Relative Density

Detailed Description

Relative density, also known as specific gravity, is the ratio of the density of a substance to the density of a reference substance at a specified temperature. It is a dimensionless quantity that provides a convenient way to compare the densities of different materials. In the context of a Safety Data Sheet (SDS), relative density provides important information for handling, storage, transportation, and emergency response planning.

The relative density (d) of a substance is defined by the formula:

d = ρ_substance / ρ_reference

Where:

d is the relative density (dimensionless)
ρ_substance is the density of the substance
ρ_reference is the density of the reference substance

For liquids and solids, the reference substance is typically water at 4°C (which has a density of 1.0000 g/cm³). For gases, the reference is usually air at standard conditions (which has a density of approximately 0.001225 g/cm³ at 20°C and 101.325 kPa).

Key concepts related to relative density include:

Specific gravity: Another term for relative density, particularly common in industrial applications.
Vapor density: The density of a vapor or gas relative to air (a special case of relative density).
API gravity: A measure used in the petroleum industry, related to relative density by the formula: API gravity = (141.5/relative density) - 131.5.
Buoyancy: Materials with relative density less than 1 will float in water; those with relative density greater than 1 will sink.
Temperature dependence: Relative density varies with temperature due to differential thermal expansion of the substance and the reference material.

Importance in Safety Data Sheets

Relative density information in an SDS is important for several reasons:

Spill response: Indicates whether a liquid will float or sink in water, affecting containment strategies.
Fire fighting: Determines if flammable liquids will float on water (potentially spreading fire) or sink (potentially being contained).
Ventilation design: Vapor density (relative to air) affects how vapors will behave in a space.
Material handling: Helps in equipment selection and handling procedures.
Storage considerations: Informs appropriate storage conditions and container selection.
Transportation: Useful for load planning and compliance with weight restrictions.
Process design: Essential for equipment sizing, flow calculations, and process control.
Quality control: Often used as a specification parameter for material acceptance.

Measurement Methods

Several techniques are used to determine relative density:

Method	Description	Typical Applications
Hydrometer	A calibrated float that sinks to different levels depending on the liquid's density	Liquids (e.g., battery acid, antifreeze, wine, milk)
Pycnometer	A container of precisely known volume used to compare the mass of the substance with the mass of an equal volume of reference substance	Liquids, solids (powders)
Digital Density Meter	Electronic devices that use oscillating U-tube technology to measure density	Precise measurements for liquids
Hydrostatic Weighing	Based on Archimedes' principle, comparing the weight of an object in air to its weight when submerged in a fluid of known density	Solids
Baumé Scale	A scale related to specific gravity, commonly used in industrial applications	Industrial liquids, acids, syrups
API Hydrometer	Specialized hydrometer calibrated in API gravity units	Petroleum products
ASTM Methods	Standardized procedures (e.g., ASTM D1298, D4052, D5355)	Petroleum products, chemicals

Relative Density vs. Absolute Density

Absolute Density

Definition: Mass per unit volume

Units: g/cm³, kg/m³, lb/ft³

Example: Water has a density of 0.9982 g/cm³ at 20°C

Advantage: Provides actual mass-volume relationship

Relative Density

Definition: Ratio of density to reference substance

Units: Dimensionless (no units)

Example: Water has a relative density of 0.9982 at 20°C (relative to water at 4°C)

Advantage: Easy comparison, directly indicates floating/sinking behavior

While both properties provide similar information, relative density offers several advantages in practical applications:

No units to convert between different measurement systems
Immediate indication of buoyancy in water (values < 1 float, values > 1 sink)
Often easier to measure accurately than absolute density
Commonly used in industry-specific applications (petroleum, brewing, etc.)

Relative Density of Common Substances

Substance	Relative Density at 20°C	State	Behavior in Water
Gasoline	0.71-0.77	Liquid	Floats
Diesel Fuel	0.82-0.86	Liquid	Floats
Ethanol	0.789	Liquid	Floats
Vegetable Oil	0.91-0.93	Liquid	Floats
Water (at 20°C)	0.9982	Liquid	N/A
Seawater	1.02-1.03	Liquid	Sinks in fresh water
Milk	1.03-1.04	Liquid	Sinks
Glycerol	1.26	Liquid	Sinks
Sulfuric Acid (98%)	1.84	Liquid	Sinks
Mercury	13.6	Liquid	Sinks
Ice	0.917	Solid	Floats
Polyethylene	0.91-0.96	Solid	Floats
Wood (pine)	0.35-0.55	Solid	Floats
Concrete	2.3-2.4	Solid	Sinks
Aluminum	2.7	Solid	Sinks
Steel	7.8-8.0	Solid	Sinks
Lead	11.34	Solid	Sinks
Methane	0.55 (vapor density)	Gas	N/A (rises in air)
Propane	1.55 (vapor density)	Gas	N/A (sinks in air)

Special Considerations

Vapor Density

Vapor density is a special case of relative density that compares the density of a gas or vapor to that of air. It is calculated as:

Vapor Density = Molecular Weight of Substance / Molecular Weight of Air (≈ 29 g/mol)

Vapor density is important for safety considerations because:

Vapors with density > 1 will tend to accumulate in low areas (basements, pits)
Vapors with density < 1 will tend to rise and accumulate near ceilings
This behavior affects ventilation design and emergency response planning

API Gravity

API gravity is a measure used specifically in the petroleum industry. It is related to relative density by the formula:

API Gravity = (141.5 / Relative Density at 60°F) - 131.5

API gravity is inversely related to relative density, meaning higher API values indicate lighter oils. This scale is used to classify petroleum products:

Light crude oil: API > 31.1° (Relative density < 0.87)
Medium crude oil: API between 22.3° and 31.1° (Relative density 0.87-0.92)
Heavy crude oil: API < 22.3° (Relative density > 0.92)
Extra heavy oil: API < 10° (Relative density > 1.0)

Temperature Effects

Relative density varies with temperature due to differential thermal expansion of the substance and the reference material. This is particularly important for precise measurements and quality control applications.

When reporting relative density in an SDS, it is important to specify:

The temperature at which the measurement was made (usually 20°C or 25°C)
The reference temperature for the reference substance (usually 4°C for water)

This is sometimes written in the format: d_20/4 = 0.998, indicating the relative density at 20°C compared to water at 4°C.

Examples of Relative Density Descriptions in SDSs

"Relative density: 0.789 at 20°C (water = 1)"
"Specific gravity: 1.84 (water = 1)"
"Relative density: 0.9982 at 20°C/4°C"
"Specific gravity: 0.82-0.86 at 15°C"
"Relative density: Not applicable for gases"
"Vapor density: 2.1 (air = 1)"
"Relative density: 1.05 ± 0.02 at 25°C"
"API gravity: 35.0° at 60°F (equivalent to relative density of 0.85)"

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), relative density 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 solids.

For transportation purposes, relative density information may be required for determining appropriate packaging, tank specifications, and weight limitations under regulations such as ADR, IMDG, and IATA.

Best Practices

When reporting relative density in an SDS:

Express the value as a dimensionless number (no units)
Clearly indicate the reference substance (typically "water = 1" for liquids and solids, "air = 1" for gases)
Specify the temperature at which the measurement was made (typically 20°C or 25°C)
For materials with variable density (e.g., ranges of composition), provide the appropriate range
For petroleum products, consider including API gravity in addition to relative density
For gases and volatile liquids, include vapor density (relative to air)
Specify the test method used, especially for standardized methods
Ensure consistency between relative density information and other related properties in the SDS (e.g., absolute density, solubility behavior)
When appropriate, include information about buoyancy in water (floats or sinks)