Solubility in Water

Detailed Description

Water solubility is the ability of a substance to dissolve in water, forming a homogeneous solution. It is a fundamental physical property that describes the maximum amount of a substance that can dissolve in a given quantity of water at specific temperature and pressure conditions. In the context of a Safety Data Sheet (SDS), water solubility provides critical information for environmental fate, exposure assessment, handling procedures, and emergency response planning.

Water solubility can be expressed in several ways:

Quantitative: Mass of solute per volume of water (e.g., g/L, mg/L, ppm)
Mass fraction: Mass of solute per mass of solution (e.g., g/kg, % w/w)
Molar: Moles of solute per volume of water (e.g., mol/L)
Qualitative: Descriptive terms (e.g., "soluble," "slightly soluble," "insoluble")

Key concepts related to water solubility include:

Saturation: The point at which no more solute can dissolve in the solvent at a given temperature and pressure.
Temperature dependence: For most solids, solubility increases with temperature; for gases, solubility typically decreases with increasing temperature.
Miscibility: The ability of liquids to mix in all proportions, forming a homogeneous solution (e.g., ethanol and water are miscible).
Immiscibility: The inability of liquids to form a homogeneous solution (e.g., oil and water are immiscible).
Hydrophilicity: The tendency of a molecule or portion of a molecule to interact with or be dissolved by water.
Hydrophobicity: The tendency of a molecule or portion of a molecule to repel water or resist being dissolved by water.

Importance in Safety Data Sheets

Water solubility information in an SDS is important for several reasons:

Environmental fate: Indicates how a substance will distribute in the environment and its potential mobility in soil and water systems.
Bioavailability: Affects the potential for a substance to be taken up by organisms.
Spill response: Guides appropriate containment and cleanup procedures for spills.
Fire fighting: Indicates whether water can be used effectively for fire suppression and whether the substance will dissolve in firefighting water runoff.
Exposure assessment: Helps evaluate potential routes of exposure, particularly for aquatic environments.
Waste disposal: Informs appropriate disposal methods and potential environmental impacts.
Process design: Essential for equipment selection, material compatibility, and process control.
Formulation: Critical for product development and stability considerations.

Measurement Methods

Several techniques are used to determine water solubility:

Method	Description	Typical Applications
Flask Method	Direct measurement of the concentration of a saturated solution	Moderately soluble substances
Column Elution Method	Passing water through a column containing the test substance and analyzing the eluate	Sparingly soluble substances
Shake-Flask Method	Mixing excess solute with water, allowing equilibration, and measuring the concentration in the aqueous phase	General purpose, widely used
Generator Column Method	Passing water through a column packed with an inert support coated with the test substance	Very slightly soluble substances
Slow-Stirring Method	Gently stirring the test substance with water to avoid formation of emulsions	Hydrophobic substances
OECD Test Guidelines	Standardized procedures (e.g., OECD 105, 120)	Regulatory testing
Computational Methods	Estimation based on structure-property relationships	Screening, preliminary assessment

Solubility Categories

Water solubility is often described using standardized qualitative terms, particularly in pharmacopoeias and chemical reference works:

Descriptive Term	Approximate Solubility Range	Parts of Solvent per Part of Solute	Example Substances
Very soluble	>1000 g/L	<1	Sodium hydroxide, sucrose, potassium acetate
Freely soluble	100-1000 g/L	1-10	Potassium chloride, citric acid, glucose
Soluble	33-100 g/L	10-30	Sodium chloride, copper sulfate, zinc chloride
Sparingly soluble	10-33 g/L	30-100	Potassium bromide, borax, potassium chlorate
Slightly soluble	1-10 g/L	100-1000	Calcium hydroxide, benzoic acid, lead chloride
Very slightly soluble	0.1-1 g/L	1000-10,000	Calcium sulfate, silver chloride, barium carbonate
Practically insoluble or insoluble	<0.1 g/L	>10,000	Calcium carbonate, silver iodide, most oils

For liquids, additional terms are used:

Miscible: Capable of mixing in all proportions (e.g., ethanol and water)
Partially miscible: Mixes to a limited extent (e.g., diethyl ether and water)
Immiscible: Does not mix to form a homogeneous solution (e.g., oil and water)

Water Solubility of Common Substances

Substance	Water Solubility at 20°C	Descriptive Term	Notes
Sodium Chloride (Table Salt)	359 g/L	Soluble	Increases slightly with temperature
Sucrose (Table Sugar)	2000 g/L	Very soluble	Increases significantly with temperature
Ethanol	Miscible	Very soluble	Mixes in all proportions
Acetone	Miscible	Very soluble	Mixes in all proportions
Methanol	Miscible	Very soluble	Mixes in all proportions
Glycerol	Miscible	Very soluble	Mixes in all proportions
Acetic Acid	Miscible	Very soluble	Mixes in all proportions
Benzene	1.8 g/L	Slightly soluble	Forms separate layer
Toluene	0.52 g/L	Very slightly soluble	Forms separate layer
n-Hexane	0.0095 g/L	Practically insoluble	Forms separate layer
Calcium Carbonate	0.013 g/L	Practically insoluble	Increases with CO₂ presence
Calcium Sulfate	2.1 g/L	Slightly soluble	Decreases with temperature
Copper Sulfate	203 g/L	Freely soluble	Increases with temperature
Vegetable Oil	<0.001 g/L	Practically insoluble	Forms separate layer
Oxygen (gas)	0.04 g/L	Very slightly soluble	Decreases with temperature
Carbon Dioxide (gas)	1.7 g/L	Slightly soluble	Decreases with temperature

Factors Affecting Water Solubility

Molecular Structure

The structure of a molecule significantly affects its water solubility:

Polar groups: Functional groups that can form hydrogen bonds with water (e.g., -OH, -NH₂, -COOH) generally increase water solubility.
Hydrocarbon chains: Longer hydrocarbon chains decrease water solubility due to their hydrophobic nature.
Ionic compounds: Generally have higher water solubility due to ion-dipole interactions with water molecules.
Molecular size: Larger molecules tend to have lower water solubility due to increased energy required for cavity formation in the water structure.

Temperature

Temperature affects water solubility in different ways depending on the nature of the solute:

Solids: Solubility typically increases with temperature due to increased kinetic energy and enhanced ability to overcome intermolecular forces.
Gases: Solubility typically decreases with increasing temperature due to increased kinetic energy of gas molecules, making it easier for them to escape from solution.
Endothermic dissolution: Substances that absorb heat when dissolving show increased solubility with temperature.
Exothermic dissolution: Substances that release heat when dissolving may show decreased solubility with temperature, though this is less common.

Pressure

Pressure effects on solubility vary by phase:

Gases: Solubility increases with pressure according to Henry's Law.
Liquids and solids: Pressure has minimal effect on solubility unless the pressure is extremely high.

pH

For ionizable compounds, pH can dramatically affect water solubility:

Acids: More soluble at higher pH (more basic conditions) due to ionization.
Bases: More soluble at lower pH (more acidic conditions) due to ionization.
Amphoteric compounds: Solubility varies complexly with pH, often with minimum solubility at the isoelectric point.

Environmental and Safety Implications

Water solubility has significant implications for environmental fate and safety:

Environmental mobility: Water-soluble substances can be transported by rainfall, groundwater, and surface water, potentially spreading contamination over large areas.
Bioavailability: Water-soluble substances are generally more bioavailable to aquatic organisms and may be more readily absorbed by terrestrial organisms.
Persistence: Water-soluble substances may be more susceptible to biodegradation and photodegradation but can also contaminate water resources more readily.
Spill response: Water-soluble spills may be more difficult to contain using conventional methods like booms and may require different cleanup approaches.
Fire hazards: Water-soluble flammable substances may spread fire through water runoff during firefighting operations.
Exposure routes: Water-soluble substances may present greater risks for dermal absorption and can contaminate drinking water sources.

Examples of Water Solubility Descriptions in SDSs

"Solubility in water: 359 g/L at 20°C"
"Water solubility: Miscible in all proportions"
"Solubility: Insoluble in water (<0.1 g/L at 20°C)"
"Water solubility: 1.8 g/L at 20°C; 2.3 g/L at 25°C"
"Solubility in water: Slightly soluble (1-10 g/L)"
"Water solubility: Forms emulsion"
"Solubility: Reacts with water, hydrolysis occurs"
"Water solubility: pH-dependent; 0.5 g/L at pH 5, 15 g/L at pH 7, 300 g/L at pH 9"

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), water solubility 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 substances and mixtures.

For environmental hazard classification, water solubility is a critical parameter for determining potential aquatic toxicity and environmental fate under regulations such as EU CLP, US EPA, and various national environmental protection laws.

Best Practices

When reporting water solubility in an SDS:

Provide quantitative data whenever possible (g/L, mg/L)
Clearly indicate the temperature at which the solubility was measured (typically 20°C or 25°C)
For pH-dependent solubility, specify the solubility at different pH values
For liquids, clearly indicate whether they are miscible, partially miscible, or immiscible with water
For substances that react with water, indicate this behavior rather than reporting a solubility value
For substances with very low solubility, use appropriate qualifiers (e.g., "practically insoluble," "<0.1 g/L")
Specify the test method used, especially for standardized methods
For calculated values, indicate the calculation method
Ensure consistency between water solubility information and other related properties in the SDS (e.g., partition coefficient, environmental fate)
For substances that form emulsions or dispersions rather than true solutions, clearly indicate this behavior