Determination of Color in Petroleum Products
The color of petroleum products is an important quality indicator, as it can provide insight into refining consistency, contamination, and aging.
1️⃣ Test Standards for Color Determination
✔ ASTM D1500 – Standard Test Method for Petroleum Products by the ASTM Color Scale
✔ ISO 2049 – Determination of Color of Petroleum Products
✔ ASTM D156 – Saybolt Color of Petroleum Products (for lighter products)
✔ ASTM D6045 – Color Determination by Spectrophotometry
2️⃣ Selection of Color Test Method
| Test Method | Best for | Color Range |
|---|---|---|
| ASTM D1500 (ASTM Color Scale) | Lubricants, base oils, diesel fuel | 0.5 to 8.0 (Dark oils) |
| ISO 2049 | Crude oil, petroleum distillates | Similar to ASTM D1500 |
| ASTM D156 (Saybolt Color) | Kerosene, aviation fuel, light oils | +30 (clear) to -16 (dark) |
| ASTM D6045 (Spectrophotometric) | Precise digital measurement | Matches ASTM D1500 & D156 |
3️⃣ ASTM D1500 – Color Determination Method
🔹 Principle
Petroleum products are visually compared to standard glass color discs or measured digitally using a colorimeter.
🔹 Apparatus & Equipment
✅ ASTM D1500 Color Comparator – Visual comparison using color discs
✅ Colorimeter / Spectrophotometer – Automated digital measurement (ASTM D6045)
✅ Glass Sample Cell – 25mm or 33mm diameter
✅ Light Source – White light for accurate viewing
🔹 Test Procedure
- Sample Preparation: Ensure the sample is clear and free of bubbles.
- Comparison with Color Discs: View the sample against standard ASTM color discs under consistent lighting.
- Digital Measurement (ASTM D6045): Insert sample into a spectrophotometer for precise color reading.
- Record the Closest Match: Report the result based on ASTM color scale (0.5 – 8.0).
4️⃣ Interpretation of Results
🔹 Lighter Color (0.5 – 3.0) – Indicates high purity, low oxidation, and minimal contamination.
🔹 Darker Color (4.0 – 8.0) – Suggests aging, oxidation, contamination, or different crude oil sources.
Typical Color Ranges for Petroleum Products
| Product | Color Range |
|---|---|
| Gasoline | Saybolt +25 to +30 (very clear) |
| Kerosene | Saybolt +15 to +30 |
| Diesel Fuel | ASTM 1.0 to 3.5 |
| Lubricating Oil | ASTM 1.5 to 8.0 |
| Crude Oil | ASTM 4.0 to 8.0+ |
5️⃣ Alternative Methods for Color Measurement
| Test | Best for |
|---|---|
| Saybolt Color (ASTM D156) | Lighter petroleum products |
| Spectrophotometric Color (ASTM D6045) | High-precision automated color analysis |
Best Test Method for Color Determination in Lubricating Oils
For lubricating oils, color determination is essential to assess purity, aging, contamination, and oxidation levels.
1️⃣ Recommended Test Methods
| Test Method | Standard | Best for | Color Range |
|---|---|---|---|
| ASTM D1500 (ASTM Color Scale) | ASTM D1500 / ISO 2049 | Lubricating oils, base oils, diesel fuel | 0.5 to 8.0 (Dark oils) |
| Spectrophotometric Method | ASTM D6045 | Digital measurement of oil color | Matches ASTM D1500 |
| Saybolt Color Test | ASTM D156 | Light-colored lubricants, base oils | +30 (clear) to -16 (dark) |
📌 For most lubricants, ASTM D1500 is the standard method used.
2️⃣ ASTM D1500 – Procedure for Lubricating Oils
🔹 Principle
The lubricating oil sample is visually compared to standard ASTM color discs or measured digitally using a colorimeter.
🔹 Required Equipment
✅ ASTM D1500 Color Comparator – Manual visual comparison using color discs
✅ Colorimeter / Spectrophotometer – For automated digital measurement
✅ Glass Sample Cell – 25mm or 33mm diameter
✅ Consistent Light Source – White light for accurate viewing
🔹 Test Steps
1️⃣ Prepare Sample – Ensure the oil is clear and free from bubbles.
2️⃣ Visual Comparison – Compare the sample against standard ASTM color discs.
3️⃣ Digital Measurement (ASTM D6045) – Place the sample in a colorimeter for precise reading.
4️⃣ Record & Report – Report the closest ASTM color number (0.5 – 8.0).
3️⃣ Interpretation of Results for Lubricating Oils
| ASTM Color Number | Lubricant Condition / Quality |
|---|---|
| 0.5 – 2.0 | Fresh, refined base oils, light engine oils |
| 2.5 – 4.0 | Mid-range lubricants, slight oxidation |
| 4.5 – 6.0 | Used lubricating oil, increased oxidation |
| 6.5 – 8.0 | Highly oxidized or contaminated oil |
✔ Lighter color (0.5 – 2.0) = High purity, fresh oil
❌ Darker color (4.0 – 8.0) = Possible contamination, oxidation, or aging
4️⃣ Alternative Methods for Lubricating Oils
| Test | Best for |
|---|---|
| ASTM D1500 (Visual Comparator) | Manual oil color grading |
| ASTM D6045 (Spectrophotometric) | Precise digital measurement |
| Saybolt Color (ASTM D156) | Lighter base oils, white mineral oils |
📌 For dark oils (heavily used or oxidized), use spectrophotometry (ASTM D6045) for more accuracy.