ISO Standard Acid Value Determination Experiment: Principles, Procedures, and Applications

The ISO standard acid value determination experiment, primarily governed by ISO 660, is the internationally recognized method for quantifying free fatty acid content in animal and vegetable fats and oils. This comprehensive technical guide explains the titration principles, sample preparation requirements, procedural steps, calculation formulas, and quality control measures essential for accurate acid value analysis in food, chemical, and biodiesel laboratories worldwide.

Technical Overview

The acid value (AV), also known as acid number or neutralization value, represents the quantity of potassium hydroxide, in milligrams, required to neutralize the free fatty acids present in one gram of a fat or oil sample. The internationally accepted procedure for this determination is specified in ISO 660 (Animal and vegetable fats and oils — Determination of acid value and acidity), with the current edition being ISO 660:2020. This standard defines the analytical protocol, reagent specifications, apparatus requirements, and precision criteria that laboratories must follow to produce valid, comparable results.

Acid value is a critical quality and freshness indicator. During storage, processing, or oxidation, triglycerides hydrolyze to release free fatty acids, causing the acid value to rise. Consequently, regulatory bodies, food manufacturers, refiners, and biodiesel producers all rely on ISO 660 compliant testing to monitor degradation, verify refining efficiency, and confirm compliance with commercial specifications. The method is based on titrating a sample dissolved in a neutralized ethanol-diethyl ether solvent mixture against standardized ethanolic potassium hydroxide solution, using phenolphthalein as the endpoint indicator, or alternatively employing potentiometric endpoint detection for darkly colored samples.

Testing Samples

ISO 660 applies to a broad range of lipid-based materials. Proper sampling and sample preparation, performed according to ISO 5555 and ISO 661 respectively, are prerequisites for reliable results.

Vegetable Oils

Crude, refined, and bleached-deodorized vegetable oils—including soybean, rapeseed (canola), sunflower, palm, olive, peanut, corn, and coconut oils—are routinely tested. Crude oils typically exhibit higher acid values (2–10 mg KOH/g), while properly refined oils should fall below 0.1–0.6 mg KOH/g depending on grade and specification.

Animal Fats and Fish Oils

Tallow, lard, poultry fat, and marine oils are covered by the standard. Fish oils, which are highly prone to hydrolysis, require careful sample handling and prompt analysis to prevent artifactual acid value elevation.

Used Frying Oils

Deep-frying oils undergo accelerated hydrolysis at elevated temperatures. Acid value measurement supports discard-point decisions and validates the effectiveness of filtration and treatment processes in food service operations.

Specialty and Industrial Lipids

The method extends to fatty acids, lecithin products, wax esters, and certain lipid derivatives. For biodiesel and fatty acid methyl esters, a related European standard (EN 14104) applies, though ISO 660 principles remain foundational.

Testing Items

The core deliverable of the experiment is the acid value, expressed in mg KOH/g, but the standard framework also encompasses several associated parameters and reporting items:

• Acid value (mg KOH/g): the primary titrimetric result calculated from titrant volume, concentration, and sample mass.
• Acidity expressed as free fatty acid (FFA) percentage: conversion of acid value to mass fraction of a reference fatty acid, typically oleic acid for vegetable oils (acidity % = AV ÷ 1.99) or palmitic acid for palm products (acidity % = AV ÷ 2.19).
• Blank titration value: correction for any acidity contributed by the solvent system.
• Repeatability check: duplicate determinations whose difference must remain within the repeatability limit (r) specified in ISO 660.
• Titrant concentration verification: confirmation of the exact molarity of the ethanolic KOH solution through standardization against a primary standard such as potassium hydrogen phthalate.

Testing Methods

The ISO 660 procedure follows a defined sequence of preparation, dissolution, titration, and calculation steps, with method variants selected according to sample color and expected acidity.

Principle of the Method

A accurately weighed test portion is dissolved in a previously neutralized 1:1 (v/v) mixture of ethanol (95% v/v) and diethyl ether. The free fatty acids in the sample are titrated with standardized aqueous or ethanolic potassium hydroxide (0.1 mol/L or 0.5 mol/L) to a phenolphthalein endpoint, indicated by a persistent faint pink color lasting at least 15 seconds. The acid value is calculated from the volume of titrant consumed.

Reagents and Apparatus

• Standardized KOH solution, 0.1 mol/L or 0.5 mol/L, in ethanol (95% v/v) or water, with concentration known to four decimal places.
• Mixed solvent: ethanol and diethyl ether (1:1), or ethanol and toluene (1:1) for samples that dissolve poorly; neutralized immediately before use with KOH and phenolphthalein.
• Phenolphthalein indicator solution, 10 g/L in ethanol.
• Analytical balance with 0.001 g readability or better.
• Burette (25 mL or 50 mL, class A) or automatic/potentiometric titrator equipped with a pH electrode.
• Conical flasks (250 mL), boiling water bath, and suitable glassware cleaned and dried before use.

Sample Mass Selection

The test portion mass is chosen so that the titration consumes a convenient titrant volume, generally targeting 1–30 mL of KOH solution. For expected acid values below 1 mg KOH/g, a sample of 20–50 g with 0.1 mol/L titrant is recommended; for acid values between 1 and 10 mg KOH/g, 5–10 g is typical; for highly acidic samples exceeding 10 mg KOH/g, 1–2 g portions with 0.5 mol/L titrant are appropriate.

Hot Ethanol Titration Procedure (Indicator Method)

• Weigh the test portion into a 250 mL conical flask and record the mass to the nearest 0.001 g.
• Add approximately 50–150 mL of the neutralized mixed solvent (scaled to sample mass) to the flask.
• Heat the flask in a boiling water bath with occasional swirling until the sample is completely dissolved; maintain the solution temperature, as the standard specifies titration of a warm solution.
• Add 0.5–1 mL of phenolphthalein indicator solution.
• Titrate with standardized KOH solution while swirling vigorously until a faint pink color persists for at least 15 seconds, indicating the endpoint.
• Perform a blank titration on the solvent alone under identical conditions.
• Execute at least duplicate determinations; the results must satisfy the repeatability limit before averaging.

Potentiometric Variant

For dark-colored oils and fats where the phenolphthalein endpoint is visually obscured—such as crude palm oil or used frying oils—ISO 660 permits potentiometric titration using a glass/reference electrode system. The titration curve is recorded, and the endpoint is identified from the inflection point, eliminating subjective color judgment and improving precision.

Calculation Formula

The acid value is computed as follows: AV = (V − V₀) × c × 56.1 / m, where V is the titrant volume consumed by the sample in mL, V₀ is the blank titration volume in mL, c is the exact molar concentration of the KOH solution in mol/L, 56.1 is the molar mass of KOH in g/mol, and m is the test portion mass in grams. The result is reported to two decimal places as the mean of acceptable duplicate determinations.

Quality Control and Precision

ISO 660 establishes precision data derived from interlaboratory studies. Repeatability (r) requires that the absolute difference between two independent single results, obtained by the same operator on identical test material, not exceed the tabulated r value more than once in twenty cases; analogous reproducibility (R) limits govern comparisons between laboratories. Regular standardization of titrant, use of carbon dioxide-free solvents, protection of KOH solution from atmospheric carbon dioxide, and avoidance of prolonged heating during dissolution are all essential safeguards against systematic bias.

Applications

ISO 660 acid value testing serves diverse commercial, regulatory, and research purposes across multiple industries:

• Edible oil refining: monitoring degumming, neutralization, and deodorization efficiency; refined oil specifications commonly cap acid value at 0.6 mg KOH/g (corresponding to 0.3% free fatty acid as oleic acid).
• Food quality assurance: verifying shelf-life stability, detecting oxidative and hydrolytic deterioration, and supporting supplier qualification programs.
• International trade: providing an objective contractual parameter for crude and refined oil shipments, customs classification, and dispute resolution under FOSFA, GAFTA, and similar trade rules.
• Biodiesel and oleochemical production: assessing feedstock quality, controlling esterification and neutralization steps, and evaluating final product conformance.
• Frying oil management: establishing discard thresholds and validating oil-life extension treatments in industrial and restaurant kitchens.
• Pharmaceutical and cosmetic manufacturing: confirming the quality of lipid excipients and ingredients such as fixed oils and emollients under pharmacopeial-aligned specifications.
• Academic and R&D laboratories: studying lipid oxidation kinetics, antioxidant efficacy, and enzymatic lipolysis with a standardized, globally comparable metric.

FAQ

What is the difference between acid value and free fatty acid content? Acid value is a titrimetric quantity (mg KOH/g), while free fatty acid content expresses acidity as a mass percentage of a reference fatty acid. The two are interconvertible: acidity as oleic acid (%) equals acid value divided by 1.99. Acid value is the fundamental measurement; FFA is a derived expression.

Why must the solvent be neutralized before use? Ethanol and diethyl ether can contain trace acidic impurities or absorb carbon dioxide, which would otherwise be titrated alongside the sample's fatty acids, inflating the result. Neutralizing the solvent immediately before titration, and running a blank correction, removes this source of positive bias.

Which titrant concentration should be selected? The choice depends on expected acidity. Using 0.1 mol/L KOH for low-acidity refined oils maximizes titration sensitivity, whereas 0.5 mol/L KOH suits crude or degraded samples so that titrant volumes remain practical and readable. ISO 660 recommends sample masses that keep titrant consumption within an optimal range.

Can the indicator method be used for dark oils? Visual phenolphthalein endpoint detection becomes unreliable in dark matrices. In such cases, ISO 660 authorizes potentiometric titration, in which the endpoint is detected electrochemically. Alternatively, laboratories sometimes dilute dark samples, though this increases uncertainty and is less preferred.

How are results judged for acceptability? Duplicate determinations are compared against the repeatability limit tabulated in ISO 660 for the relevant acidity range. If the difference exceeds r, the determination is repeated. Accepted results are averaged, and the laboratory report should state the standard applied, the titration mode, the titrant concentration, and the individual and mean acid values.

Is ISO 660 equivalent to other national methods? ISO 660 is technically aligned with AOCS Cd 3d-63, the European Pharmacopoeia methods for fixed oils, and the Chinese national standard GB/T 5530 for animal and vegetable fats and oils. Minor differences in solvent composition, titrant medium, or endpoint criteria may exist, so the governing standard must always be cited when comparing data between laboratories or across trade contracts.