💧 Water content by distillation method
In short
Determination of water in petroleum products, bitumens, tars and products derived from these materials (excluding emulsions) by heating the sample under reflux with a water-immiscible solvent. The test is performed according to PN-ISO 3733:2008 (Polish version; adopts ISO 3733:1999 [IDT]); Range: up to 25 % water; above 25 % the method may be used, but without determined precision. The procedure comprises 10 steps; it is used for: Bitumens, coal tars and preparations from them, excluding emulsions — with an aromatic solvent, Road oils, fuel oils, lubricating oils and petroleum sulfonates — with a petroleum distillate solvent, Lubricating greases — with a paraffinic solvent.
At a glance
- Standard: PN-ISO 3733:2008 (Polish version; adopts ISO 3733:1999 [IDT])
- Category: Petrochemistry
- Procedure steps: 10
- Range: up to 25 % water; above 25 % the method may be used, but without determined precision
- Traps: 2, 5, 10 and 25 ml; smallest scale division 0.05 ml (2 and 5 ml), 0.1 ml or 0.2 ml (10 and 25 ml, Table 1)
- Distillation rate: 2 to 9 drops of condensate per second
Overview
STATUS (as of 06.11.2026). According to the PKN catalogue card (read on 06.11.2026) PN-ISO 3733:2008 is a Polish version published on 08.02.2008 (17 pages), implementing ISO 3733:1999 [IDT], with no withdrawal note. ISO 3733:1999 is the second edition (15.09.1999), which, as a technical revision, replaced the 1976 edition; the Standard Norge catalogue lists it as the current edition with no successor. The description is based on the full text of ISO 3733:1999 in English.
SCOPE. The standard specifies a distillation method for determining water up to 25 % in petroleum products, bitumens, tars and products derived from these materials, excluding emulsions. The method may also be used above 25 %, but no precision has been determined for such contents. Volatile water-soluble material, if present, is measured as water. According to the notes to the scope, knowledge of the water content is important in the refining, purchase, sale and transfer of products, and the result can be used to correct the volume of product transferred.
CHOICE OF SOLVENT (Table 3 of the standard). Aromatic solvent — bitumens, coal tars and preparations from them (excluding emulsions); petroleum distillate — road oils, fuel oils, lubricating oils and petroleum sulfonates; paraffinic solvent — lubricating greases. The solvent must be free of water, and a blank is run for every new batch of solvent.
APPARATUS. A glass still (Dean and Stark apparatus) or a metal still, a heater, a reflux condenser and a graduated glass trap with a capacity of 2, 5, 10 or 25 ml. An apparatus is deemed suitable if it gives correct results in the trap verification and in the water recovery test described in the standard.
The same subject is also covered by ASTM D95, described in a separate Knowledge Base entry; a comparison of the two documents is not given here.
WHAT WE DO NOT GIVE HERE. The standard does not give the distillation time, the mass or volume of the test portion in grams or millilitres (only the rule that the water from the portion must fit in the trap), the boiling temperature of the water–solvent mixture or a comparison with the Karl Fischer method — we do not give such figures or assessments here. For the 2 ml trap the standard has not determined the precision.
Method principle
The sample is heated under reflux with a water-immiscible solvent, which co-distils with the water in the sample. The condensed solvent and water separate continuously in the trap: the water settles in its graduated section and the solvent returns to the still. Distillation is carried out at a rate of 2 to 9 drops of condensate per second until no water is visible outside the trap and its volume in the trap remains constant for 5 min. After cooling, the volume of water is read, the solvent blank is subtracted and the result is converted to a percentage by volume or by mass of the sample, taking the density of water as 1 g/ml.
Applications
- Bitumens, coal tars and preparations from them, excluding emulsions — with an aromatic solvent
- Road oils, fuel oils, lubricating oils and petroleum sulfonates — with a petroleum distillate solvent
- Lubricating greases — with a paraffinic solvent
- Refining, purchase, sale and transfer of petroleum products — correction of volume for water content
Key parameters
| Parameter | Value |
|---|---|
| Range | up to 25 % water; above 25 % the method may be used, but without determined precision |
| Traps | 2, 5, 10 and 25 ml; smallest scale division 0.05 ml (2 and 5 ml), 0.1 ml or 0.2 ml (10 and 25 ml, Table 1) |
| Distillation rate | 2 to 9 drops of condensate per second |
| End of distillation | no water visible outside the trap and a constant volume of water in the trap for 5 min |
| Result | to the nearest 0.05 % (2 ml trap) or 0.1 % (10 and 25 ml traps with a 100 ml or 100 g portion) |
| Precision (10 and 25 ml traps) | water collected 0.0 to 1.0 ml: repeatability 0.1 ml, reproducibility 0.2 ml; 1.1 to 25 ml: repeatability 0.1 ml or 2 % of the mean, reproducibility 0.2 ml or 10 % of the mean (whichever is greater) |
Standard
- Standard number
- PN-ISO 3733:2008 (Polish version; adopts ISO 3733:1999 [IDT])
- Title (PL)
- Przetwory naftowe i materiały asfaltowe — Oznaczanie wody — Metoda destylacyjna
- Title (EN)
- Petroleum products and bituminous materials — Determination of water — Distillation method
Step-by-step procedure
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Verification of the trap graduation
Before first use, add distilled water to the trap in 0.05 ml increments from a 5 ml microburette or a micropipette readable to 0.01 ml. If the difference between the water added and the water observed exceeds 0.05 ml, reject the trap or recalibrate it.
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Water recovery test
Before determinations, introduce 250 to 1 000 ml of dry xylene (0.02 % water maximum), according to the size of the still, and distil as in the determination. Discard the contents of the trap and, after the still has cooled, add distilled water from a burette in the amount given in Table 2 of the standard (e.g. 1.00 ml with a 2 ml trap — permissible 1.0 ± 0.025 ml; 5.0 ml with a 10 ml trap — 5 ± 0.25 ml; 12.0 ml with a 25 ml trap — 12 ± 0.25 ml) and distil again. A result outside the limits indicates vapour leaks, too rapid boiling or ingress of extraneous moisture.
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Homogenization of the sample
Crush and mix thoroughly solid samples that are sufficiently brittle. Mix liquid samples in the original container, warming them if necessary, in the way verified according to Annex A of the standard. If homogeneity is in doubt, determine water in the whole sample or in at least three portions and report the average.
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Measuring the test portion
Choose the portion so that its water does not exceed the capacity of the trap (unless it has a stopcock) and measure it to ± 1 %. Measure liquid samples with a graduated cylinder and rinse it with one 50 ml and two 25 ml portions of the solvent; weigh solid and viscous samples directly into the still and add 100 ml of solvent (more for low water contents and large portions).
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Assembling the apparatus
Add a magnetic stirrer or glass beads or other boiling aids. Assemble the apparatus vapour- and liquid-tight with a trap chosen for the expected water content; with a metal still with a removable cover, insert a gasket of heavy paper moistened with solvent. The condenser and trap must be chemically clean, with a loose cotton plug at the top of the condenser; cold water circulates through the condenser jacket.
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Distillation
Heat so that the condensate discharges from the condenser at 2 to 9 drops per second; with a metal still start with the burner about 75 mm above the bottom and lower it gradually. Distil until no water is visible outside the trap and its volume in the trap remains constant for 5 min. If a persistent ring of water forms in the condenser tube, carefully increase or cut off the condenser water for a few minutes.
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Reading
Allow the trap and contents to cool to room temperature. Transfer water drops adhering to the walls to the water layer with a glass rod or by other suitable means. Read the volume of water to the nearest scale division.
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Solvent blank
For every new batch of solvent, place in the still the volume of solvent used in the determinations and carry out steps 5–7 without a sample.
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Calculation
Water content in % (V/V) = (V₁ − V₂) / V₀ × 100 or (V₁ − V₂) / (m/ρ) × 100, and in % (m/m) = (V₁ − V₂) / m × 100, where V₁ — water in the trap in the determination [ml], V₂ — water in the blank [ml], V₀ — volume of the portion [ml], m — mass of the portion [g], ρ — density of the sample at 15 °C [g/ml]; the density of water is taken as 1 g/ml.
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Result and test report
Report the water content to the nearest 0.05 % (2 ml trap) or 0.1 % (10 and 25 ml traps with a 100 ml or 100 g portion). The test report contains at least a reference to the standard, the type and complete identification of the product, the result, any deviation from the procedure and the date of the test.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Glass or metal still | Vessel with a short neck and a joint for the reflux tube of the trap; capacities of 500, 1 000 and 2 000 ml have proved satisfactory; metal still with a heat-resistant gasket of a material other than asbestos | — |
| Graduated trap | 2, 5, 10 or 25 ml, requirements of Table 1 of the standard; chosen for the expected water content; if more than 25 ml of water may be collected — a 25 ml trap with a stopcock, the excess water being drained into a graduated cylinder | — |
| Reflux condenser | A straight water-cooled condenser 400 mm long is recommended; connections vapour- and liquid-tight (ground joints for glass, O-rings for metal) | — |
| Heater | Electric heater with the glass still (combined with a magnetic stirrer it effectively prevents bumping); with the metal still a gas ring burner with ports on the inside circumference, movable along the vessel | — |
| 5 ml microburette or micropipette | Readable to 0.01 ml — for verifying the trap graduation before first use | — |
| Graduated cylinder and balance | Measuring the test portion to ± 1 %: liquid samples in a cylinder, solid and viscous samples by weighing directly into the still | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Aromatic solvent | 1330-20-7 | Free of water (0.02 % maximum): industrial grade xylene (mixed xylenes), a blend of 20 % (V/V) industrial grade toluene and 80 % (V/V) xylene, or a petroleum distillate giving not more than 5 % (V/V) at 125 °C and not less than 20 % (V/V) at 160 °C (ISO 3405), with a density not lower than 855 kg/m³ at 15 °C |
| Petroleum distillate solvent | — | Free of water; 5 % (V/V) boils between 90 °C and 100 °C and 90 % (V/V) distils below 210 °C |
| Paraffinic solvent | 540-84-1 | Free of water: light petroleum with a boiling range of 100 °C to 120 °C or 2,2,4-trimethylpentane (isooctane) of at least 95 % purity |
| Distilled water | 7732-18-5 | For verifying the trap graduation and for the water recovery test |
Health and safety (OHS)
- The standard contains a general warning: its use may involve hazardous materials, operations and equipment, and it is the responsibility of the user to establish safety practices and regulatory limitations
- Xylene (harmonised CLP classification): flammable liquid and vapour (H226), harmful in contact with skin and if inhaled (H312, H332), causes skin irritation (H315)
- Toluene (component of the 20/80 blend): highly flammable (H225), may be fatal if swallowed and enters airways (H304), causes skin irritation (H315), may cause drowsiness or dizziness (H336), suspected of damaging the unborn child (H361d), may cause damage to organs through prolonged or repeated exposure (H373)
- 2,2,4-Trimethylpentane (CLP): H225, H304, H315, H336, very toxic to aquatic life with long-lasting effects (H400, H410)
- Bumping is prevented by a magnetic stirrer or glass beads; with a metal still the ring burner is moved along the vessel for materials likely to foam or solidify
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 3733:2008 (Polish version; adopts ISO 3733:1999 [IDT]) — “Petroleum products and bituminous materials — Determination of water — Distillation method”.
How does this method work?
The sample is heated under reflux with a water-immiscible solvent, which co-distils with the water in the sample. The condensed solvent and water separate continuously in the trap: the water settles in its graduated section and the solvent returns to the still.
What is the measuring range and accuracy?
Range: up to 25 % water; above 25 % the method may be used, but without determined precision; Traps: 2, 5, 10 and 25 ml; smallest scale division 0.05 ml (2 and 5 ml), 0.1 ml or 0.2 ml (10 and 25 ml, Table 1); Distillation rate: 2 to 9 drops of condensate per second; End of distillation: no water visible outside the trap and a constant volume of water in the trap for 5 min.
How long does the test take?
The procedure comprises 10 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Glass or metal still, Graduated trap, Reflux condenser, Heater, 5 ml microburette or micropipette, Graduated cylinder and balance.
Where is this test used?
Bitumens, coal tars and preparations from them, excluding emulsions — with an aromatic solvent; Road oils, fuel oils, lubricating oils and petroleum sulfonates — with a petroleum distillate solvent; Lubricating greases — with a paraffinic solvent; Refining, purchase, sale and transfer of petroleum products — correction of volume for water content.
What safety precautions apply?
The standard contains a general warning: its use may involve hazardous materials, operations and equipment, and it is the responsibility of the user to establish safety practices and regulatory limitations; Xylene (harmonised CLP classification): flammable liquid and vapour (H226), harmful in contact with skin and if inhaled (H312, H332), causes skin irritation (H315); Toluene (component of the 20/80 blend): highly flammable (H225), may be fatal if swallowed and enters airways (H304), causes skin irritation (H315), may cause drowsiness or dizziness (H336), suspected of damaging the unborn child (H361d), may cause damage to organs through prolonged or repeated exposure (H373); 2,2,4-Trimethylpentane (CLP): H225, H304, H315, H336, very toxic to aquatic life with long-lasting effects (H400, H410); Bumping is prevented by a magnetic stirrer or glass beads; with a metal still the ring burner is moved along the vessel for materials likely to foam or solidify.
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-ISO 3733.