Conductivity

Detailed Description

Conductivity is a measure of a material's ability to conduct electric current. It is the reciprocal of resistivity and quantifies how easily electric charges can flow through a material when subjected to an electric field. In the context of a Safety Data Sheet (SDS), conductivity information is important for assessing potential hazards related to static electricity, electrical safety, and material handling considerations.

There are two main types of conductivity relevant to chemical safety:

Electrical conductivity: The ability to conduct electric current, typically measured in siemens per meter (S/m) or microsiemens per centimeter (μS/cm).
Thermal conductivity: The ability to conduct heat, typically measured in watts per meter-kelvin (W/(m·K)), which is not covered in this document.

Electrical conductivity (σ) is defined by Ohm's law and is related to resistivity (ρ) by:

σ = 1/ρ = J/E

Where:

σ is the electrical conductivity (S/m)
ρ is the electrical resistivity (Ω·m)
J is the current density (A/m²)
E is the electric field strength (V/m)

For solutions, conductivity is often expressed as specific conductance (κ), which is the conductivity of the solution normalized to standard conditions.

Importance in Safety Data Sheets

Conductivity information in an SDS is important for several reasons:

Static electricity hazards: Materials with low conductivity can accumulate static charges, potentially leading to discharges that could ignite flammable atmospheres.
Grounding and bonding requirements: Helps determine appropriate grounding and bonding procedures for handling and storage.
Electrical classification: Relevant for determining appropriate electrical equipment in hazardous areas.
Corrosion potential: For electrolyte solutions, conductivity correlates with ionic strength and potential corrosivity.
Material compatibility: Affects compatibility with electrical equipment and potential for galvanic corrosion.
Process safety: Important for processes involving electrical heating, electrostatic operations, or electrochemical reactions.
Environmental fate: For aqueous solutions, conductivity can indicate dissolved solids content and potential environmental impact.
Quality control: Often used as a specification parameter for water purity, solution concentration, or material consistency.

Measurement Methods

Several techniques are used to measure electrical conductivity:

Method	Description	Typical Applications
Conductivity Cell (AC)	Uses alternating current and two or four electrodes to measure conductance of a solution	Electrolyte solutions, water quality testing
Four-Point Probe	Uses four aligned probes to measure sheet resistance of thin films or solid surfaces	Semiconductors, thin films, solid materials
Eddy Current Testing	Non-contact method using electromagnetic induction	Metals, alloys, conductive materials
Van der Pauw Method	Uses four contacts at the periphery of a flat sample	Thin films, semiconductor wafers
Impedance Spectroscopy	Measures impedance over a range of frequencies	Complex materials, interfaces, electrochemical systems
Toroidal Conductivity	Inductive method using two toroidal coils	Highly conductive or corrosive solutions
Resistivity Meter	Direct measurement of resistance, converted to resistivity	Solids, powders (with compression)
ASTM Methods	Standardized procedures (e.g., ASTM D1125, D5391)	Regulatory testing, specification compliance

Conductivity Units and Typical Values

Electrical conductivity can be expressed in various units:

Unit	Symbol	Equivalent in SI Units	Common Usage
Siemens per meter	S/m	1 S/m	SI unit, scientific applications
Millisiemens per centimeter	mS/cm	0.1 S/m	Highly conductive solutions
Microsiemens per centimeter	μS/cm	0.0001 S/m	Moderately conductive solutions, water quality
Ohm-meter	Ω·m	Resistivity (1/(S/m))	Resistivity of solids and poorly conductive materials
Ohm-centimeter	Ω·cm	0.01 Ω·m	Semiconductor industry, materials science

Typical conductivity values for various materials:

Material Category	Conductivity Range (S/m)	Classification	Examples
Metals	10⁶ - 10⁸	Conductor	Silver (6.3×10⁷ S/m), Copper (5.9×10⁷ S/m), Aluminum (3.5×10⁷ S/m)
Semiconductors	10⁻⁶ - 10⁴	Semiconductor	Silicon (4.3×10⁻⁴ S/m), Germanium (2.2 S/m), Graphite (3×10⁴ S/m)
Strong Electrolyte Solutions	1 - 30	Strong Electrolyte	Seawater (5 S/m), 5% NaCl solution (8.2 S/m)
Weak Electrolyte Solutions	0.001 - 1	Weak Electrolyte	Tap water (0.005-0.05 S/m), Weak acids and bases
Insulators	<10⁻¹²	Insulator	Glass (10⁻¹⁰-10⁻¹⁴ S/m), Rubber (10⁻¹⁴ S/m), Most plastics (10⁻¹⁶-10⁻¹² S/m)

Conductivity of common aqueous solutions at 25°C:

Solution	Conductivity (μS/cm)	Notes
Ultrapure Water	0.055	Theoretical minimum for water due to self-ionization
Distilled Water	0.5-3	Varies with CO₂ absorption and trace impurities
Drinking Water	50-1000	Varies with mineral content
Seawater	50,000	High salt content
0.1 M HCl	39,200	Strong acid
0.1 M NaOH	22,700	Strong base
0.1 M Acetic Acid	570	Weak acid
0.1 M NaCl	10,600	Strong electrolyte

Factors Affecting Electrical Conductivity

For Solutions

Several factors affect the electrical conductivity of solutions:

Concentration: Generally, conductivity increases with electrolyte concentration, though not always linearly due to ion-ion interactions at higher concentrations.
Temperature: Conductivity typically increases with temperature (about 1-3% per °C for aqueous solutions) due to increased ion mobility.
Nature of electrolyte: Strong electrolytes (completely dissociated) have higher conductivity than weak electrolytes at the same concentration.
Ion mobility: Smaller ions and ions with higher charge generally have higher mobility and contribute more to conductivity.
Viscosity: Higher viscosity reduces ion mobility and decreases conductivity.
Solvent properties: Dielectric constant and other solvent properties affect ion dissociation and mobility.

For Solids

For solid materials, conductivity is affected by:

Temperature: For metals, conductivity decreases with increasing temperature due to increased electron scattering. For semiconductors, conductivity typically increases with temperature due to increased charge carrier concentration.
Crystal structure: Defects, grain boundaries, and crystal orientation affect electron mobility.
Impurities: Even small amounts of impurities can significantly affect conductivity, especially in semiconductors.
Pressure: Higher pressure generally increases conductivity by reducing atomic spacing.
Moisture content: For many materials, absorbed moisture can significantly increase conductivity.
Particle size and compaction: For powders and granular materials, conductivity depends on particle-to-particle contact and compaction.

For Mixtures and Composites

For heterogeneous materials:

Composition: Relative amounts of conductive and non-conductive components.
Dispersion: Distribution of conductive components within the matrix.
Percolation threshold: Minimum concentration of conductive component needed to form a continuous conductive network.
Interface properties: Contact resistance between different phases or components.
Orientation: For anisotropic materials, conductivity may vary with direction.

Static Electricity Hazards and Conductivity

Conductivity is a critical factor in assessing static electricity hazards:

Insulating materials (conductivity <10⁻⁸ S/m): Can accumulate static charges that may persist for long periods. Examples include most plastics, glass, and dry powders.
Static dissipative materials (conductivity 10⁻⁸ to 10⁻⁴ S/m): Allow charges to flow slowly, reducing accumulation. Examples include anti-static plastics and some rubbers.
Conductive materials (conductivity >10⁻⁴ S/m): Rapidly dissipate static charges when properly grounded. Examples include metals, carbon-filled plastics, and aqueous solutions.

For liquids, the following guidelines are often used:

High static risk: Conductivity <50 pS/m (e.g., ultra-low sulfur diesel, toluene)
Moderate static risk: Conductivity 50-10,000 pS/m (e.g., kerosene, xylene)
Low static risk: Conductivity >10,000 pS/m (e.g., alcohols, aqueous solutions)

Static electricity control measures should be implemented based on conductivity and other factors such as flash point, flammable range, and minimum ignition energy.

Examples of Conductivity Descriptions in SDSs

"Electrical conductivity: 5.8×10⁷ S/m at 20°C"
"Conductivity: 150 μS/cm (0.015 S/m) at 25°C"
"Specific conductance: <10 pS/m (requires static electricity precautions)"
"Electrical resistivity: 1.68×10⁻⁸ Ω·m at 20°C"
"Conductivity: Not applicable for non-ionizing substances"
"Electrical conductivity: Increases with moisture content (dry: <10⁻¹² S/m; wet: ~10⁻⁶ S/m)"
"Conductivity: 5.2 mS/cm at 20°C (1% solution in water)"
"Electrical conductivity: Non-conductive (insulator)"

Regulatory Considerations

While electrical conductivity is not specifically mandated by GHS for all substances, it is often included in Section 9 of Safety Data Sheets as supplementary information that helps users assess potential hazards and appropriate handling procedures.

Conductivity information is particularly relevant for:

Flammable liquids: Low conductivity (<50 pS/m) may require special handling to prevent static electricity hazards.
Powders and dusts: Conductivity affects potential for dust explosion hazards and static accumulation.
Electrolyte solutions: Conductivity correlates with concentration and potential corrosivity.
Materials used in electrical applications: Conductivity is critical for proper functioning and safety.

In some industries (e.g., petroleum, chemical manufacturing), specific conductivity requirements may be mandated by industry standards or company policies to ensure safe handling and processing.

Best Practices

When reporting conductivity in an SDS:

Express the value in standard units (preferably S/m, mS/cm, or μS/cm)
Clearly indicate the temperature at which the measurement was made
For solutions, specify the concentration if the measurement was made on a diluted sample
For materials with variable conductivity (e.g., dependent on moisture content), provide appropriate ranges or conditions
For substances where conductivity is not applicable or not relevant, clearly state this
For materials with very low conductivity that may present static electricity hazards, include appropriate warnings and precautions in Section 7 (Handling and Storage)
For electrolyte solutions, consider providing information on specific conductance at standard concentration
For semiconductors or materials with strongly temperature-dependent conductivity, consider providing values at multiple temperatures or temperature coefficients
Ensure consistency between conductivity information and other related properties in the SDS (e.g., corrosivity, static electricity hazards)