The acid value is one of the most important quality parameters for evaluating the level of free fatty acids and oxidative degradation in edible oils, mineral oils, lubricants, and insulating oils. This article presents a complete oil acid value inspection procedure based on international and national standards such as ISO 660, ASTM D664, ASTM D974, and the corresponding GB series, covering sample preparation, titration methods, calculation formulas, precision requirements, and result evaluation. It serves as a practical technical reference for laboratory analysts, quality control engineers, and equipment maintenance personnel.
Technical Overview
The acid value (AV), also called the acid number in petroleum applications, is defined as the mass of potassium hydroxide (KOH), in milligrams, required to neutralize the free acidic constituents in one gram of sample. It is expressed in mg KOH/g and directly reflects the concentration of free fatty acids in fats and oils, or of acidic oxidation and degradation products in mineral oils and lubricants. A rising acid value indicates hydrolysis, oxidation, thermal stress, or contamination, and it is therefore used both as a product quality specification and as a condition-monitoring indicator.
The principal standards governing acid value determination include ISO 660 and GB/T 5530 for animal and vegetable fats and oils, GB 5009.229 for acid value determination in foods, ASTM D974 and GB/T 4945 for color-indicator titration of petroleum products and lubricants, ASTM D664 and GB/T 7304 for potentiometric titration, ASTM D8045 for thermometric titration, GB/T 264 for petroleum products by the boiling ethanol method, JIS K 0070 for general chemical products, and GB/T 7599 together with IEC 60422 guidance for in-service transformer oils. Sampling and sample preparation are covered by ISO 5555, ISO 661, GB/T 5524, ASTM D4057, and GB/T 4756.
An acid value of 1 mg KOH/g corresponds approximately to 0.5 percent free fatty acid content expressed as oleic acid, which allows results to be converted between reporting conventions depending on the industry sector.
Testing Samples
Representative sampling and correct sample handling are prerequisites for valid acid value results. Samples shall be taken into clean, dry, light-protective containers filled with minimal headspace, labeled with identification, date, and batch information, and analyzed as soon as practicable. Solid fats shall be homogenized by gentle warming just above their melting point, avoiding excessive temperature that could accelerate hydrolysis or oxidation before testing.
Edible Vegetable Oils and Animal Fats
This category includes soybean oil, rapeseed oil, palm oil, sunflower oil, peanut oil, olive oil, tallow, lard, and fish oil. Typical acid values range from below 0.2 mg KOH/g for fully refined oils to several mg KOH/g for crude oils, cold-pressed oils, and oils subjected to prolonged frying.
Mineral Oils and Industrial Lubricants
Turbine oils, hydraulic fluids, gear oils, engine oils, and compressor oils are tested as new products against specification limits and as in-service fluids for condition monitoring. In this sector the result is usually reported as the total acid number (TAN), and the change in TAN relative to the new-oil baseline is more diagnostic than the absolute value.
Transformer and Other Insulating Oils
Unused mineral insulating oils are required to have very low acidity, typically not exceeding 0.03 mg KOH/g. In-service oils are monitored periodically, and a progressive increase in acid value reflects cellulose and oil ageing, with in-service action levels commonly set around 0.1 to 0.15 mg KOH/g depending on voltage class and equipment criticality.
Fatty Acids, Biodiesel, and Oleochemical Products
Crude and distilled fatty acids, biodiesel (fatty acid methyl esters), biodiesel feedstocks, and ester-based products can exhibit acid values from below 0.5 mg KOH/g for finished biodiesel to well above 100 mg KOH/g for acidic oleochemical intermediates, requiring appropriate sample masses and titrant concentrations.
Testing Items
The inspection procedure primarily determines the following quantities, selected according to the sample type and applicable specification:
- Acid value (AV) of animal and vegetable fats and oils, in mg KOH/g.
- Free fatty acid (FFA) content, calculated from the acid value and conventionally expressed as percent oleic acid, percent lauric acid, or percent palmitic acid.
- Total acid number (TAN) and, where required, strong acid number of petroleum products and lubricants.
- Acid value of in-service insulating oils as an ageing indicator within a maintenance program.
- Supporting observations recorded at the time of testing, including sample appearance, color, moisture condition, and any endpoint anomalies that may affect result validity.
Testing Methods
Method selection depends on the product type, expected acidity range, sample color, and regulatory or contractual requirements. The two classical families are visual color-indicator titration and instrumental potentiometric titration, supplemented by thermometric titration and rapid screening techniques.
Reagents and Apparatus
- Standard volumetric potassium hydroxide solution (typically 0.1 mol/L or 0.5 mol/L) in ethanol or isopropanol, standardized against primary-standard potassium hydrogen phthalate.
- Neutralized 95 percent ethanol, or a toluene-isopropanol mixture with a small proportion of water for dark and viscous petroleum samples.
- Indicators such as phenolphthalein for light-colored edible oils, alkali blue 6B for dark oils in GB/T 264 and GB/T 7599, and para-naphtholbenzein for ASTM D974.
- Analytical balance readable to 0.001 g or better, class A burettes or an automated titrator, conical flasks, reflux condensers, water bath or hot plate, and a potentiometric titrator with glass and reference electrodes where applicable.
Sample Mass Selection
The sample mass is chosen so that the titration consumes a suitable volume of titrant, generally between about 0.5 and 10 mL for indicator methods. As guidance aligned with ISO 660, an expected acid value below 1 mg KOH/g calls for approximately 20 g of sample, 1 to 4 mg KOH/g for 10 g, 4 to 15 mg KOH/g for 2.5 g, 15 to 75 mg KOH/g for 0.5 g, and values above 75 mg KOH/g for about 0.1 g titrated with more concentrated KOH solution.
Color-Indicator Titration (Hot Ethanol Method)
The weighed sample is dissolved in hot neutralized ethanol under gentle boiling with reflux or on a water bath to ensure complete extraction of the acidic constituents. Indicator is added, and the solution is titrated with standard KOH solution while hot, with the flask swirled continuously, until the endpoint persists for at least fifteen to thirty seconds. A blank titration of the solvent and indicator is performed under identical conditions. This approach, described in ISO 660 and GB/T 5530 and adapted in GB/T 264 and GB/T 7599, is suitable for samples whose color permits reliable visual endpoint detection.
Cold Solvent Indicator Titration
For liquid oils that are fully miscible with the solvent at room temperature, the sample is dissolved in a neutralized ethanol-diethyl ether or ethanol-light petroleum mixture, phenolphthalein is added, and titration is carried out at ambient temperature to a faint pink endpoint. This rapid variant, included in ISO 660, GB/T 5530, and GB 5009.229, is widely used for routine control of edible oils.
Potentiometric Titration
The sample is dissolved in the prescribed toluene-isopropanol-water or alcoholic solvent and titrated with standard alcoholic KOH using a glass indicating electrode and a reference electrode. The endpoint is located from the inflection of the titration curve, either manually or automatically. This method, specified in ASTM D664, GB/T 7304, and ISO 660, is preferred for dark-colored, highly viscous, or additive-containing samples where visual endpoints are unreliable, and it distinguishes weak and strong acidic constituents.
Thermometric Titration and Rapid Screening
Thermometric titration per ASTM D8045 detects the endpoint through the enthalpy change of neutralization, offering fast analysis and stable results for crude oils and used lubricants. Portable test kits and instrumental quick methods may be used for on-site screening, provided that any acceptance decision is confirmed by a standard laboratory method.
Calculation of Results
The acid value is calculated as AV = (V minus V0) x c x 56.11 divided by m, where V is the titrant volume consumed by the sample in mL, V0 is the blank volume in mL, c is the exact molar concentration of the KOH solution in mol/L, 56.11 is the molar mass of KOH in g/mol, and m is the sample mass in grams. Free fatty acid content as percent oleic acid is obtained by multiplying the acid value by 0.503; the corresponding factors are approximately 0.357 for lauric acid and 0.457 for palmitic acid.
Precision, Quality Control, and Reporting
Duplicate determinations shall be performed and shall agree within the repeatability limits of the applicable standard, which are typically about 0.05 to 0.1 mg KOH/g for low-acidity refined oils and a small relative percentage for crude and high-acidity samples. The burette shall be protected from atmospheric carbon dioxide with a soda-lime guard tube, the KOH solution shall be restandardized periodically, and blank values shall remain negligible. The report shall identify the method and standard used, the titrant concentration, the individual and mean results with units, the temperature of testing, and any deviations or observations. Results below 1 mg KOH/g are conventionally reported to two decimal places.
Safety requirements include protective gloves and eyewear when handling corrosive KOH solutions, operation of flammable solvents such as ethanol, toluene, and diethyl ether away from ignition sources or under a fume hood, and disposal of spent solvent and titration waste through authorized chemical waste channels.
Applications
Acid value testing is applied across the entire life cycle of oils, from refining and acceptance inspection to in-service condition assessment:
- Refining control of edible oils, where neutralization, degumming, and deacidification efficiency are verified against product specifications such as GB 2716, which limits most edible vegetable oils to 3 mg KOH/g and frying oils to 5 mg KOH/g, and trade standards that restrict extra virgin olive oil to 0.8 mg KOH/g.
- Shelf-life studies and rancidity assessment of stored fats, oils, snack foods, and oil-based ingredients.
- Quality acceptance of biodiesel and its feedstocks against acid number limits of 0.5 mg KOH/g specified in EN 14214 and ASTM D6751.
- Condition monitoring of turbine, hydraulic, and gear oils, where a significant TAN increase relative to the new-oil baseline signals oxidation and the need for oil change or reclamation.
- Transformer oil maintenance programs, where acid value trending supports ageing classification and decisions on filtration, regeneration, or oil replacement.
- Oleochemical production control for fatty acids, esters, surfactants, cosmetic oils, and pharmaceutical excipients.
- Regulatory compliance testing and dispute resolution performed by accredited third-party laboratories using standardized methods.
FAQ
What is the difference between acid value and acid number?
They are the same quantity expressed in mg KOH/g. The term acid value is customary for fats and oils, while acid number, total acid number, or TAN is customary in the petroleum and lubricant sector.
Which test method should be selected?
For light-colored edible oils, the cold solvent or hot ethanol indicator method is economical and adequate. For dark oils, used lubricants, additive-containing products, and transformer oils, potentiometric titration per ASTM D664 or GB/T 7304 is preferred because the endpoint is detected instrumentally and is not obscured by sample color.
Why must the solvent be neutralized and the ethanol heated?
Neutralization removes dissolved acidic impurities and carbon dioxide that would otherwise inflate the blank, while hot ethanol ensures complete dissolution and extraction of the free fatty acids from the sample so that the titration measures the full acidity.
How is acid value converted to free fatty acid content?
Multiply the acid value by the ratio of the fatty acid molar mass to that of KOH, giving approximately 0.503 for oleic acid, 0.357 for lauric acid, and 0.457 for palmitic acid, expressed as percent by mass.
What are typical acceptance limits?
Limits depend on the product: fully refined edible oils are commonly below 0.6 mg KOH/g, extra virgin olive oil must not exceed 0.8 mg KOH/g, biodiesel must not exceed 0.5 mg KOH/g, unused transformer oil is typically limited to about 0.03 mg KOH/g, and in-service limits are set by the applicable maintenance code and equipment class.
How often should in-service oils be tested?
Insulating oils and critical lubricants are typically tested annually or according to the established maintenance schedule, with increased frequency once an upward acid value trend appears, because trending provides earlier and more reliable warning than any single value.
Can a high acid value alone justify rejecting an oil?
No. The acid value should be interpreted together with related indicators such as peroxide value, water content, dielectric properties, viscosity, and color, since a complete degradation assessment requires a multi-parameter evaluation against the relevant specification.
