Acid Value, Iodine Value and Hydroxyl Value: How to Read an Oleochemical Specification Sheet

A specification sheet for a fatty acid, fatty alcohol or glyceride usually lists a dozen numbers. Buyers tend to fixate on purity and price and skim past the rest, yet three of those numbers - acid value, iodine value and hydroxyl value - explain most of the batch-to-batch variation that later shows up on the production line. Read together, they tell you more about how a material will behave than any single assay figure.

Acid value: the free fatty acid you did not order

Acid value (AV) reports the milligrams of potassium hydroxide needed to neutralise the free fatty acid in one gram of sample, so it is a direct measure of hydrolysis. For a refined fatty acid the number is large by design, because it is titrating the carboxyl groups that make the product a fatty acid in the first place; there the useful signal is drift away from the historical mean rather than the absolute figure. For glycerin, propylene glycol or a fatty alcohol the same test is a purity indicator. A food-grade glycerin held below an AV of about 0.1 behaves very differently in an esterification or a cosmetic emulsion from one that has crept towards 0.5 after a summer in a warm warehouse. Rising AV also brings odour, because the short-chain free acids released by hydrolysis are volatile and perceptible long before the material looks any different.

Iodine value: unsaturation, colour and shelf life

Iodine value (IV) counts double bonds, expressed as grams of iodine absorbed per 100 g of sample. It predicts oxidation. A fully hydrogenated stearic acid with an IV below 1 is essentially saturated and will not yellow in storage. Palm-based stearic grades in the low single digits behave almost as well. A soft grade with an IV in the teens will develop rancidity and colour far faster, particularly in hot and humid climates, and the effect accelerates once the material is flaked and exposed to air. IV is also the practical dial for physical form: higher IV means a lower titre, a softer flake, weaker cake strength in soap noodles and a lower melting point in candle and personal care work. When a supplier quotes two grades at the same stated purity, the IV difference is usually the real reason one costs more.

Hydroxyl value: the figure alcohol and ester buyers overlook

Hydroxyl value (OHV) is the KOH equivalent of the hydroxyl groups present, and it converts fairly directly into average molecular weight. For a C16-C18 fatty alcohol, an OHV in the low two hundreds points to the expected blend; a noticeably higher figure suggests a shorter average chain or residual moisture, and a lower figure suggests heavier tails or hydrocarbon contamination. In ester production OHV is the reaction monitor. It falls as acid is consumed, and the residual value at the end of the cook tells you how far the esterification actually went, which is why two lots with identical appearance and near-identical ester content can behave differently downstream - one simply carries more unreacted hydroxyl than the other.

Saponification value, titre and colour: the supporting cast

Saponification value (SV) measures total ester plus acid content and is most useful as a cross-check: SV minus AV gives the ester value, which should agree with the composition the supplier claims. Titre, the solidification point, governs pumping, flaking and storage behaviour and cannot be inferred from purity alone. Colour on the Gardner or APHA scale flags heat exposure during processing and predicts how the material will look in a finished white or clear product. A lot with clean AV and IV but colour two steps above normal has usually been overheated somewhere, and peroxide value will often confirm it.

Reading the sheet against your own process

The practical rule is to buy against the property your process actually feels. A soap maker cares about titre and IV because they set hardness and colour. An emulsifier plant cares about OHV and IV because they set ethoxylation stoichiometry and oxidation risk. A food or pharmaceutical user cares about AV, colour and the absence of odour precursors. Ask for the test method next to each figure as well: AV by titration and AV by near-infrared can differ by a few tenths, and a certificate that omits the method leaves you comparing two different measurements. Retain a sample of every lot and trend the three values instead of judging a single certificate in isolation, because drift, not the absolute number, is what causes surprises on the line.

SINOFO Group supplies imported glycerin, propylene glycol, fatty acids, fatty alcohols and food ingredients from established producers in Malaysia and Indonesia, with full batch documentation and consistent specification sheets. If you would like a specification sheet, a retained sample or a quotation for a trial order, please send your target grades and volumes through the contact form on this site and our team will respond.