💧 Water content by distillation method
In short
Determination of water content (0–25% v/v) in petroleum products, tars and bituminous materials by azeotropic distillation with Dean-Stark receiver. The test is performed according to PN-EN ISO 3733:2019; Determination range: 0.01–25% v/v (0–25% m/m). The procedure comprises 10 steps (about 3 h 28 min–4 h 28 min in total); it is used for: Quality control of lubricating and hydraulic oils, Testing of fuels — diesel oil, heavy fuel oils, Analysis of asphalts and asphalt emulsions (road construction).
At a glance
- Standard: PN-EN ISO 3733:2019
- Category: Petrochemistry
- Procedure steps: 10
- Total time of stages: 3 h 28 min–4 h 28 min
- Determination range: 0.01–25% v/v (0–25% m/m)
- Reading resolution: 0.05 mL (10 mL receiver), 0.1 mL (25 mL)
- Repeatability: ±0.03 mL (up to 1 mL water), ±5% of value (>1 mL)
Overview
Dean-Stark distillation method is a classical technique for determination of water content in petroleum products, bituminous materials, tars and emulsions. It is based on azeotropic distillation phenomenon — water and organic solvent (most commonly xylene or toluene) form azeotrope boiling at lower temperature than each component separately, enabling quantitative water distillation from sample.
Method is applicable to samples with water content from 0.01% to 25% by volume. It is used wherever water presence affects product quality: lubricating oils (bearing corrosion, emulsification), fuels (tank corrosion, freezing), asphalts (pavement blisters), transformer oils (dielectric strength reduction) and waste oils (difficult regeneration).
Dean-Stark apparatus was developed in 1920 by Ernest W. Dean and David D. Stark and remains reference method in ISO, ASTM and EN standards to this day. Method advantages include simplicity, low cost and ability to determine water in dark, viscous and heterogeneous samples where Karl Fischer method may give false results.
PN-EN ISO 3733 standard is equivalent to ASTM D95 with certain procedural differences (e.g. solvent choice). Both methods give comparable results and are accepted in international trade.
Method principle
Weighed sample of petroleum product is placed in distillation flask together with solvent not miscible with water (xylene or toluene). Flask is heated — solvent and water co-distill as azeotrope. Vapors condense in reflux condenser and flow to calibrated Dean-Stark receiver. Water (heavier) settles at receiver bottom in lower layer, while excess solvent (lighter) overflows back to flask. Distillation continues until water volume in receiver stops increasing. Water content is read volumetrically from receiver scale and calculated as percentage of sample mass or volume.
Applications
- Quality control of lubricating and hydraulic oils
- Testing of fuels — diesel oil, heavy fuel oils
- Analysis of asphalts and asphalt emulsions (road construction)
- Assessment of transformer oils (power industry)
- Control of waste oils before regeneration
- Testing of plastic greases and technical waxes
- Crude oil analysis (acceptance control)
Key parameters
| Parameter | Value |
|---|---|
| Determination range | 0.01–25% v/v (0–25% m/m) |
| Reading resolution | 0.05 mL (10 mL receiver), 0.1 mL (25 mL) |
| Repeatability | ±0.03 mL (up to 1 mL water), ±5% of value (>1 mL) |
| Solvent | Xylene (b.p. 138–144°C) or toluene (b.p. 111°C) |
| Distillation temperature | 110–160°C (depending on solvent and sample) |
Standard
- Standard number
- PN-EN ISO 3733:2019
- Title (PL)
- Przetwory naftowe i materiały asfaltowe — Oznaczanie zawartości wody — Metoda destylacyjna
- Title (EN)
- Petroleum products and bituminous materials — Determination of water — Distillation method
Step-by-step procedure
-
Apparatus preparation
Assemble Dean-Stark distillation set: flask + Dean-Stark receiver + reflux condenser. Check tightness of ground joints. Clean and dry receiver.
-
Sample weighing
Weigh 100–200 g petroleum product sample (mass depends on expected water content). For low water content samples — larger weight.
-
Solvent addition
Add 100–200 mL xylene (or toluene) to flask. Solvent to sample ratio ~1:1. Mix gently.
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Condenser filling
Connect cooling water circulation (temp. 15–25°C, flow ~0.5 L/min). Check condenser proper operation.
-
Distillation
Heat flask with heating mantle. Set heating power so distillation rate is 2–5 drops of condensate/s. Water-xylene vapors condense in condenser and flow to receiver.
-
Distillation monitoring
Observe water level in receiver. Water collects at bottom (lower layer), solvent overflows back to flask. Distill until water volume does not change for 5 min.
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Receiver wall cleaning
If water droplets deposited on receiver walls, rinse with few mL hot solvent from condenser or use PTFE wire.
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Cooling and reading
Allow apparatus to cool to room temperature. Read water volume in receiver with accuracy 0.05 mL (10 mL receiver) or 0.1 mL (25 mL).
-
Calculations
Water content [% v/m] = (V_water [mL] / m_sample [g]) × 100. Alternatively [% v/v] = (V_water / V_sample) × 100. Result with accuracy to 0.05%.
-
Quality control
Perform blank test (solvent only) — result ≤0.05 mL. Check recovery: add known amount of water to solvent, distill, recovery ≥95%.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Dean-Stark distillation set | Koehler K45000, Normalab NDS 450, DURAN laboratory glassware | 3,000–15,000 PLN |
| Calibrated Dean-Stark receiver | Capacity 10 mL or 25 mL, graduation 0.05 / 0.1 mL, DURAN/Simax | 200–600 PLN |
| Round bottom flask 500 mL | DURAN / Simax, ground joint NS 29/32, with side tubulus | 80–200 PLN |
| Reflux condenser (Liebig/Graham) | Length 400–600 mm, ground joint NS 29/32, DURAN | 150–400 PLN |
| Heating mantle or electric hot plate | Electrothermal EME, Bibby EM, Falc MA | 1,500–5,000 PLN |
| Laboratory balance (0.1 g) | Kern EW, Radwag PS | 1,500–4,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Xylene (mixture of isomers) | 1330-20-7 | Distillation solvent, analytical grade, b.p. 138–144°C, immiscible with water |
| Toluene (alternative solvent) | 108-88-3 | Analytical grade, b.p. 110.6°C, used interchangeably with xylene |
| Distilled water (for calibration) | 7732-18-5 | For checking tightness and receiver calibration — add known water volume to solvent |
Health and safety (OHS)
- Xylene / toluene — flammable, vapors form explosive mixtures with air, work under fume hood
- Xylene — harmful by inhalation (H312, H332), irritant, use nitrile gloves and goggles
- Hot apparatus — burn risk, thermal gloves when handling heating mantle
- Petroleum products — often flammable, avoid open flame nearby
- Post-distillation waste — dispose as hazardous organic waste
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 3733:2019 — “Petroleum products and bituminous materials — Determination of water — Distillation method”.
How does this method work?
Weighed sample of petroleum product is placed in distillation flask together with solvent not miscible with water (xylene or toluene). Flask is heated — solvent and water co-distill as azeotrope.
What is the measuring range and accuracy?
Determination range: 0.01–25% v/v (0–25% m/m); Reading resolution: 0.05 mL (10 mL receiver), 0.1 mL (25 mL); Repeatability: ±0.03 mL (up to 1 mL water), ±5% of value (>1 mL); Solvent: Xylene (b.p. 138–144°C) or toluene (b.p. 111°C).
How long does the test take?
The times given for the individual stages add up to approximately 3 h 28 min–4 h 28 min. The procedure comprises 10 steps.
What equipment is required?
Dean-Stark distillation set, Calibrated Dean-Stark receiver, Round bottom flask 500 mL, Reflux condenser (Liebig/Graham), Heating mantle or electric hot plate, Laboratory balance (0.1 g). Indicative cost of the core instrument (Dean-Stark distillation set): 3,000–15,000 PLN.
Where is this test used?
Quality control of lubricating and hydraulic oils; Testing of fuels — diesel oil, heavy fuel oils; Analysis of asphalts and asphalt emulsions (road construction); Assessment of transformer oils (power industry); Control of waste oils before regeneration; Testing of plastic greases and technical waxes; Crude oil analysis (acceptance control).
What safety precautions apply?
Xylene / toluene — flammable, vapors form explosive mixtures with air, work under fume hood; Xylene — harmful by inhalation (H312, H332), irritant, use nitrile gloves and goggles; Hot apparatus — burn risk, thermal gloves when handling heating mantle; Petroleum products — often flammable, avoid open flame nearby; Post-distillation waste — dispose as hazardous organic waste.
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 PN-EN ISO 3733.