Pure 12-hydroxystearic acid has a theoretical acid value of about 187 mg KOH/g, based on its molecular weight of about 300. Commercial grades usually sit a little lower, often from the high 170s to the mid 180s.
A good result is one close to the theoretical value, with a hydroxyl value that also stays high. The two figures only make sense together.
Why commercial 12-HSA is not pure
12-HSA is made by splitting hydrogenated castor oil. The product carries the other fatty acids from castor oil, mainly stearic and palmitic acid. It can also contain unsplit glycerides and estolides, which form when 12-HSA molecules react with each other.
- A low acid value suggests unsplit glycerides or estolides, which reduce grease yield.
- A normal acid value with a low hydroxyl value suggests more stearic acid or loss of hydroxyl groups during hydrogenation.
- A high iodine value shows incomplete hydrogenation of the HCO feedstock.
- A low melting point usually goes with more impurities.
What to ask for
Set a minimum acid value and a minimum hydroxyl value, each with a test method. Add a maximum iodine value, a melting point range, a colour limit and a moisture limit. If you make grease, consider a trial soap batch on a sample, because soap yield is the result you care about.
Ask the plant for certificates from several recent lots, not only the one tied to your sample. This shows how much the figures vary. Our guide on batch consistency explains how to read a spread of results.
Testing and companion limits
Acid value is measured by titration with potassium hydroxide, following standard methods from bodies such as AOCS or ISO. Make sure your lab and the plant use the same method and solvent. Small differences in method can move the result by a few units.
For more on the full set of limits, see how to write a specification for HCO or 12-HSA.
Acid value is one line in a longer specification. Grease makers usually read it alongside the items below:
- Hydroxyl value, set as a minimum, to confirm the hydroxyl groups survived hydrogenation.
- Iodine value, set as a maximum, to confirm the feedstock was fully hydrogenated.
- Melting point, set as a range, with the method named.
- Colour on a named scale, for light-coloured greases and cosmetic uses.
- Moisture, set as a maximum.
Reading certificates over time
One certificate tells you little. Ask for results from the last ten or so lots and plot acid value and hydroxyl value side by side. A steady pair of figures shows a well-run splitting and hydrogenation process. A drifting acid value, or a hydroxyl value that falls while the acid value holds, is a sign to ask questions before you order more.
Keep a retained sample of each lot you receive. If a grease batch underperforms, you can retest the 12-HSA and see whether the cause lies there or in your own process.
Next step
We can have 12-HSA samples tested independently against your limits and check each lot before loading. Book a free sourcing review to start.