Acid Value: mg KOH per Gram Indicator of Free Fatty Acids
The acid value expresses how many milligrams of potassium hydroxide (KOH) are needed to neutralise the free fatty acids present in one gram of oil or fat. Because it captures hydrolytic breakdown before rancid flavours emerge, the metric anchors incoming inspection, refining control, and shelf-life modelling across edible oil, cosmetic, and biodiesel supply chains. Use this guide alongside the saponification value explainer and the peroxide value primer to build a full picture of lipid stability.
Laboratory teams can rehearse titration planning with the serial dilution calculator and check reagent strengths using the pH from concentration tool before executing an accredited method such as ISO 660 or AOCS Cd 3d-63.
Definition and Standard Methods
Acid value (AV) is defined as the mass of KOH in milligrams required to neutralise free fatty acids in one gram of sample. It is numerically equal to the acid number in mg KOH/g. Internationally harmonised protocols—including ISO 660, AOAC 940.28, and AOCS Cd 3d-63—specify reagents (typically ethanolic KOH), indicators (phenolphthalein or potentiometric probes), and titration procedures to achieve reproducible endpoints.
The measured volume of standardised base is multiplied by its molar concentration and the molar mass of KOH (56.1 g/mol) before division by the sample mass. Expressed algebraically:
AV = (VKOH × CKOH × 56.1) / msample.
Results may also be converted to percent oleic acid by multiplying by 0.503 for commodity benchmarks. Maintaining SI notation with clear subscripts avoids ambiguity during audits and aligns with guidance in ISO/IEC 17025 quality manuals.
Historical Development
Acid value testing emerged during the late nineteenth century when chemists such as Jean-Baptiste Dumas and Wilhelm Normann investigated fat hydrolysis for soap making and hydrogenation. Early titrations used sodium hydroxide in ethanol, but variability in base strength complicated inter-laboratory comparisons. Adoption of potassium hydroxide, coupled with the availability of primary-standard potassium hydrogen phthalate, improved accuracy and reproducibility.
Trade bodies including the Society of Public Analysts (precursor to today’s Royal Society of Chemistry) published consensus methods in the early 1900s. These eventually informed Codex Alimentarius specifications that limit free fatty acid content in edible oils, providing the regulatory impetus for the meticulous titration workflows in use today.
Conceptual Foundations
Stoichiometry of the Neutralisation
Each mole of KOH neutralises one mole of monobasic free fatty acid. Although food oils contain diverse acids, expressing results as equivalent milligrams of KOH standardises reporting. When strong bases such as NaOH are substituted, labs must document equivalent factors to keep mg KOH/g traceable to SI units.
Blank Corrections and Solvent Effects
Solvents and reagents often introduce slight alkalinity or acidity. Running a blank titration and subtracting its volume protects accuracy, especially when testing refined oils with AV below 0.1 mg KOH/g. Drying the solvent, or including molecular sieves, minimises water uptake that could bias the titration.
Calculating Uncertainty
Analysts propagate uncertainty from volumetric glassware tolerances, titrant molarity, and balance readability. Documenting these contributions keeps acid value certificates defensible and aligns with ISO GUM principles. The molarity explainer provides additional background on concentration traceability.
Applications
Edible Oil Refining
Refiners monitor crude palm, soybean, or sunflower oil to schedule neutralisation and deodorisation stages. Elevated acid value correlates with yield loss because neutralisation removes both free acids and entrained neutral oil. Tracking AV alongside iodine value helps optimise processing parameters for specific cultivars.
Biodiesel Feedstock Qualification
In transesterification, high free fatty acid content triggers soap formation and catalyst consumption. ASTM D6751 and EN 14214 therefore limit acid value of finished biodiesel to ≤0.5 mg KOH/g. Pretreatments such as acid esterification rely on baseline AV data to confirm whether an additional neutralisation step is required.
Cosmetic and Pharmaceutical Stability
Emulsion-based creams and ointments use acid value trending to detect hydrolysis that could shift pH outside stability windows. Coupling AV with water activity and peroxide value testing provides early warning of rancidity before customer complaints emerge.
Importance and Future Outlook
Automation is modernising acid value analysis through potentiometric titrators, autosamplers, and LIMS integrations. These systems still depend on careful calibration of titrant concentration, making the principles outlined here enduringly relevant. Inline spectroscopy and mid-infrared probes show promise for rapid screening, but titration remains the reference method that validators use to qualify alternative technologies.
As sustainable sourcing drives interest in novel lipids—from algal oils to upcycled cooking grease—the acid value will remain a critical metric for negotiating contracts, scheduling refining loads, and assuring product quality. Mastery of the unit’s calculation and context ensures that data scientists, QA leads, and procurement teams speak a common analytical language.