🧫 Water content — xylene distillation method
In short
Determination of water content (0–25% v/v) in petroleum products, tars and bituminous materials by azeotropic distillation with xylene in Dean-Stark apparatus. The test is performed according to ASTM D95-13(2018); Determination range: 0.02–25% v/v water. The procedure comprises 11 steps (about 5 h 28 min–6 h 28 min in total); it is used for: Crude petroleum quality control (reservoir acceptance), Testing of new and used lubricating oils, Control of transformer oils (power industry).
At a glance
- Standard: ASTM D95-13(2018)
- Category: Petrochemistry
- Procedure steps: 11
- Total time of stages: 5 h 28 min–6 h 28 min
- Determination range: 0.02–25% v/v water
- Reading resolution: 0.05 mL (10 mL collector), 0.1 mL (25 mL)
- Repeatability (0–1% water): ±0.05% v/v
Overview
ASTM D95 is American equivalent of European standard ISO 3733, describing distillation determination of water content in petroleum products and bituminous materials. Method is based on same azeotropic distillation phenomenon as ISO 3733, but contains certain procedural differences in apparatus selection, solvent volume and evaluation criteria.
Water is one of most common contaminants in petroleum products, appearing at extraction stage (formation water), processing (technological processes), storage (condensation, tank leaks) and distribution (rain water in tankers). Its presence causes: tank and pipeline corrosion, freezing in fuel filters (diesel oil), degradation of refining additives, microorganism multiplication (biofilm in fuel tanks), dielectric strength drop in transformer oils and pavement defects in asphalts.
D95 distillation method measures both free and emulsified water. It is simpler and cheaper than Karl Fischer method (ASTM D6304), but has higher detection threshold (~0.02% v/v). For samples with very low water content (<0.05%), coulometric Karl Fischer method is recommended. D95 method is however irreplaceable for dark, high-viscosity samples containing asphaltenes or sulfur (which interfere with KF titration).
ASTM D95 standard has long history — first edition dates from 1930, making it one of oldest standard methods for petroleum product testing. Despite passage of time, method principle remains unchanged, confirming its reliability and universality.
Method principle
Weighed sample of petroleum product is placed in distillation flask and mixed with excess of water-immiscible solvent (xylene, toluene or other hydrocarbon — ASTM D95 prefers xylene). Flask is heated — xylene forms with water azeotrope boiling at ~95°C (lower than pure xylene ~140°C). Azeotrope vapors condense in reflux condenser and flow to calibrated Dean-Stark collector. Water (denser) collects at collector bottom in lower layer, while excess solvent (lighter) returns to flask through overflow. Distillation continues until water collection ceases (constant level for min. 5 min). Water volume is read from scale and calculated as percentage of sample.
Applications
- Crude petroleum quality control (reservoir acceptance)
- Testing of new and used lubricating oils
- Control of transformer oils (power industry)
- Testing of fuels — diesel oil, heavy fuel oils
- Analysis of road asphalts and asphalt emulsions
- Testing of plastic greases and technical waxes
- Control of waste oils before regeneration
- Testing of tars and bituminous insulation materials
Key parameters
| Parameter | Value |
|---|---|
| Determination range | 0.02–25% v/v water |
| Reading resolution | 0.05 mL (10 mL collector), 0.1 mL (25 mL) |
| Repeatability (0–1% water) | ±0.05% v/v |
| Repeatability (1–10% water) | ±0.1% v/v |
| Solvent | Xylene (ASTM D95 preferred) |
| Sample mass | 100–200 g (depending on expected water content) |
Standard
- Standard number
- ASTM D95-13(2018)
- Title (PL)
- Zawartość wody w produktach naftowych i materiałach bitumicznych metodą destylacji
- Title (EN)
- Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation
Step-by-step procedure
-
Apparatus assembly and check
Assemble Dean-Stark distillation set: 500 mL flask + collector + reflux condenser. Check ground joint tightness (silicone grease). Check cooling water circulation.
-
Collector cleaning
Clean collector with chromosulfuric mixture or acetone. Rinse with water, then acetone. Air dry. Collector must be absolutely clean and dry.
-
Collector calibration (optional)
Add precisely measured water volume (e.g. 1.0 mL with pipette) to 100 mL xylene. Distill. Reading in collector should be 1.0 ±0.05 mL (recovery ≥95%).
-
Sample weighing
Weigh 100–200 g petroleum product sample into distillation flask. For high expected water content — smaller weight (50–100 g). Record mass with accuracy 0.1 g.
-
Solvent addition
Add 100–200 mL xylene (ratio ~1:1 v/m with sample). For viscous samples (asphaltenes, HFO) — add more solvent. Add 2–3 boiling stones.
-
Distillation
Heat flask slowly. Set heating power so condensation rate is 2–5 drops/s. Too fast distillation may cause sample carryover into collector (error).
-
Water collection monitoring
Observe water collecting at collector bottom. Xylene (upper layer) returns to flask through overflow. Continue distillation until water level does not change for min. 5 min.
-
Water layer clarification
If water/xylene interface is unclear (emulsion), add 1–2 drops surfactant or wait longer. Can also cool collector in cold water stream.
-
Reading and calculations
Cool collector to room temperature. Read water volume [mL] from scale. Calculate: water content [% v/m] = (V_water × 100) / m_sample [g]. For % v/v divide by sample density.
-
Post-analysis cleaning
Pour flask contents (after cooling!) into waste container. Rinse flask and collector with xylene, then acetone. Dry.
-
Quality control
Blank test (xylene only, 100 mL): water reading ≤0.05 mL. Spiked sample with known water amount: recovery 95–105%. Duplicate: agreement ±0.05 mL.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Dean-Stark apparatus (complete) | Koehler K45000, Ayalytical WDS, Humboldt H-4236 | 3,000–15,000 PLN |
| Calibrated Dean-Stark collector | Capacity 10 mL or 25 mL, graduation 0.05 / 0.1 mL, borosilicate | 200–600 PLN |
| Round bottom flask 500 mL | DURAN / Simax, ground joint NS 24/29 or 29/32 | 60–150 PLN |
| Reflux condenser (Liebig) | Length 400–500 mm, ground joint NS 24/29, DURAN | 120–350 PLN |
| Heat source | Heating mantle Electrothermal, Bibby EM or gas burner (not recommended) | 1,500–5,000 PLN |
| Laboratory balance (0.1 g) | Kern PCB, Radwag WLC | 1,000–3,000 PLN |
| Boiling stones / anti-bumping granules | Ceramic or PTFE, reusable | 30–100 PLN / 100 pcs |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Xylene (mixture of isomers) | 1330-20-7 | Distillation solvent, analytical grade, b.p. 138–144°C, water-free (<0.01%) |
| Toluene (alternative) | 108-88-3 | Analytical grade, b.p. 110.6°C, used interchangeably with xylene per ASTM D95 |
| Distilled water | 7732-18-5 | For calibration and apparatus tightness check — add known volume, distill, check recovery |
| Petroleum solvent (for cleaning) | 64742-49-0 | For preliminary flask rinsing after bituminous products |
Health and safety (OHS)
- Xylene — flammable, harmful (H312, H332), neurotoxic, work only under fume hood
- Xylene vapors — heavier than air, accumulate at floor level, ventilation mandatory
- Hot apparatus — flask, condenser, heating mantle — burn risk, thermal gloves
- Petroleum products — flammable, avoid ignition sources (burner, spark)
- Asphalts and HFO — may release H₂S when heated, fume hood mandatory
- Post-distillation waste — dispose as hazardous waste (code 16 07)
Frequently asked questions
Which standard describes this test?
The test is performed according to ASTM D95-13(2018) — “Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation”.
How does this method work?
Weighed sample of petroleum product is placed in distillation flask and mixed with excess of water-immiscible solvent (xylene, toluene or other hydrocarbon — ASTM D95 prefers xylene). Flask is heated — xylene forms with water azeotrope boiling at ~95°C (lower than pure xylene ~140°C).
What is the measuring range and accuracy?
Determination range: 0.02–25% v/v water; Reading resolution: 0.05 mL (10 mL collector), 0.1 mL (25 mL); Repeatability (0–1% water): ±0.05% v/v; Repeatability (1–10% water): ±0.1% v/v.
How long does the test take?
The times given for the individual stages add up to approximately 5 h 28 min–6 h 28 min. The procedure comprises 11 steps.
What equipment is required?
Dean-Stark apparatus (complete), Calibrated Dean-Stark collector, Round bottom flask 500 mL, Reflux condenser (Liebig), Heat source, Laboratory balance (0.1 g), Boiling stones / anti-bumping granules. Indicative cost of the core instrument (Dean-Stark apparatus (complete)): 3,000–15,000 PLN.
Where is this test used?
Crude petroleum quality control (reservoir acceptance); Testing of new and used lubricating oils; Control of transformer oils (power industry); Testing of fuels — diesel oil, heavy fuel oils; Analysis of road asphalts and asphalt emulsions; Testing of plastic greases and technical waxes; Control of waste oils before regeneration; Testing of tars and bituminous insulation materials.
What safety precautions apply?
Xylene — flammable, harmful (H312, H332), neurotoxic, work only under fume hood; Xylene vapors — heavier than air, accumulate at floor level, ventilation mandatory; Hot apparatus — flask, condenser, heating mantle — burn risk, thermal gloves; Petroleum products — flammable, avoid ignition sources (burner, spark); Asphalts and HFO — may release H₂S when heated, fume hood mandatory; Post-distillation waste — dispose as hazardous waste (code 16 07).
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number ASTM D95.