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D2081-92 – Standard Test Method Technical Guide
This standard, jointly developed by ASTM and the American Oil Chemists’ Society (AOCS), establishes a procedure for determining the pH of fatty quaternary ammonium chlorides through the preparation and measurement of a 5% solution.
🧪 Reagents and Equipment Specifications
The standard mandates the use of specific high-purity reagents and calibrated equipment to ensure reproducible results.
| 🟦 Item | 📐 Specification |
|---|---|
| pH Meter | Glass electrode system conforming to Test Method E 70. Must be standardized at pH 7.0. |
| Magnetic Stirrer | Equipped with an inert, plastic-coated stirring bar to avoid contamination. |
| Reagent Water | Must conform to Specification D 1193, Type II for purity. |
| Solvent Solution | Isopropyl Alcohol (1 + 1): Prepared by mixing equal volumes of 99% isopropyl alcohol and water. |
⚙️ Test Procedure and Sample Preparation
The test requires precise calculation of the specimen mass based on its nonvolatile solids content to create an exact 5% test solution.
First, determine the percent nonvolatile matter using Test Method D 2079. The required sample weight for a 200 g solution is calculated using the following equation from Section 6.2:
Grams of sample = (200 × 5) / % Nonvolatile Matter
Melt the sample if it is not liquid, mix thoroughly, and weigh the calculated amount into a 250-mL low-form beaker. Dissolve the specimen in 100 g of isopropyl alcohol solution, warming as needed. Cool the solution to room temperature and add additional solvent to bring the total mass to exactly 200 g.
| 📏 Parameter | 🎯 Requirement |
|---|---|
| Total Solution Mass | 200 g |
| Solution Concentration | 5% based on nonvolatile (solid) matter |
| Electrode Immersion | Lower half of each electrode submerged |
| Stirring Condition | Vigorous agitation without spattering |
| Reading Criteria | Record pH when instrument displays a stable reading |
📊 Reporting and Critical Notes
Per Section 7, the final pH value must be reported to the nearest 0.1 pH unit. Several important factors regarding precision and sample chemistry must be considered when interpreting results.
⚠️ Precision and Bias: According to Section 8 of the standard, formal precision and bias statements were not established at the time of publication. Results, particularly across different laboratories, should be interpreted with an understanding of this limitation.
💡 Sample Stability: Note 1 highlights that dissolved methyl chloride in commercial products can hydrolyze to form hydrochloric acid. This process gradually decreases the pH of the material during storage and can cause significant discrepancies between results obtained at different times or in different laboratories.
❓ Frequently Asked Questions
🔍 Why is the test performed on a 5% solution?
© 2026 TNLab — This article is a technical interpretation for reference only. The original standard as published by ASTM International takes precedence.