Chemical Classification Vapor Pressure Test: Methods, Standards, and Regulatory Applications

Vapor pressure testing is a core physical-chemical measurement used to classify chemicals under the GHS framework, UN transport regulations, and national hazard communication standards. This technical article explains the measurement principle, applicable sample types, internationally recognized test methods, and the regulatory applications of vapor pressure data, helping manufacturers and regulatory professionals select the most appropriate testing strategy for hazard classification and SDS documentation.

Technical Overview

Vapor pressure is the pressure exerted by a vapor when it is in thermodynamic equilibrium with its condensed phase (liquid or solid) inside a closed system at a specified temperature. It is one of the most important physical-chemical properties in chemical hazard classification because it determines how readily a substance evaporates, influences flammability behavior, and defines the boundary between the gaseous and liquid states under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS).

Under GHS criteria, a substance is regarded as a gas if its vapor pressure is equal to or greater than 300 kPa at 50 °C, or if it is completely gaseous at 20 °C and a standard pressure of 101.3 kPa. Vapor pressure data therefore directly determine whether a product falls into the hazard class of gases under pressure (compressed, liquefied, dissolved, or liquefied refrigerated gases) or is treated as a liquid for further classification such as flammable liquids.

Vapor pressure is normally reported in Pa or kPa at reference temperatures of 20 °C, 25 °C, or 50 °C, and it appears in Section 9 (Physical and Chemical Properties) of the Safety Data Sheet (SDS). The principal methodological references include OECD Test Guideline 104 (Vapour Pressure Curve), ASTM D2879, ASTM D323, ASTM D5191, ASTM D6377, ISO 3007, EN 13016, and the Chinese national standards GB/T 8017 and the GB 30000 series that implement GHS classification and labelling criteria.

Testing Samples

Pure and Technical-Grade Organic Chemicals

Solvents, intermediates, monomers, and specialty chemicals in liquid or low-melting solid form are the most common samples. Only a few milliliters of material are generally required. Purity and thermal stability are assessed before testing because impurities and dissolved gases can distort equilibrium pressure readings.

Low-Volatility Solids and High-Boiling Substances

Substances with expected vapor pressures below approximately 1 Pa, such as certain pesticides, plasticizers, flame retardants, and polymer additives, require high-sensitivity techniques and larger sample quantities to achieve reliable results.

Petroleum Products and Volatile Mixtures

Gasoline, crude oil, naphtha, condensates, and other hydrocarbon mixtures are tested with Reid-type or automated mini-methods at 37.8 °C to obtain vapor pressure values for quality control, storage management, and transport classification.

Liquefied Gases and Pressurized Products

Liquefied petroleum gas, aerosol products, and liquefied compressed gases are evaluated using dedicated pressure apparatus at ambient and elevated temperatures to confirm their classification as gases under pressure and to assess container pressure build-up behavior.

Testing Items

Typical test items delivered by accredited third-party laboratories and in-house physical-chemical testing facilities include the following:

• Single-point vapor pressure determination at 20 °C, 25 °C, and/or 50 °C
• Full vapor pressure curve across a defined temperature range
• Extrapolated vapor pressure at 50 °C for GHS gas-versus-liquid state confirmation
• Derived thermodynamic parameters, including enthalpy of vaporization and Antoine equation constants
• Reid vapor pressure or dry vapor pressure equivalent (DVPE) for fuels and crude oils
• Boiling temperature at reduced pressure for correlative analysis

Testing Methods

Static Method

The sample is sealed in a thermostated equilibrium cell fitted with a calibrated pressure sensor, and the pressure is recorded at each temperature step once equilibrium is reached. The method covers roughly 10 to 100,000 Pa and is well suited to multi-temperature vapor pressure curves, provided the sample is thoroughly degassed beforehand.

Isoteniscope Method

Described in ASTM D2879 and OECD Test Guideline 104, this differential technique balances the vapor pressure of the sample against a reference column while excluding dissolved gases. It is applicable from about 100 to 100,000 Pa, requires only a small sample, and is widely used for regulatory submission data.

Dynamic (Boiling Point) Method

The boiling temperature of the sample is measured at several controlled pressures, and the vapor pressure curve is derived from the Clausius-Clapeyron relationship. The method is suitable for thermally stable liquids in the range of approximately 1,000 to 100,000 Pa.

Gas Saturation Method

An inert carrier gas is passed over or through the sample at constant temperature until saturated, and the transported vapor is quantified, typically by gas chromatography. Recognized in OECD 104, the technique covers approximately 10^-7 to 1 Pa and is the preferred approach for low-volatility environmental chemicals.

Effusion Methods

Knudsen effusion measures mass loss through a calibrated orifice under vacuum, while the vapor pressure balance continuously weighs the effusing vapor. These techniques extend measurement capability down to about 10^-8 Pa and are ideal for solids and high-molecular-weight substances.

Reid and Mini-Methods for Petroleum Products

The wet Reid method (ASTM D323, ISO 3007, GB/T 8017) determines vapor pressure at 37.8 °C using a twin-chamber apparatus. Automated procedures such as ASTM D5191 (mini method), ASTM D6377 (expansion method for crude oils), and EN 13016 deliver equivalent dry vapor pressure values with smaller samples, faster turnaround, and improved precision.

Data Treatment and Extrapolation

Measured data points are fitted using the Antoine equation or the Clausius-Clapeyron relation so that the vapor pressure at 50 °C can be extrapolated for GHS state-of-matter confirmation. When extrapolation is not scientifically justified, for example due to decomposition, classification proceeds on the basis of the best available measured data.

Applications

Vapor pressure test results support a wide range of regulatory, safety, and commercial purposes across the chemical supply chain:

• GHS classification and labelling, including confirmation of gas-versus-liquid state and assignment of the gases under pressure hazard class under the GB 30000 series and equivalent national implementations
• Dangerous goods transport classification under the UN Model Regulations, particularly for UN Class 2 gases and pressure receptacle requirements
• Preparation of Section 9 physical and chemical properties data in Safety Data Sheets
• Chemical registration dossiers under programs such as REACH, where vapor pressure is a standard endpoint
• Process, packaging, and storage safety, including pressure build-up evaluation and venting design for closed containers
• Fuel quality control and crude oil stabilization in the petroleum industry
• Environmental fate and occupational exposure assessment, where volatility governs emission and inhalation behavior

FAQ

At which temperature should vapor pressure be reported for classification?

For GHS purposes the decisive value is the vapor pressure at 50 °C compared with the 300 kPa threshold, while SDS documentation commonly cites values at 20 °C or 25 °C. Test reports should clearly state the measurement temperatures and any extrapolation applied.

How much sample is required?

Conventional liquid methods typically need 5 to 50 mL, whereas low-pressure techniques such as gas saturation or effusion may require several grams of solid. Exact quantities depend on the selected method and the expected volatility of the substance.

How is the appropriate method selected?

Method selection is driven by the expected vapor pressure range, the physical state of the sample, its thermal stability, and the intended regulatory use. A preliminary estimation based on boiling point or structure often guides the choice before testing begins.

Can vapor pressure be estimated instead of measured?

Quantitative structure-property relationships and gas chromatographic retention correlation are accepted as screening tools, but formal hazard classification generally requires measured data generated by accredited third-party laboratories, especially when the value lies near the 300 kPa GHS threshold.

How long does the test take?

A single-temperature determination typically takes several working days once equilibrium and calibration checks are complete, while a full multi-temperature curve with extrapolated parameters usually requires one to three weeks depending on sample behavior and reporting requirements.