Vapor Pressure

Detailed Description

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. In simpler terms, it is a measure of a substance's tendency to evaporate. In the context of a Safety Data Sheet (SDS), vapor pressure is a critical physical property that provides information about volatility, which directly relates to inhalation exposure potential, fire hazards, and other safety considerations.

Key concepts related to vapor pressure include:

Equilibrium vapor pressure: The pressure reached when the rate of evaporation equals the rate of condensation in a closed system.
Temperature dependence: Vapor pressure increases exponentially with temperature according to the Clausius-Clapeyron equation.
Volatility: Substances with higher vapor pressures are more volatile and evaporate more readily.
Partial pressure: In mixtures, each component contributes its own partial vapor pressure based on its concentration and intrinsic vapor pressure.
Saturation: When air contains the maximum amount of vapor possible at a given temperature, it is saturated.

The vapor pressure of a substance is influenced by several factors:

Molecular structure: Affects intermolecular forces and thus the energy required for molecules to escape to the vapor phase.
Intermolecular forces: Stronger forces (hydrogen bonding, dipole-dipole interactions) generally result in lower vapor pressures.
Temperature: Higher temperatures increase molecular kinetic energy, leading to higher vapor pressures.
Surface area: While not affecting the equilibrium vapor pressure value itself, larger surface areas allow faster approach to equilibrium.

Importance in Safety Data Sheets

Vapor pressure information in an SDS is critical for several reasons:

Inhalation exposure assessment: Indicates the potential for a substance to become airborne and create inhalation hazards.
Fire and explosion hazard evaluation: High vapor pressure substances can create flammable atmospheres more readily.
Storage requirements: Informs appropriate container design, venting needs, and temperature control requirements.
Ventilation needs: Helps determine appropriate ventilation requirements for safe handling.
PPE selection: Guides selection of appropriate respiratory protection and other PPE.
Emergency response planning: Critical for assessing vapor hazards during spills or releases.
Environmental fate prediction: Helps predict environmental partitioning and transport.

Measurement Methods

Several techniques are used to determine vapor pressure:

Method	Description	Typical Applications
Static Method	Direct measurement of pressure in a closed system at equilibrium	Medium to high vapor pressure substances
Dynamic Method (Isoteniscope)	Balancing vapor pressure against an inert gas pressure	Low to medium vapor pressure liquids
Gas Saturation Method	Passing inert gas through the substance and measuring weight loss	Very low vapor pressure substances
Knudsen Effusion Method	Measuring rate of effusion through a small orifice	Very low vapor pressure solids
Ebulliometry	Measuring boiling point elevation under different pressures	High vapor pressure liquids
Transpiration Method	Measuring mass transport in a carrier gas stream	Low vapor pressure substances
ASTM D323 (Reid Vapor Pressure)	Standardized method for petroleum products	Fuels and volatile petroleum products
OECD Test Guideline 104	Standardized methods for regulatory purposes	Chemical registration and classification

Mathematical Relationship

The temperature dependence of vapor pressure can be described by the Clausius-Clapeyron equation:

ln(P₂/P₁) = (ΔH_vap/R) × (1/T₁ - 1/T₂)

Where:

P₁ and P₂ are vapor pressures at temperatures T₁ and T₂ (in Kelvin)
ΔH_vap is the enthalpy of vaporization
R is the gas constant

For practical applications, the Antoine equation is often used:

log₁₀(P) = A - B/(C + T)

Where:

P is the vapor pressure
T is the temperature
A, B, and C are substance-specific constants

These relationships allow for the estimation of vapor pressure at different temperatures when experimental data is limited.

Vapor Pressure Ranges and Hazard Implications

Vapor Pressure Range (at 20°C)	Volatility Classification	Hazard Implications	Examples
>50 kPa	Very High	Rapid evaporation; high inhalation exposure potential; may form explosive atmospheres; may cause frostbite upon rapid evaporation	Propane (850 kPa), butane (213 kPa), dimethyl ether (530 kPa)
10-50 kPa	High	Significant evaporation at room temperature; substantial inhalation exposure potential; readily forms flammable mixtures with air	Acetone (24 kPa), methanol (12.8 kPa), ethyl acetate (10 kPa)
1-10 kPa	Moderate	Moderate evaporation rate; significant inhalation exposure potential in poorly ventilated areas; can form flammable mixtures in confined spaces	Toluene (2.9 kPa), xylene (0.8-1.2 kPa), ethylene glycol (0.12 kPa)
0.1-1 kPa	Low	Slow evaporation; moderate inhalation exposure potential; less likely to form flammable atmospheres except in confined spaces	Butyl acetate (0.15 kPa), phenol (0.2 kPa), water (2.3 kPa)
<0.1 kPa	Very Low	Minimal evaporation at room temperature; low inhalation exposure potential; unlikely to form flammable atmospheres	Mineral oil (0.0001 kPa), diethylene glycol (0.008 kPa), most solids

Note: The classifications and ranges above are approximate and may vary depending on regulatory framework and specific application context.

Vapor Pressure of Common Substances

Substance	Vapor Pressure at 20°C (kPa)	Vapor Pressure at 50°C (kPa)	Volatility Classification
Water	2.3	12.3	Moderate
Acetone	24.7	80.0	High
Ethanol	5.8	30.0	Moderate
Toluene	2.9	12.0	Moderate
n-Hexane	17.6	73.0	High
Methanol	12.8	53.3	High
Benzene	10.0	36.7	High
Xylene (mixed isomers)	0.8-1.2	5.0-6.0	Moderate
Glycerol	0.0003	0.0033	Very Low
Mercury	0.00017	0.0026	Very Low

Vapor Pressure and Safety Considerations

Understanding vapor pressure is critical for several safety aspects:

Inhalation exposure: High vapor pressure substances can quickly reach harmful airborne concentrations.
Fire and explosion hazards: Substances with high vapor pressures can rapidly form flammable or explosive mixtures with air.
Container pressurization: Sealed containers of volatile liquids can build up pressure at elevated temperatures.
Cold injury potential: Rapid evaporation of very high vapor pressure liquids can cause freezing injuries.
Confined space hazards: Even moderate vapor pressure substances can create hazardous atmospheres in confined spaces.
Environmental release concerns: High vapor pressure substances can disperse widely in the environment.
Storage considerations: Temperature control is critical for substances with high vapor pressure.

Examples of Vapor Pressure Descriptions in SDSs

"Vapor pressure: 24.7 kPa (20°C)"
"Vapor pressure: 169.3 mm Hg at 25°C"
"Vapor pressure: <0.01 kPa (20°C)"
"Vapor pressure: 2.3 kPa (20°C); 12.3 kPa (50°C)"
"Vapor pressure: 7.0 psi (Reid method)"
"Vapor pressure: Negligible at room temperature"
"Vapor pressure: Not applicable (high melting point solid)"
"Vapor pressure: 0.13 kPa at 20°C (calculated using Antoine equation)"

Vapor Pressure Units and Conversions

Vapor pressure can be expressed in various units. Common conversions include:

Unit	Symbol	Conversion to kPa
Kilopascal	kPa	1 kPa = 1 kPa
Pascal	Pa	1 Pa = 0.001 kPa
Bar	bar	1 bar = 100 kPa
Millibar	mbar	1 mbar = 0.1 kPa
Atmosphere	atm	1 atm = 101.325 kPa
Millimeter of mercury	mmHg or Torr	1 mmHg = 0.133322 kPa
Pound per square inch	psi	1 psi = 6.89476 kPa

In SDSs, it is good practice to provide vapor pressure in kPa or mmHg, with the temperature clearly specified.

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), vapor pressure should be indicated in Section 9 of the Safety Data Sheet as part of the description of basic physical and chemical properties. This information is considered mandatory for liquids and gases, though it may be reported as "not applicable" for solids with negligible vapor pressure.

For transportation purposes, vapor pressure information may be required for determining appropriate packaging, tank specifications, and pressure relief requirements under regulations such as ADR, IMDG, and IATA.

Best Practices

When reporting vapor pressure in an SDS:

Specify the value in standard units (preferably kPa or mmHg)
Clearly indicate the temperature at which the vapor pressure was measured or calculated (typically 20°C or 25°C)
When relevant, provide vapor pressure at multiple temperatures (e.g., 20°C and 50°C) to show temperature dependence
For mixtures, consider reporting the vapor pressure of the most volatile component if it dominates the overall vapor pressure
For substances with very low vapor pressure, use appropriate qualifiers (e.g., "negligible," "<0.01 kPa")
Specify the test method used, especially for standardized methods like Reid Vapor Pressure
For calculated values, indicate the calculation method (e.g., "estimated using Antoine equation")
Ensure consistency between vapor pressure information and other volatility-related properties in the SDS (e.g., boiling point, evaporation rate)
For substances with high vapor pressure, include appropriate warnings about inhalation and fire hazards in the relevant SDS sections