For commercial castor oil, the acid value should be low, the iodine value usually sits in a natural range of roughly 82 to 90, and the colour should be pale. Exact limits depend on the grade, such as FSG or BSS, and on your use.
Treat these three tests differently. Acid value and colour are quality measures you set limits for. Iodine value is mainly an identity check.
Acid value
Acid value is the milligrams of potassium hydroxide needed to neutralise the free acids in one gram of oil. A low figure means the oil has little free fatty acid.
High acid value usually comes from damaged or damp seed, slow processing or poor storage. It matters because free acids can affect reactions in your process and corrode equipment. For illustration, first special grade is commonly traded with a maximum of around 2, and BSS grade lower, often around 1. Agree your own limit.
Iodine value
Iodine value measures unsaturation, in grams of iodine absorbed per 100 grams of oil. Castor oil has a fairly narrow natural range, roughly 82 to 90, because its fatty acid mix is dominated by ricinoleic acid.
A figure well outside this range suggests the oil is not pure castor oil, has been blended with another oil, or has been modified. Hydrogenated castor oil, for example, has a very low iodine value. So use iodine value to confirm identity, together with hydroxyl value and saponification value.
Colour
Colour is measured on the Gardner scale or the Lovibond scale. Make sure your spec names the scale, cell size for Lovibond, and method. Paler oil generally means better seed, careful processing and less heat damage. Colour matters if it carries through to your product, as in some coatings, inks and personal care uses.
Colour can darken with heat and storage, so test at loading, not only at the plant.
Other tests to include
All figures here are typical values for illustration. Specify test methods, such as AOCS or ISO methods, and use a lab both sides accept. The grade differences are explained in BSS grade castor oil and FSG, and common problems in quality complaints with castor and edible oils.
- Hydroxyl value: roughly in the 160s for castor oil, and the key figure for polyurethane and derivative makers.
- Saponification value: roughly 175 to 187.
- Moisture and volatile matter: a low maximum.
- Insoluble impurities: a low maximum.
- Specific gravity and refractive index: identity checks.
- Ricinoleic acid content by gas chromatography, where your use needs it.
How we help
We write castor oil specifications with you, draw sealed samples at the crusher and at loading, and send them to an NABL-accredited lab for acid value, iodine value, hydroxyl value and colour. You see the original reports before the container leaves. Ask for a free sourcing review.