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D1839-14 – Standard Test Method Technical Guide
🔬 Test Method Principle and Scope
ASTM D1839 −14 (Reapproved 2019) provides a standardized procedure for determining the concentration of amyl nitrate in diesel fuels, specifically targeting the range of 0.1 % to 0.5 % by volume. Alkyl nitrates are added to diesel fuels to improve the cetane number, but their presence can lead to erroneous conclusions regarding the fuel’s coke-forming propensity if not properly identified.
The method operates by the simultaneous hydrolysis of the nitrate ester in 62.5 % sulfuric acid and the nitration of m-xylenol by the liberated nitric acid. The resulting nitroxylenol is steam distilled from the reaction mixture and reacted with sodium hydroxide to form a distinct yellow sodium salt. Following the removal of residual diesel fuel through ether extraction, the color intensity of the aqueous solution is measured spectrophotometrically at 452 nm. The concentration of amyl nitrate is calculated by comparing this absorbance to a pre-established standard curve.
💡 Note on Hexyl Nitrate: This test method has been utilized for the determination of hexyl nitrate in diesel fuels. For calculations involving hexyl nitrate, a density of 0.97 g/mL should be used. It is important to note that this specific application has not been validated through cooperative interlaboratory testing per the standard’s current scope.
⚙️ Key Apparatus and Specifications
Precision in this analysis demands specific equipment configurations. The standard references several critical apparatus requirements, detailed below. Proper sampling in accordance with ASTM D4057 is also essential for obtaining representative results.
| 🟦 Component | 📏 Specification | 🎯 Material / Notes |
|---|---|---|
| Absorption Cells | 1.000 ± 0.002 cm path length | Matched pairs are required |
| Round-Bottom Flask | 300 mL capacity | Borosilicate glass |
| Reflux Condenser | 300 mm length | Allihn-type, standard taper joints |
| Lunge Pipet | 2 mL capacity | Alternate: 2.5 mL hypodermic syringe with scabbard |
| Separatory Funnel | 125 mL capacity | Squibb-type |
| Distillate Collector | Sealed-in funnel design | Borosilicate glass, standard taper joints |
| Heating Mantle | Fits 300 mL flask | Must be controlled with a variable transformer |
📊 Procedure, Interferences, and Calculations
The procedure begins with careful hydrolysis and nitration in a 300 mL round-bottom flask fitted with the Allihn condenser. After the reaction, the nitroxylenol is distilled into the collector, reacted with sodium hydroxide, and the residual diesel fuel is removed via ether extraction in the separatory funnel. The spectrophotometric measurement is then taken. Reagent water must conform to ASTM D1193 to ensure no background interference.
⚠️ Critical Interferences: The presence of certain substances will yield erroneously high results. Specific interferents include other nitrate esters (e.g., hexyl or octyl nitrate), inorganic nitrate ions (NO₃⁻), and nitrogen oxides (NOₓ). Care must be taken to ensure the purity of the 62.5 % sulfuric acid and all reagents.
| 🟦 Parameter | 📐 Value / Requirement |
|---|---|
| Reaction Acid Concentration | 62.5 % Sulfuric Acid |
| Detection Wavelength | 452 nm |
| Analytical Range | 0.1 % to 0.5 % by volume |
| Hexyl Nitrate Calculation Density | 0.97 g/mL |
| Primary Calibration | Standard curve of absorbance vs. concentration |
The standard emphasizes that this test method is applicable as a basis for judging compliance with specifications covering amyl nitrate in diesel fuels.
❓ Frequently Asked Questions
🔍 What is the specific concentration range covered by ASTM D1839 −14?
The standard is explicitly designed to cover the determination of amyl nitrate in diesel fuels within the range of 0.1 % to 0.5 % by volume.
💡 How is the characteristic yellow color developed in this test?
The method relies on a two-step reaction. First, the amyl nitrate ester is hydrolyzed by 62.5 % sulfuric acid to release nitric acid. This nitric acid instantly nitrates m-xylenol to form nitroxylenol. After distillation, the nitroxylenol reacts with sodium hydroxide to produce a stable yellow sodium salt that is suitable for spectrophotometric analysis.
⚡ At what wavelength is the absorbance of the solution measured?
The absorbance of the yellow sodium salt solution is measured at 452 nm using a spectrophotometer equipped with matched absorption cells having a light path of 1.000 ± 0.002 cm.
📌 What substances are cited as direct interferences with this test method?
Nitrate esters other than the target analyte, inorganic nitrate ions (NO₃⁻), and nitrogen oxides (NOₓ) are all known to interfere. Their presence in the sample or reagents will contribute to a higher colorimetric response, resulting in falsely elevated amyl nitrate readings.