Stearic Acid
Properties
| State | Solid (waxy) |
| Color | White to faint yellow |
| Solubility | Insoluble in water (3 mg/L at 20 °C); soluble in ethanol, chloroform, diethyl ether |
| Melting Point | 69.3 °C |
| Boiling Point | 383 °C |
About Stearic Acid
Stearic acid (octadecanoic acid, CH3(CH2)16COOH, 284.478 g/mol) is the saturated 18-carbon fatty acid that gives beef tallow its waxy texture and cocoa butter its sharp melting curve at 34 to 35 °C — a property essential to chocolate's snap when it breaks. The name comes from the Greek 'stear' (tallow), reflecting how it was first isolated by Michel-Eugene Chevreul in 1823 from animal fats during the breakthrough work that established the chemistry of glycerol esters and laid the foundation for modern lipid biochemistry. Industrial production today still relies on hydrolysis of triglycerides — a fat splitting plant pressure-cooks tallow or palm stearin with steam at 250 °C and 50 atm to give a mixture of free fatty acids (mostly stearic and palmitic in the C16-C18 range) which is then fractionated by vacuum distillation. Global production runs above 1 million tonnes per year. The biggest single use is rubber compounding, where stearic acid functions as the activator that transforms zinc oxide into the soluble zinc stearate complex needed to catalyze sulfur vulcanization — every car tire in the world contains 1 to 3% stearic acid by weight in the tread compound. The second biggest use is soap and cosmetics: sodium stearate is the principal component of bar soap (the saponification reaction CH3(CH2)16COOH + NaOH -> CH3(CH2)16COONa + H2O is the textbook example taught in every introductory organic class), and stearic acid itself is the structural backbone of most face creams and lotions, where it acts as the oily phase of the oil-in-water emulsion stabilized by triethanolamine.
Where you'll encounter it
If you have ever bitten into a piece of dark chocolate and noticed that distinctive sharp snap rather than a soft yield, the credit goes largely to stearic acid in cocoa butter — the polymorphic crystallization of stearate-rich triglycerides into the stable Form V crystal during tempering is what gives chocolate its glossy surface, audible snap, and mouth-feel that melts cleanly at body temperature. In a candle factory, stearic acid is added to paraffin wax at 5 to 15% to raise the melting point, harden the wax, and reduce drip — the same chemistry that gave the Industrial Revolution its 'stearin candle' that replaced smelly tallow dips in the 1830s. In a tire-compounding lab, the small flakes of waxy white solid added to the carbon black, polymer, sulfur, and zinc oxide on the two-roll mill are stearic acid; they melt during mixing at 70 °C and form zinc distearate in situ with the ZnO, and that zinc soap is what activates the cyclic sulfur vulcanization accelerators (CBS, MBS, TBBS) during cure.
Common Uses
- Vulcanization activator in rubber tire and conveyor-belt compounds at 1 to 3% loading
- Soap manufacture as the precursor to sodium stearate, the primary component of bar soap
- Candle hardening agent at 5 to 15% in paraffin to raise melting point and reduce drip
- Cosmetic emulsifier and thickener in face creams, lotions, and foundation makeup
- Lubricant and mold-release agent for plastic injection molding and metal die-casting
- Dispersant for carbon black, titanium dioxide, and iron oxide pigments in paints and inks
- Coating agent for hard-candy production to give a smooth surface and slow moisture pickup
- Tableting lubricant in pharmaceutical formulations as magnesium stearate at 0.25 to 1%
Safety Information
Low toxicity. GHS: not classified as hazardous. Generally Recognized as Safe (GRAS) by the US FDA for direct food use; permitted in cosmetics, food contact, and pharmaceutical excipient applications. No OSHA PEL specifically for stearic acid; the OSHA particulates not otherwise regulated PEL of 15 mg/m3 total dust applies to the powdered form. Acute oral LD50 is greater than 21 g/kg in rats — essentially nontoxic. The molten material at 70 to 100 °C poses a thermal-burn hazard during industrial handling; nitrile gloves and a face shield are standard. Dust from powdered or flaked stearic acid can form combustible dust clouds at concentrations above 50 g/m3, so industrial fat-splitting plants and tablet-pressing facilities use dust collection and grounding to manage explosion risk. Skin contact is well tolerated and stearic acid is in fact a recognized skin emollient.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.