Iodine value (IV) is the grams of iodine absorbed by 100 g of sample - a direct count of carbon-carbon double bonds per unit mass, measured by the Wijs method (AOCS Cd 1d / GB/T 5532). It sits on almost every fatty acid and ester certificate, yet it is the most misread number on the page: buyers compare IV across chemically unrelated esters, or read a falling IV as "more saturated" when nothing was hydrogenated. Both mistakes cost money.
The identity formula: IV = 25,380 x n / MW
For a pure compound with n double bonds and molecular weight M, the theoretical IV is 25,380 x n / M (253.8 g iodine per mole of C=C, expressed per 100 g). The worked examples across our range:
| Material | CAS | Formula / MW | Double bonds | Theoretical IV | Typical spec |
|---|---|---|---|---|---|
| Stearic acid | 57-11-4 | C18H36O2 / 284.5 | 0 | 0 | 2 max |
| Oleic acid | 112-80-1 | C18H34O2 / 282.5 | 1 | 89.9 | about 85-95, drifting lower in lower-purity grades |
| Methyl oleate | 112-62-9 | C19H36O2 / 296.5 | 1 | 85.6 | mid-80s |
| Butyl oleate | 142-77-8 | C22H42O2 / 338.6 | 1 | 74.9 | 70s |
| Sorbitan monooleate | 1338-43-8 | C24H44O6 / 428.6 | 1 | 59.2 | 62-76 (NF; anhydrosorbitol isomer mixture) |
| Sorbitan trioleate | 26266-58-4 | C60H108O8 / 957.5 | 3 | 79.5 | 77-85 |
| Castor oil | 8001-79-4 | triglyceride, ~932 | 3 | ~82 | 82-90 |
| Hydrogenated castor oil | 8001-78-3 | triglyceride, ~932 | 0 | 0 | 5 max |
One check is worth committing to memory: the theoretical IV closes the identity loop the same way saponification value does. A "sorbitan trioleate" certified at IV 40 is not sorbitan trioleate - the arithmetic forbids it.
Trap 1: the same unsaturation, four different IVs
Oleic acid, methyl oleate, butyl oleate and sorbitan monooleate all carry exactly one double bond per molecule, yet their IVs fall from 90 to the low 60s. Nothing became more saturated - the molecule simply got heavier, so each 100 g contains fewer molecules and fewer double bonds. IV is a per-gram count, not a per-molecule one. The discipline that follows: compare IV only within the same ester family. A polyglyceryl oleate (for example Polyglyceryl-2 Oleate, CAS 49553-76-6; 9007-48-1 or Polyglyceryl-10 Oleate, CAS 79665-93-3) will sit lower than methyl oleate for structural reasons, and that tells you nothing about which is "more stable" per double bond.
Trap 2: IV also moves when the fatty-acid distribution moves
Commercial "oleic acid" is a blend, not a molecule. Higher-purity grades (90% oleic) cluster near the theoretical 89.9; tallow-based 65-80% grades carry more palmitic and stearic acid, and their IV drifts downward even though the double bonds that remain are just as oxidisable. Conversely, a rising IV in a nominally identical grade means the supplier shifted the oleate/linoleate balance - linoleic acid has two double bonds and pulls IV up while quietly worsening oxidation behaviour. IV is a blend meter before it is a purity meter.
Hydrogenation is the only move that truly collapses IV
Castor oil reads IV 82-90 with a hydroxyl value of about 160-168; run it through hydrogenation and IV collapses to 5 max while the hydroxyl value stays around 145-165 - the C=C count went to zero, the OH count did not. The two numbers measure different things, which is why both appear on the certificate. Note also that hydrogenation changes the legal identity: castor oil is CAS 8001-79-4, hydrogenated castor oil is CAS 8001-78-3, and it arrives as a 86-88 °C wax. That hydrogenated base is the starter for PEG-40 Hydrogenated Castor Oil and PPG-3 Hydrogenated Castor Oil - and once ethoxylated, those materials carry a different risk to check on the COA, the one described in our 1,4-dioxane residue guide.
What IV predicts: drying behaviour and oxidation
The classic paint-industry classification still explains why cosmetic oleates store well: oils above IV 130 are drying (they auto-oxidise into a solid film - linseed oil sits above 170), IV 115-130 is semi-drying, and below 115 is non-drying. Every oleate ester on our range - methyl, butyl, sorbitan, polyglyceryl - sits in the non-drying zone, which is exactly what you want in an emollient. But IV also counts the sites where oxidation starts: each additional double bond multiplies the oxidation rate, so a linoleate-rich batch oxidises far faster than an oleate-rich one even at the same storage temperature. Monitor ageing with the peroxide value, anisidine value and TOTOX triangle described in our oxidation indices guide; use IV to predict the risk class before the first drum is opened.
Reception checks that actually work
- Ask for the method: Wijs (AOCS Cd 1d, GB/T 5532) is the default; results only hold for non-conjugated double bonds, so conjugated systems need a different approach.
- Triangulate with saponification value (average MW) and hydroxyl value (free OH): the three together separate "wrong material" from "wrong grade" in one pass.
- Compare only within a family: sorbitan oleate against sorbitan oleate, polyglyceryl oleate against polyglyceryl oleate - never a monoester against a triester.
- For saturated anchors, stearic acid (CAS 57-11-4) should read IV 2 max and methyl cocoate C12-14 (CAS 61788-59-8) stays in the single-to-low-teens, inherited from coconut oil's IV of roughly 6-12.
Where our unsaturated range sits
From acids to esters to ethoxylates, with CAS as stated: Oleic Acid (112-80-1), Stearic Acid (57-11-4), Methyl Oleate (112-62-9), Butyl Oleate (142-77-8), Sorbitan Oleate (1338-43-8), Sorbitan Trioleate (26266-58-4), Sorbitan Sesquioleate (8007-43-0), Polyglyceryl-2 Oleate, Polyglyceryl-10 Oleate, Polyglyceryl-3 Polyricinoleate (29894-35-7; 235783-76-3), Castor Oil (8001-79-4), PEG-40 Hydrogenated Castor Oil (61788-85-0), PPG-3 Hydrogenated Castor Oil (111870-68-9) and Methyl Cocoate C12-14 (61788-59-8). Sample packs ship worldwide; request the full COA with iodine, saponification and hydroxyl values per lot.