⛽ Oxidation stability of diesel fuel
Assessment of diesel fuel resistance to oxidation by 16-hour oxygen bubbling at 95°C and determination of insoluble sediment amount.
Overview
Oxidation stability is a key quality parameter of diesel fuel and other middle distillates, which determines their resistance to chemical degradation under oxygen influence. Fuel oxidation leads to formation of resins, sediments and organic acids, which clog fuel filters, cause wear of injection elements and corrosion of storage tanks.
The method according to ISO 12205 is an accelerated test — it simulates a one-year storage period of diesel fuel by bubbling oxygen through the fuel sample at 95°C for 16 hours. After aging, two types of sediments are determined: insoluble filterable (retained on membrane filter) and insoluble adherent (deposited on glass vessel and cooler walls, washed out with three-component solvent). The sum of both fractions constitutes total insoluble sediments.
According to PN-EN 590 standard, the maximum permissible amount of total insoluble sediments is 25 g/m³. Fuels with FAME addition (biodiesel) typically show lower oxidation stability due to the presence of unsaturated double bonds in esters, which accelerates autoxidation processes.
The test is mandatory in diesel fuel quality certificates and is an important element of quality control at every stage of the logistics chain — from refinery, through storage depots, to fuel stations.
Method principle
350 mL of filtered diesel fuel is placed in an oxidation tube and heated to 95°C ± 1°C in a heating block. Oxygen is bubbled through the sample at a flow rate of 3 L/h for 16 hours. A reflux condenser prevents loss of volatile fractions. After aging, the sample is cooled, filtered through a 0.8 µm pore membrane (filtered sediments) and vessels are washed with three-component solvent (toluene/acetone/methanol) to collect adherent sediments. After solvent evaporation, both types of sediments are weighed and summed.
Applications
- Diesel fuel quality certificate (PN-EN 590) — limit 25 g/m³
- Oxidation stability assessment of diesel/FAME blends (B7, B10, B20)
- Quality control of fuels at storage depots
- Testing the effect of antioxidant additives on fuel durability
- Predicting storage durability of fuels for strategic reserves
- Compatibility assessment of fuel blends from different sources
- R&D research on new fuel formulations
Key parameters
| Parameter | Value |
|---|---|
| Oxidation temperature | 95°C ± 1°C |
| Oxidation time | 16 h |
| Oxygen flow | 3 L/h |
| Sample volume | 350 mL (after filtration) |
| PN-EN 590 limit | ≤25 g/m³ total sediments |
| Repeatability | 8 g/m³ (at 25 g/m³ level) |
Standard
- Standard number
- PN-EN ISO 12205:2006
- Title (PL)
- Przetwory naftowe — Oznaczanie stabilności oksydacyjnej średnich destylatów naftowych
- Title (EN)
- Petroleum products — Determination of the oxidation stability of middle-distillate fuels
Step-by-step procedure
1. Sample preparation
Filter diesel fuel sample through 0.8 µm membrane to remove solid contaminants. Measure 350 mL of filtered fuel into glass tube.
2. Apparatus preparation
Wash tubes and condensers with three-component solvent, then acetone. Dry. Mount tube with sample in heating block.
3. Condenser and oxygen connection
Mount reflux condenser on tube. Connect oxygen line with glass tube reaching bottom of tube. Set O₂ flow to 3 L/h.
4. Heating to 95°C
Set block temperature to 95°C ± 1°C. Wait for sample to reach set temperature.
5. Oxidation — 16 hours
Start time counting from reaching 95°C. Bubble oxygen continuously for 16 h. Monitor flow and temperature.
6. Sample cooling
After 16 h turn off heating and oxygen supply. Cool sample to room temperature without dismantling system.
7. Filtration — filtered sediments
Filter cooled sample through pre-weighed 0.8 µm membrane under vacuum. Rinse filtrate with n-heptane.
8. Vessel washing — adherent sediments
Wash tube, condenser and tubes with three-component solvent (toluene:acetone:methanol 1:1:1). Collect rinsings into weighing vessel.
9. Drying and weighing
Dry membrane and vessel with rinsings in oven (110°C). Cool in desiccator. Weigh to 0.1 mg accuracy.
10. Result calculation
Filtered sediments [g/m³] = (sediment mass on filter / 0.350) × 1000. Adherent sediments — similarly. Total = sum of both fractions.
11. Result evaluation
Compare result with PN-EN 590 limit (≤25 g/m³). For duplicates check repeatability. Round result to 1 g/m³.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Oxidation stability apparatus | Koehler K12100/K12200 (4–8 positions), Normalab OSFM 436, Stanhope-Seta 15450-2 | 40 000–90 000 PLN |
| Reflux condenser | Glass condenser matched to apparatus, cooling water | included with apparatus |
| Oxygen delivery system | Medical O₂ cylinder 5.0, regulator, rotameter 3 L/h | 500–2 000 PLN + cylinder |
| Vacuum filtration set | Suction flask, Büchner funnel, 0.8 µm membranes (Millipore, Whatman) | 2 000–5 000 PLN |
| Analytical balance (d=0.1 mg) | Mettler Toledo ME204, Sartorius Secura 224 | 5 000–15 000 PLN |
| Laboratory oven | Binder ED 56, Memmert UF55 | 5 000–12 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Oxygen gas (purity ≥99.5%) | 7782-44-7 | For bubbling through sample, cylinder with regulator and rotameter |
| Toluene | 108-88-3 | Analytical grade, component of three-component solvent for washing vessels |
| Acetone | 67-64-1 | Analytical grade, component of three-component solvent |
| Methanol | 67-56-1 | Analytical grade, component of three-component solvent (proportions 1:1:1) |
| Filter membranes 0.8 µm | — | Cellulose or PTFE membranes, Ø 47 mm, for sediment filtration |
| n-Heptane | 142-82-5 | For preliminary rinsing of filters and vessels from fuel |
Health and safety (OHS)
- Oxygen — supports combustion, do not use near flammable materials and open flame
- Temperature 95°C — burn risk, use heat-resistant gloves
- Toluene, acetone, methanol — flammable and toxic, work under fume hood
- Diesel fuel — flammable, avoid ignition sources
- N-heptane — highly flammable and irritating, use in digestorium
- Safety glasses, nitrile gloves and coat mandatory