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D4046-14 – Standard Test Method Technical Guide
📜 Overview and Scope
ASTM D4046-14 is a spectrophotometric test method for determining alkyl nitrate content in diesel fuels, specifically covering concentrations from 0.03% to 0.30% by volume. Originally approved in 1981 and reapproved in 2019, this standard is under the jurisdiction of ASTM Committee D02. The method applies to any alkyl nitrate such as amyl, hexyl, or octyl nitrate, provided the calibration standards contain the same alkyl nitrate ester as the test specimens. Values are stated in SI units, and the test is essential for assessing cetane number improvers and verifying fuel specification compliance.
⚗️ Test Procedure and Equipment Specifications
The determination involves hydrolysis of the alkyl nitrate in 65% sulfuric acid, liberating nitric acid which nitrates m-xylenol. The resulting nitroxylenol is extracted from the reaction mixture and reacted with sodium hydroxide to form a yellow salt. Absorbance is measured spectrophotometrically at 452 nm, and the alkyl nitrate concentration is determined from the slope of a calibration curve. The following table lists critical apparatus required with specified tolerances:
| 🟦 Apparatus | 📏 Requirement | 🎯 Tolerance |
|---|---|---|
| Absorption Cells | 1.000 cm path length | ±0.002 cm |
| Separatory Funnel | 125 mL, Squibb-type | TFE-fluorocarbon stopcock |
| Shaking Machine | 250 oscillations/min | Automatic for multiple samples |
| Spectrophotometer | 452 nm measurement | ±5 nm |
| Volumetric Flasks | 10 mL and 100 mL | Standard |
| Measuring Pipets | 1 mL, 3 mL, 4 mL, 10 mL | Standard |
| Buret | 10 mL capacity | Standard |
All chemicals must be reagent grade conforming to ACS specifications. Reagent water shall meet ASTM D1193 requirements. The sample is shaken at 250 oscillations per minute for consistent extraction. Note that other nitrate esters, inorganic nitrate ions, and nitrogen oxides interfere with the test method and must be absent.
🟦 Calibration Note: Use the same alkyl nitrate ester in both calibration standards and test specimens. This ensures identical hydrolysis and nitration rates, providing accurate quantification.
⚡ Safety Note: The procedure involves corrosive 65% sulfuric acid and organic solvents. Review Section 7 of the standard for specific hazard statements and use appropriate protective equipment.
❓ Frequently Asked Questions
🔍 What concentration range does this test method cover?
D4046-14 is applicable for alkyl nitrate concentrations from 0.03% to 0.30% by volume in diesel fuels.
💡 Which compounds cause interference in the determination?
Interferences include other nitrate esters, inorganic nitrate ions, and nitrogen oxides. These must be absent from the test sample for valid results.
⚡ At what wavelength is the absorbance measured?
The yellow salt formed from the nitroxylenol is measured spectrophotometrically at 452 nm ± 5 nm.
📌 Why must the calibration standard contain the same alkyl nitrate as the sample?
Different alkyl nitrates exhibit varying reaction rates. Matching the specific ester in the calibration ensures the hydrolysis was equivalent and the concentration determination is accurate.