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Three numbers do most of the work on a castor oil spec. Acid value shows how much free fatty acid is present — lower means fresher, more refined oil. Iodine value measures unsaturation (reactive double bonds), which drives oxidation and drying behaviour. Hydroxyl value measures the –OH groups that make castor oil unique — it is what enables polyurethane and ester chemistry. Read together, they tell you the oil's quality, reactivity and suitability for your process.
Castor oil is bought to a specification, not by appearance alone. Most of that specification comes down to a handful of titration values that describe the oil's chemistry in numbers. Acid value, iodine value and hydroxyl value are the three headline figures — between them they capture how refined the oil is, how reactive it is, and whether it will perform in the application you have in mind. Everything else on the sheet supports these three.
What it measures: the quantity of free fatty acids in the oil, reported as the milligrams of potassium hydroxide (KOH) needed to neutralise them in one gram of oil.
Why it matters: free fatty acids form when an oil is exposed to moisture, heat or age. A low acid value signals a fresh, well-refined oil; a high acid value can point to less refining or some degradation, and may cause problems in acid-sensitive reactions. Premium grades such as First Special Grade carry a low acid value, while Commercial Grade sits higher.
| Grade | Typical acid value (mg KOH/g) |
|---|---|
| First Special Grade (FSG) | ~1.5–2.0 |
| Pale Pressed / FPD | ~2.0–3.0 |
| Commercial Grade (CCO) | ~3.0–5.0 |
What it measures: the degree of unsaturation — how many carbon-carbon double bonds the oil contains — expressed as grams of iodine absorbed per 100 g of oil.
Why it matters: double bonds are reactive sites. A higher iodine value means more unsaturation, which affects how readily the oil oxidises, polymerises or "dries." For castor oil the value typically sits around 82–90. This figure becomes especially important if the oil is destined for dehydration into dehydrated castor oil (DCO), where the unsaturation is deliberately increased to create a drying oil for coatings.
What it measures: the amount of available hydroxyl (–OH) groups, reported as an equivalent in mg KOH/g.
Why it matters: this is the value that sets castor oil apart from almost every other vegetable oil. Most oils have a hydroxyl value near zero; castor oil sits around 160–168 because of the hydroxyl group on its dominant fatty acid, ricinoleic acid. Those –OH groups are what let castor oil act as a natural polyol in polyurethanes and undergo the esterification used to make lubricant esters and surfactants. If your process relies on castor's reactivity, this is the number to watch. (For the chemistry behind it, see what is ricinoleic acid.)
Around the three headline figures sit several confirming parameters:
| Parameter | What it confirms | Indicative range |
|---|---|---|
| Saponification value | Average fatty-acid chain length / ester content | ~176–187 mg KOH/g |
| Moisture & volatiles | Water content; stability and shelf life | Max ~0.20–0.30% |
| Colour (Gardner) | Appearance and grade | ~1–3 (FSG), higher for CCO |
| Specific gravity @ 25°C | Density / identity | ~0.957–0.961 |
| Refractive index @ 25°C | Purity / identity | ~1.473–1.477 |
No single number tells the whole story. A pale oil with a low acid value but an unexpected hydroxyl or iodine value may not behave as you expect in a reaction. The practical approach is to agree the two or three values that matter most for your application up front, then check each batch's Certificate of Analysis against them. Our guide on understanding a castor oil CoA walks through how to verify a certificate, and how to choose the right grade maps these values to grades.
For premium First Special Grade, an acid value around 1.5–2.0 mg KOH/g is typical; commercial grade runs higher. Lower generally indicates fresher, more refined oil. Always confirm against your agreed specification.
It measures unsaturation — the number of reactive double bonds. Castor oil sits around 82–90. Higher values mean more reactivity to oxidation and drying, which matters for products like dehydrated castor oil.
Because roughly 85–90% of its fatty acids are ricinoleic acid, which carries a hydroxyl group. Most vegetable oils have a hydroxyl value near zero; castor oil sits around 160–168, which enables its polyurethane and ester chemistry.
They are determined by standard titration methods (for example AOCS methods). A credible Certificate of Analysis states the method used alongside each measured result.
