Density

Detailed Description

Density is a fundamental physical property that describes the mass of a substance per unit volume. It is a measure of how tightly matter is packed together. In the context of a Safety Data Sheet (SDS), density provides important information for handling, storage, transportation, and emergency response planning.

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

ρ = m/V

Where:

ρ (rho) is the density
m is the mass
V is the volume

Key concepts related to density include:

Absolute density: The mass per unit volume of a substance.
Relative density (specific gravity): The ratio of the density of a substance to the density of a reference substance (usually water at 4°C, which has a density of 1 g/cm³).
Bulk density: The mass of a material (including voids) per unit volume, particularly relevant for powders, granules, and other particulate materials.
Temperature dependence: Most substances expand when heated, resulting in decreased density at higher temperatures.
Pressure dependence: Liquids and solids are only slightly compressible, so their density changes minimally with pressure. Gases, however, show significant density changes with pressure.

Importance in Safety Data Sheets

Density information in an SDS is important for several reasons:

Material handling: Affects equipment selection and handling procedures.
Storage considerations: Determines stacking limitations, floor loading requirements, and container selection.
Transportation: Critical for load planning, vehicle selection, and compliance with weight restrictions.
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.
Process design: Essential for equipment sizing, flow calculations, and process control.
Formulation: Important for recipe calculations when components are measured by volume but formulations are specified by mass.

Measurement Methods

Several techniques are used to determine density:

Method	Description	Typical Applications
Pycnometry	Uses a container of precisely known volume (pycnometer) to determine mass-to-volume ratio	Liquids, solids (powders)
Hydrometry	Uses a calibrated float (hydrometer) that sinks to different levels depending on liquid density	Liquids (e.g., battery acid, antifreeze)
Oscillating U-tube	Measures the resonant frequency of a U-tube filled with the sample	Precise measurements for liquids and gases
Archimedes' Principle	Based on buoyancy and displacement of a fluid	Solids, especially irregularly shaped objects
Gas Pycnometry	Uses gas displacement to measure volume precisely	Porous solids, powders
Hydrostatic Weighing	Weighing an object in air and then in a fluid of known density	Solids
Digital Density Meters	Electronic devices that use various principles to measure density	Industrial quality control, laboratory analysis
ASTM Methods	Standardized procedures (e.g., ASTM D1298, D4052)	Petroleum products, chemicals

Density Units and Conversions

Density can be expressed in various units. Common units and conversions include:

System	Common Units	Conversion to g/cm³
SI (metric)	kg/m³	1 kg/m³ = 0.001 g/cm³
CGS	g/cm³	1 g/cm³ = 1 g/cm³
Imperial/US	lb/ft³	1 lb/ft³ = 0.016018 g/cm³
Imperial/US	lb/gal (US)	1 lb/gal = 0.119826 g/cm³
Imperial/US	lb/gal (UK)	1 lb/gal = 0.099776 g/cm³

In SDSs, density is typically reported in g/cm³ or kg/m³, with the temperature specified (usually 20°C or 25°C).

Density of Common Substances

Substance	Density at 20°C (g/cm³)	State	Notes
Air	0.001225	Gas	At sea level, standard pressure
Water	0.9982	Liquid	Maximum density at 4°C (1.0000 g/cm³)
Ethanol	0.789	Liquid	Less dense than water, miscible
Gasoline	0.71-0.77	Liquid	Floats on water
Diesel Fuel	0.82-0.86	Liquid	Floats on water
Sulfuric Acid (98%)	1.84	Liquid	Sinks in water
Mercury	13.6	Liquid	Very high density for a liquid
Sodium Chloride	2.16	Solid	Table salt
Iron	7.87	Solid	Common structural metal
Lead	11.34	Solid	High-density metal
Polyethylene	0.91-0.96	Solid	Common plastic, floats in water
Wood (pine)	0.35-0.55	Solid	Varies with species and moisture content

Density-Related Properties

Relative Density (Specific Gravity)

Relative density, also known as specific gravity, is the ratio of the density of a substance to the density of a reference substance. For liquids and solids, the reference substance is typically water at 4°C (1.0000 g/cm³). For gases, the reference is usually air at standard conditions.

Relative Density = ρ_substance / ρ_reference

Relative density is a dimensionless quantity and is particularly useful for:

Quickly determining if a liquid will float or sink in water (relative density < 1 will float, > 1 will sink)
Quality control in industries where composition affects density
Calculating mass from volume measurements

Vapor Density

Vapor density is the density of a vapor or gas compared to 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

Bulk Density

Bulk density is the mass of a material (including voids) per unit volume. It is particularly relevant for powders, granules, and other particulate materials. Bulk density is always less than absolute density due to the presence of voids between particles.

Bulk density is important for:

Storage space calculations
Transportation planning
Process equipment design
Packaging specifications

Temperature and Pressure Effects

Density varies with temperature and pressure:

Temperature effect: Most substances expand when heated, resulting in decreased density. Water is a notable exception between 0°C and 4°C, where it increases in density as temperature rises.
Pressure effect: Increased pressure generally increases density, with the effect being most significant for gases, moderate for liquids, and minimal for solids.

When reporting density in an SDS, it is important to specify the temperature (and for gases, the pressure) at which the measurement was made.

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), density or 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, 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 density in an SDS:

Specify the value in standard units (preferably g/cm³ or kg/m³)
Clearly indicate the temperature at which the density was measured (typically 20°C or 25°C)
For gases, include the pressure at which the density was determined
When relevant, provide both absolute density and relative density (specific gravity)
For materials with variable density (e.g., ranges of composition), provide the appropriate range
For powders and granular materials, specify whether the value is absolute density or bulk density
For mixtures with significant temperature-dependent density, consider providing values at multiple temperatures
Specify the test method used, especially for standardized methods
For substances where vapor density is relevant for safety (e.g., volatile substances), include this information
Ensure consistency between density information and other related properties in the SDS (e.g., solubility behavior)