Iodine Value: Unsaturation Index for Oils
The iodine value (IV) measures the grams of iodine absorbed by 100 grams of fat or oil under controlled conditions. Because iodine adds across carbon–carbon double bonds, the metric quantifies total unsaturation—a key determinant of oxidative stability, melting behaviour, and nutritional profile. Use this explainer alongside the peroxide value guide to link unsaturation with observed oxidation.
Laboratories typically follow Wijs or Hanus methods, each relying on iodination followed by sodium thiosulfate back-titration. Review the serial dilution calculator to design suitable thiosulfate concentrations and ensure volumetric precision consistent with ISO 3961.
Definition and Test Chemistry
The iodine value is calculated using the expression IV = [(B − S) × N × 12.69] / m, where B and S are the blank and sample titration volumes of sodium thiosulfate in millilitres, N is the normality of the thiosulfate solution, 12.69 is the conversion factor relating iodine uptake to titrant consumption, and m is the sample mass in grams. The factor 12.69 derives from the molar mass of iodine (253.8 g/mol) and the stoichiometry of the iodometric reaction.
Maintaining samples at 20–25 °C and shielding them from light prevents side reactions with atmospheric oxygen. Laboratories also ensure that the glacial acetic acid or cyclohexane solvent contains minimal water to avoid premature hydrolysis.
Historical Development
The iodine value originated in the late 1800s as chemists sought rapid ways to classify drying oils used in paints and varnishes. Arthur Wijs introduced the iodine monochloride reagent in 1898, providing a more controlled reaction than elemental iodine alone. Later, Jan Hanus developed an iodine bromide solution that improved stability for highly unsaturated oils.
Organisations such as the AOCS, ISO, and Codex Alimentarius codified Wijs and Hanus methods, refining reaction times, solvent systems, and endpoint detection. Modern updates emphasise photometric verification of thiosulfate normality and blank corrections to meet ISO/IEC 17025 requirements.
Conceptual Foundations
Relation to Degree of Unsaturation
Each mole of iodine reacts with one mole of double bonds. Therefore, the iodine value is proportional to the total number of unsaturated bonds per unit mass. Highly unsaturated oils such as linseed exhibit IV above 180 g I2/100 g, while saturated fats like coconut oil record IV below 10.
Impact on Oxidative Stability
More double bonds increase susceptibility to peroxidation. Trending iodine value alongside peroxide value provides early insight into storage stability and informs antioxidant dosing strategies.
Integration with Other Quality Metrics
Comparing iodine value with saponification value reveals average chain length versus unsaturation, enabling rapid detection of adulteration or blending inconsistencies. Coupling IV with acid value further distinguishes hydrolysis from oxidative change.
Applications
Drying Oils and Coatings
Paint formulators classify oils as drying (IV > 130), semi-drying (IV 100–130), or non-drying (IV < 100). The classification governs curing time, film hardness, and susceptibility to yellowing. Spec sheets often state iodine value alongside viscosity and colour metrics.
Food Industry and Nutrition
Dietitians and regulatory bodies use iodine value to infer the proportion of mono- and polyunsaturated fatty acids. High IV oils support soft margarines and cold-weather biodiesel but require antioxidants to manage oxidative rancidity.
Biodiesel Classification
European biodiesel standard EN 14214 limits iodine value to ≤120 g I2/100 g to prevent polymerisation in fuel systems. Producers therefore blend feedstocks such as rapeseed and tallow to meet both IV limits and cold-flow requirements.
Importance and Future Trends
While gas chromatography delivers detailed fatty acid profiles, iodine value remains a rapid, low-cost proxy for unsaturation. Portable spectroscopy and chemometric models now estimate IV in minutes, yet laboratories continue to validate these tools against the classic iodometric titration.
As alternative fats—such as precision-fermented lipids—enter markets, iodine value helps benchmark their performance against conventional oils. Pairing IV data with digital quality dashboards enables procurement teams, formulators, and sustainability leads to monitor sourcing decisions in real time.