⛽ Sulfated ash of oils
Determination of sulfated ash content in lubricating oils with metalorganic additives. Measure of metal content (Ca, Mg, Zn, Ba) in additive packages.
Overview
Sulfated ash is the inorganic (mineral) residue after burning lubricating oil and treating the residue with sulfuric acid followed by ignition at 775°C. This is an important quality parameter for engine and industrial oils because it represents the content of metalorganic additive packages — primarily detergents (Ca, Mg compounds), dispersants, and antioxidants (Zn compounds — ZDDP).
Sulfated ash content is crucial for oil compatibility with exhaust aftertreatment systems: DPF/FAP filters (diesel engines) and TWC catalysts (gasoline engines). Too high sulfated ash leads to faster clogging of particulate filters and catalyst poisoning. Therefore, modern engine oil specifications (ACEA C1–C5, API SP) introduced sulfated ash limits — e.g., ACEA C1 requires ≤0.5% m/m, and ACEA C3 allows ≤0.8% m/m.
The ISO 3987 method covers the range from 0.005% to 25% m/m and applies to unused lubricating oils with additive packages and to additive concentrates themselves. It is not suitable for used oils containing lead, nor for base oils without additives (for these, ISO 6245 — ordinary ash is used).
Test results are expressed as mass percent (% m/m) of sulfated ash. Typical values: conventional engine oil 0.8–1.5%, Low-SAPS oil (ACEA C) 0.5–0.8%, base oil without additives <0.005%.
Method principle
A weighed oil sample is burned in a platinum or porcelain crucible until complete charring. The carbon residue is moistened with sulfuric acid (H2SO4), which reacts with metal oxides forming sulfates. Then the crucible is ignited in a muffle furnace at 775°C until complete carbon combustion and constant mass is obtained. Metal sulfates (CaSO4, MgSO4, ZnSO4, BaSO4) constitute the sulfated ash. The process of sulfuric acid treatment and ignition is repeated until constant mass is achieved (difference between successive weighings ≤0.5 mg).
Applications
- Quality control of engine oils — ACEA C1–C5, API SP/SN specifications
- Testing metalorganic additive content in oils
- Production control of Low-SAPS oils (low sulfated ash, sulfur, phosphorus)
- Assessment of oil compatibility with DPF/FAP filters and catalysts
- Quality control of additive package concentrates
- Monitoring oil quality in service (ash increase = degradation)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.005–25% m/m |
| Ignition temperature | 775 ± 25°C |
| ACEA C1 limit | ≤0.5% m/m |
| ACEA C3 limit | ≤0.8% m/m |
| Typical engine oil | 0.8–1.5% m/m |
| Repeatability | 0.016 × X (X = result in %) |
Standard
- Standard number
- PN-EN ISO 3987:2010
- Title (PL)
- Przetwory naftowe — Oznaczanie popiolu siarczanowego w olejach smarowych i dodatkach uszlachetniajacych
- Title (EN)
- Petroleum products — Determination of sulfated ash in lubricating oils and additives
Step-by-step procedure
1. Crucible preparation
Ignite crucible in muffle furnace (775°C, 30 min). Cool in desiccator. Weigh with 0.1 mg accuracy.
2. Sample weighing
Weigh oil sample into crucible: 5–10 g for oils with expected ash <1%, 1–5 g for higher contents. Record mass with 0.1 mg accuracy.
3. Preliminary burning
Ignite sample with burner or on hot plate. Burn oil carefully, avoiding spattering. Continue until charring.
4. Sulfuric acid treatment
After cooling, add a few drops of water, then 1–2 mL concentrated sulfuric acid. Carefully heat until smoke ceases.
5. Ignition in muffle furnace
Place crucible in muffle furnace heated to 775°C. Ignite for 30 min. Cool in desiccator to room temperature.
6. Repeated H2SO4 treatment
Weigh crucible. If result is unstable: add H2SO4 again and repeat ignition (775°C, 30 min). Continue until constant mass (±0.5 mg).
7. Final weighing
After obtaining constant mass, weigh crucible with ash on analytical balance. Record final mass.
8. Blank test
Perform blank test — add 1 mL H2SO4 to empty crucible, ignite as above. Weigh residue (correction).
9. Result calculation
Sulfated ash [% m/m] = [(m3 - m1) - B] × 100 / m_sample, where m3 = crucible mass with ash, m1 = empty crucible mass, B = blank value.
10. Reporting
Report result in % m/m with 0.001% accuracy (for values <0.1%) or 0.01% (for values >0.1%). Compare with oil specification requirements.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Muffle furnace (775°C) | Nabertherm L 3/11, Carbolite CWF 1200, POL-EKO SLF 50 | 8,000–25,000 PLN |
| Platinum or porcelain crucibles | Platinum crucible 50 mL (best accuracy) or porcelain | 5,000–15,000 PLN (Pt) / 50–200 PLN (porcelain) |
| Analytical balance (d = 0.1 mg) | Mettler Toledo XPR205, Sartorius Secura 225D | 8,000–25,000 PLN |
| Gas burner or hot plate | Bunsen / Teclu burner or hot plate for preliminary burning | 200–2,000 PLN |
| Desiccator without desiccant | Glass desiccator for cooling crucibles after ignition | 300–800 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Concentrated sulfuric acid (95–98%) | 7664-93-9 | For treating residue after burning, analytical grade |
| Distilled water | 7732-18-5 | For moistening residue before adding sulfuric acid |
| Ethanol or methanol | 64-17-5 | For moistening sample before burning (facilitates even burning) |
Health and safety (OHS)
- Concentrated sulfuric acid — strongly corrosive (H314), chemical burns, work under fume hood, acid-resistant gloves
- Muffle furnace 775°C — extremely hot surface, use tongs and heat-resistant gloves
- Oil burning — flame and smoke, work under fume hood or in fume cupboard
- Platinum crucibles — do not use with hydrochloric acid or metallic sodium (Pt corrosion)
- Safety glasses, laboratory coat and gloves mandatory
- Oil fire extinguishing — sand or powder extinguisher, NEVER water