⚖️ Density by pycnometer
Determination of density of crude petroleum and petroleum products using capillary or bicapillary pycnometer. Classical high-accuracy reference method.
Overview
Density is one of basic physical parameters of crude petroleum and petroleum products, determining their commercial classification, volume-mass settlements, process calculations in refineries and product identification. Crude petroleum density is expressed in kg/m³ at 15°C temperature (or as API gravity — American Petroleum Institute scale).
Pycnometric method (PN-EN ISO 3838) is classical reference method, consisting of precise filling of calibrated glass vessel (pycnometer) with tested liquid and weighing on analytical balance. Density is calculated from liquid mass occupying known pycnometer volume at controlled temperature. Method is characterized by highest accuracy among density determination methods (±0.1 kg/m³) and is used as arbitration method in trade disputes.
ISO 3838 standard describes two variants: capillary pycnometer (Hubbard type — stopper with capillary, excess liquid flows out) and bicapillary pycnometer (two capillaries — level equalization). For routine analyses in refinery laboratories, faster oscillatory U-tube method (ISO 12185, ASTM D4052) is increasingly used, however pycnometry remains reference method required in dispute cases.
Density results are used for converting volume to mass (and vice versa) in commercial settlements of crude petroleum and products, calculating volume at reference temperature 15°C (ASTM/API conversion tables — tables 54) and calculating other parameters (e.g. refractive index, approximate molecular weight, K_UOP characterization factor).
Method principle
Glass pycnometer of precisely known volume (typically 10–50 mL) is washed, dried and weighed (m₀). Then filled with distilled water and weighed (m_w) to determine exact pycnometer volume at given temperature (V = [m_w − m₀] / ρ_water). After drying, pycnometer is filled with tested petroleum product, thermostated in water bath to set temperature (15.0 ± 0.1°C or 20.0 ± 0.1°C) and weighed (m_p). Sample density is calculated from formula: ρ = (m_p − m₀) / V. Capillary pycnometer has stopper with narrow capillary (ø ~1 mm), through which excess liquid flows during thermostating, ensuring repeatable filling.
Applications
- Commercial classification of crude petroleum (API gravity)
- Volume-mass settlements in fuel depots and refineries
- Determination of petroleum product density — gasoline, diesel oil, naphtha, lubricating oils
- Arbitration method in trade disputes regarding fuel quality
- Quality control of asphalts and bituminous binders
- Laboratory testing of crude petroleum (reservoir characterization)
- Petroleum product identification (comparison with product standards)
Key parameters
| Parameter | Value |
|---|---|
| Density range | 600–1,100 kg/m³ (petroleum products) |
| Accuracy | ±0.1 kg/m³ (capillary pycnometer) |
| Measurement temperature | 15.0 ± 0.1°C or 20.0 ± 0.1°C |
| Pycnometer capacity | 10–50 mL (typically 25 mL) |
| Repeatability | ±0.1 kg/m³ |
| Reproducibility | ±0.5 kg/m³ |
Standard
- Standard number
- PN-EN ISO 3838:2008
- Title (PL)
- Ropa naftowa i ciekłe lub stałe przetwory naftowe — Oznaczanie gęstości lub gęstości względnej — Metody piknometru kapilarnego i piknometru bikapilarnego
- Title (EN)
- Crude petroleum and liquid or solid petroleum products — Determination of density or relative density — Capillary-stoppered pyknometer and graduated bicapillary pyknometer methods
Step-by-step procedure
1. Pycnometer washing
Wash pycnometer with chromosulfuric mixture or laboratory detergent. Rinse repeatedly with distilled water. Rinse with acetone. Dry in oven (50°C) or air stream.
2. Empty pycnometer weighing (m₀)
Cool pycnometer to room temperature (in desiccator 15 min). Weigh on analytical balance with accuracy 0.1 mg. Repeat to constant mass.
3. Volume calibration (with water)
Fill pycnometer with freshly degassed distilled water. Attach capillary stopper — excess water flows out. Thermostat in bath 15.0°C ± 0.1°C for 30 min.
4. Weighing with water (m_w)
Remove pycnometer from bath, dry externally with tissue (without touching capillary). Weigh immediately. V = (m_w − m₀) / ρ_water(15°C). ρ_water(15°C) = 999.099 kg/m³.
5. Pycnometer drying
Empty pycnometer, rinse with acetone (2×), dry in stream of dry air or in oven (50°C/15 min).
6. Sample filling
Fill pycnometer with tested petroleum product (using syringe or pouring). Attach capillary stopper. Excess flows through capillary.
7. Thermostating
Place pycnometer in water bath 15.0 ± 0.1°C for 30 min. Check that no more liquid exits through capillary (thermal equilibrium reached).
8. Weighing with sample (m_p)
Remove pycnometer, dry externally. Weigh immediately on analytical balance (0.1 mg). Record mass m_p.
9. Density calculations
ρ = (m_p − m₀) / V [kg/m³]. Alternatively: relative density d¹⁵₁₅ = (m_p − m₀) / (m_w − m₀). Convert to API gravity: °API = 141.5 / d₆₀ₓ₆₀ − 131.5.
10. Quality control
Measure density of reference liquid (CRM). Result must be within ±0.3 kg/m³ from certified value. Sample duplicate: difference ≤0.1 kg/m³.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Capillary pycnometer (Hubbard type) | Brand, Witeg, DURAN — 25 mL pycnometer with capillary stopper, calibrated | 200–800 PLN |
| Bicapillary pycnometer (Gay-Lussac) | Brand, Carl Roth, DURAN — with two capillaries, 10–50 mL | 150–600 PLN |
| Thermostatic water bath | Julabo CORIO CD, Lauda ECO RE 420, precision ±0.01°C | 8,000–25,000 PLN |
| Analytical balance (0.1 mg) | Mettler Toledo ME204, Sartorius Quintix 224, Radwag AS 220.R2 | 5,000–15,000 PLN |
| Calibrated thermometer | ASTM mercury thermometer or digital Pt100 with certificate, resolution 0.01°C | 300–2,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Distilled water (calibration) | 7732-18-5 | Freshly distilled or deionized, degassed — for pycnometer volume determination |
| Acetone (pycnometer drying) | 67-64-1 | Analytical grade, for rinsing and accelerating pycnometer drying after water washing |
| Naphtha or petroleum ether | 8032-32-4 | For preliminary pycnometer rinsing after petroleum products before water washing |
| Reference liquid of known density | — | Certified density standard (CRM) — for method verification, e.g. pure toluene (ρ = 866.9 kg/m³ @ 15°C) |
Health and safety (OHS)
- Petroleum products — flammable, work away from ignition sources
- Acetone — flammable, eye irritant, work under fume hood
- Water bath — electricity + water, check grounding, do not spill
- Glass pycnometer — fragile, cut risk, handle carefully
- Protective gloves and goggles mandatory