. The endpoint is precisely defined as the first persistent pink color that remains for at least 10 seconds while the flask is swirled vigorously. If wax precipitation occurs during titration, the sample must be reheated. The titration should be carried out as quickly as possible to ensure a sharp endpoint.
| 🟦 Reagent | 📏 Specification and Preparation |
|---|---|
| Ethanolic Potassium Hydroxide | Dissolve approx. 5.6 g KOH in 5.6 g water. Dilute to 1000 mL with 95% ethanol. Standardize with 0.1 N HCl per Practice E200. |
| Phenolphthalein Indicator | Dissolve 1 g of phenolphthalein in 100 mL of denatured or 95% ethanol (10 g/L). |
| Xylene | Reagent-grade, used as the primary solvent (40 mL per sample). |
⚠️ Safety Considerations
Heating xylene and handling hot potassium hydroxide solutions require appropriate safety protocols. As per Section 1.4, it is the responsibility of the user to establish suitable safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
📊 Essential Apparatus Specifications
The accuracy of the acid number determination relies on specific lab equipment as defined in Section 5 of the standard.
| 🟦 Equipment | 📐 Required Specification |
|---|---|
| Analytical Balance | Capable of weighing to the nearest 0.001 g. |
| Buret | 50 mL capacity with 0.1 mL graduations. |
| Acid-Value Flasks | 250 mL capacity for dissolution and titration. |
❓ Frequently Asked Questions
🔍 What exactly is measured by the acid number in D1386-15?
The acid number measures the free acid content in a wax sample. It is defined as the milligrams of potassium hydroxide (KOH) required to neutralize the acidic constituents in exactly one gram of the sample (Section 3.1.1).
💡 Which materials are explicitly covered by this test method?
Standard D1386-15 is applicable to all natural waxes, including carnauba, as well as oxidized microcrystalline waxes, oxidized Fischer-Tropsch waxes, oxidized polyethylene, and montan esters (Section 1.2