Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
API MPMS 10.2 (2013): Standard Test Method for Water in Crude Oil by Distillation
Essential requirements, apparatus specifications, and compliance guidance for accurate water content determination in petroleum crude oil
1. Scope and Application
API MPMS 10.2 (2013), formally titled Standard Test Method for Water in Crude Oil by Distillation, is a key component of the American Petroleum Institute’s Manual of Petroleum Measurement Standards. It specifies a distillation procedure for determining the water content in crude oil and is technically equivalent to ASTM D4006-11. This method is applicable to crude oils with water contents ranging from 0.02 % to 25 % by volume. It is widely used in custody transfer operations, refinery feedstock evaluation, and pipeline quality assurance. The test relies on the co-distillation of water with a high-boiling aromatic solvent, followed by condensation and volumetric measurement of the separated water in a calibrated trap.
2. Technical Requirements and Apparatus
2.1 Apparatus Specifications
The standard mandates the use of specific glassware to ensure reproducibility and accuracy. The principal components are a round-bottom distillation flask (typically 500 mL or 1000 mL), a heating device with proportional temperature control, a Liebig or Graham condenser, and a graduated water trap. The trap must be graduated in 0.02 mL increments for capacities up to 2 mL, and in 0.1 mL increments for larger traps. All joints must be standard-taper glass to allow tight sealing. The apparatus must be thoroughly cleaned and dried before each test to avoid contamination.
| Component | Requirement | Remarks |
|---|---|---|
| Distillation flask | Round-bottom, 500 mL or 1000 mL | Borosilicate glass |
| Water trap (Dean–Stark) | Graduated, 0–2 mL or 0–10 mL | 0.02 mL graduation for ≤2 mL traps |
| Condenser | Liebig or Graham (water-cooled) | Effective cooling to room temperature |
| Heater | Electric mantle or hot plate with variable transformer | Must control distillation rate |
| Solvent | Xylene (or toluene with care) | Boiling point 137–140 °C |
2.2 Reagents and Solvent Purity
The standard specifies a high-boiling aromatic solvent, typically xylene, to carry the water steam-distilling from the sample. The solvent must be free of water; a blank distillation is required before use to verify dryness. The sample portion should weigh approximately 100 g (or 50 g if the expected water content is high). The solvent volume is usually 200 mL for a 500 mL flask. The distillation rate must be controlled to 2–5 drops per second during the main distillation to ensure complete water removal without excessive bumping.
3. Implementation Highlights
Tip: Use a fresh, dry trap and rinse all glassware with the solvent immediately before assembly. This prevents residual water from biasing the reading.
The procedure begins by assembling the apparatus with a layer of absorbent cotton or glass wool around the condenser to stabilize temperature. The sample is weighed into the flask, solvent is added, and the flask is heated. The distillate vapor rises to the condenser, and condensed liquid collects in the trap. Water settles in the graduated portion due to density differences. Distillation continues until no further water is collected (typically 60–90 minutes). The volume of water in the trap is read after the trap cools to room temperature. The water content is calculated as a percentage by volume (or mass) relative to the original sample volume.
Accuracy Note: When performed correctly, the method yields a repeatability of 0.02 % (absolute) for water contents up to 1 % and a reproducibility of 0.04 % across laboratories. This is essential for custody transfer agreement.
3.1 Interferences and Mitigation
Emulsified water is recovered efficiently by this method, as the solvent breaks the emulsion. However, very viscous crude oils may require dilution with additional solvent. The standard advises against using toluene as a solvent if the sample contains significant amounts of light hydrocarbons, because toluene may co-distill and affect the trap reading. Xylene is preferred for most crude oil matrices.
Warning: Water droplets adhering to the condenser wall or the trap neck must be dislodged with a wire or Teflon scraper. Failure to do so results in low bias readings. The standard provides a procedure for dislodging droplets using a bent wire.
Safety Warning: The solvent (xylene/toluene) and the sample are flammable. All work must be performed in a fume hood. The flask must be vented through the condenser to avoid pressure buildup. Never heat a sealed system.
4. Compliance and Quality Assurance
Compliance with API MPMS 10.2 (2013) is mandatory for laboratories that require API–IP–ASTM accreditation for water determination in crude oil. Key compliance points include:
- Initial qualification of the apparatus: The trap must be calibrated by the laboratory or certified by the manufacturer. A verification test with a known water standard (e.g., a reference emulsion) must be performed periodically.
- Operator training: The standard states that the analyst must have demonstrated ability to achieve the precision limits of the method. Proficiency testing records should be maintained.
- Solvent blank: A blank distillation must be performed for each new lot of solvent. The measured blank water volume must be less than 0.02 mL; if greater, the solvent must be rejected or re-dried.
- Sample handling: Samples must be mixed thoroughly at a temperature above the pour point to assure homogeneity, and the test must be started within 30 minutes of mixing to avoid stratification.
- Record keeping: All raw data, including sample mass, water trap reading, ambient temperature, and any corrective actions taken, should be recorded in a bound logbook or electronic system.
The standard is reaffirmed regularly. API MPMS 10.2 was designated as a national standard for water in crude oil under the U.S. Petroleum Measurement Laws. The 2013 edition includes clarifications on the use of electrically heated mantles and updates to the precision and bias statements based on an interlaboratory study involving 10 laboratories. Laboratories using this method must comply with the latest edition or the edition referenced in the applicable regulatory framework.
This guide is prepared for informational purposes. The complete document, API MPMS Chapter 10.2 (2013), should be consulted before conducting any testing. Review cycle reaffirmed as of 2026.
Q: What is the principle of API MPMS 10.2 for determining water in crude oil?
A: The method employs co-distillation with a water-immiscible solvent (xylene). Water from the crude oil is carried over by the solvent vapor, condensed, and collected in a graduated trap. The water volume is measured directly and expressed as a percentage of the sample volume.
A: The method employs co-distillation with a water-immiscible solvent (xylene). Water from the crude oil is carried over by the solvent vapor, condensed, and collected in a graduated trap. The water volume is measured directly and expressed as a percentage of the sample volume.
Q: Can this method be used for crude oils containing emulsified water?
A: Yes. The solvent breaks the emulsion, allowing the water to be distilled and measured. The method is effective for water contents up to 25 % by volume. For higher water contents, the sample size may be reduced or the sample can be diluted with additional solvent.
A: Yes. The solvent breaks the emulsion, allowing the water to be distilled and measured. The method is effective for water contents up to 25 % by volume. For higher water contents, the sample size may be reduced or the sample can be diluted with additional solvent.
Q: What are the main sources of error in this distillation test?
A: Common errors include incomplete dislodgement of water from the condenser, inadequate drying of the solvent (high blank), and improper distillation rate leading to water carry-over or incomplete condensation. Using a trap with incorrect graduations or a flask that is too small for the sample volume can also affect accuracy.
A: Common errors include incomplete dislodgement of water from the condenser, inadequate drying of the solvent (high blank), and improper distillation rate leading to water carry-over or incomplete condensation. Using a trap with incorrect graduations or a flask that is too small for the sample volume can also affect accuracy.
Q: How does API MPMS 10.2 relate to other standards like ASTM D4006?
A: API MPMS 10.2 is technically identical to ASTM D4006. The two standards are jointly maintained. Many custody transfer contracts reference either API MPMS 10.2 or ASTM D4006, and test results from either are considered equivalent when performed in conformance with the specified edition.
A: API MPMS 10.2 is technically identical to ASTM D4006. The two standards are jointly maintained. Many custody transfer contracts reference either API MPMS 10.2 or ASTM D4006, and test results from either are considered equivalent when performed in conformance with the specified edition.