In-Service Oil Acid Number Testing: Methods, Standards, and Condition Monitoring Applications

In-service oil acid number testing is a critical diagnostic technique used to evaluate the oxidation degradation and remaining useful life of lubricating and insulating oils in operating equipment. By measuring the total acid number (TAN) according to recognized standards such as ASTM D664, ASTM D974, ISO 6618, IEC 62021, and GB/T 7304, maintenance teams can detect early-stage oil aging, prevent corrosion and sludge formation, and optimize oil change intervals. This article provides a comprehensive technical overview of acid number testing for in-service oils, covering sample types, test methods, result interpretation, and practical applications in predictive maintenance programs.

Technical Overview

The acid number (AN), also referred to as the total acid number (TAN), is defined as the quantity of base, expressed in milligrams of potassium hydroxide (mg KOH), required to neutralize the acidic constituents present in one gram of oil. For in-service oils, the acid number is one of the most widely monitored parameters in oil condition monitoring programs because it provides a direct indication of oxidative degradation occurring during equipment operation.

Fresh lubricating and insulating oils typically exhibit a low acid number, generally below 0.05 to 0.30 mg KOH/g depending on the oil type, refining process, and additive package. As oil ages in service, hydrocarbon molecules react with oxygen at elevated temperatures, in the presence of catalysts such as wear metals (copper, iron) and moisture, to form primary oxidation products including carboxylic acids, peroxides, and aldehydes. These acidic compounds accumulate progressively, causing the acid number to rise. A sustained increase in acid number signals ongoing oxidation, depletion of antioxidant additives, and an elevated risk of sludge formation, varnish deposition, filter plugging, and corrosion of bearing surfaces and yellow metal components.

In condition-based maintenance practice, the absolute acid number value is less significant than the trend over time. Most equipment manufacturers and oil suppliers define warning and alarm thresholds relative to the new-oil baseline, with common action limits set at an increase of 0.5 to 2.0 mg KOH/g above the virgin oil value, depending on oil type and application severity. Acid number testing is frequently combined with complementary analyses such as viscosity, water content, FTIR spectroscopy, membrane patch colorimetry (varnish potential), and elemental analysis to build a complete picture of oil health.

Testing Samples

Turbine and Hydraulic Oils

Steam turbine, gas turbine, and hydraulic system oils are among the most common samples submitted for in-service acid number testing. These oils are formulated with refined base stocks and antioxidant additives, and their service life is largely governed by oxidation stability. Gas turbine oils, which operate at high bulk temperatures, are particularly susceptible to oxidation, making regular acid number trending essential for combined-cycle and peaking installations. Hydraulic fluids, including high-RVI anti-wear formulations, are monitored to prevent valve sticking, servo-valve varnishing, and pump corrosion.

Gear and Compressor Oils

Industrial gear oils, wind turbine gearbox oils, and reciprocating or rotary compressor oils are tested for acid number to assess oxidation under boundary lubrication and high-temperature conditions. Wind turbine gearbox oils deserve special attention because of the high cost of replacement, difficult access, and the criticality of extending drain intervals. Acid number trending, combined with varnish potential testing, supports informed decisions about oil renewal, top-up strategies, and filtration interventions.

Transformer and Insulating Oils

Mineral insulating oils used in power transformers, instrument transformers, tap changers, and switchgear are routinely tested for acidity in accordance with IEC 62021 series standards and ASTM D664 or D974 methods. The acidity of insulating oil is a key aging indicator because acidic products accelerate cellulose insulation degradation, increase oil conductivity and dissipation factor, and promote sludge deposition on windings, which impairs heat dissipation. For in-service transformer oils, an acidity limit of approximately 0.15 mg KOH/g is commonly applied as a caution threshold, with oil reclamation or replacement considered as values approach 0.30 to 0.40 mg KOH/g.

Engine and Circulating Oils

Diesel and gasoline engine oils are complex formulations in which both acid and base numbers must be interpreted carefully. Combustion by-products, nitrogen oxides, and sulfur oxidation products contribute acidity, while overbased detergent additives provide reserve alkalinity measured as base number (BN). Acid number testing of engine oils is normally performed alongside base number testing, since apparent acidity in highly additivated engine oils can reflect additive chemistry as well as degradation. Circulating system oils in paper mills, steel plants, and marine applications are similarly monitored.

Specialty and Fire-Resistant Fluids

Phosphate ester fire-resistant hydraulic fluids used in power generation and aviation ground systems, as well as refrigeration compressor oils and food-grade lubricants, are also tested for acid number. Phosphate esters hydrolyze in the presence of water to produce strong acidic species, so acid number testing is central to maintaining these fluids within safe operating limits.

Testing Items

Accredited third-party laboratories offering in-service oil acid number testing typically provide the following analytical items:

• Total Acid Number (TAN) — the total acidity of the oil sample, including both weak organic acids from oxidation and any strong acids, expressed in mg KOH/g.
• Strong Acid Number (SAN) — the portion of acidity attributable to strong inorganic acids, typically associated with contamination by combustion gases or sulfuric acid formation; a significant SAN indicates abnormal contamination.
• Acid number change versus new oil specification — the net increase relative to the virgin oil baseline, which is the primary basis for maintenance decision limits.
• Complementary supporting tests frequently requested with acid number, including kinematic viscosity at 40°C and 100°C, water content (Karl Fischer, ASTM D6304), FTIR oxidation/nitration peak integration, membrane patch colorimetry (MPC) for varnish potential, rotating pressure vessel oxidation test (RPVOT, ASTM D2272), and elemental analysis by ICP or XRF (ASTM D5185).

Testing Methods

Potentiometric Titration — ASTM D664 / GB/T 7304 / ISO 6619 Equivalent Practice

The potentiometric method is the reference technique for determining the acid number of petroleum products and lubricants, including dark-colored and opaque in-service oils where visual indicators cannot be used. The sample is dissolved in a mixed solvent of toluene and isopropanol with a small amount of water, and titrated with standardized alcoholic potassium hydroxide solution while the potential between a glass electrode and a reference electrode is monitored. The endpoint is located from inflection points in the titration curve. The method covers both total acid number and strong acid number determination, with reporting precision typically to the nearest 0.01 mg KOH/g. Potentiometric titration is preferred for used oil analysis because it is objective, automated, and unaffected by sample color.

Colorimetric Titration — ASTM D974 / GB/T 4945 / ISO 6618

The color-indicator method titrates the sample dissolved in isopropanol-toluene solvent against alcoholic KOH in the presence of p-naphtholbenzein indicator. The endpoint is the color transition from orange to green in basic solution. This method is suitable for relatively light-colored oils with acid numbers above approximately 0.05 mg KOH/g and is widely applied to turbine oils, hydraulic oils, and new insulating oils. It is not recommended for dark, heavily degraded, or strongly colored used oils because the visual endpoint becomes ambiguous.

Semi-Micro Color Indicator Method — ASTM D3339

ASTM D3339 is a semi-micro variation of the colorimetric technique requiring a smaller sample mass of approximately 2 to 3 grams. It is useful when sample volume is limited, such as in sampling programs for small sumps, instrument transformers, or research-scale oxidation studies, and is applicable to samples with acid numbers in the range of roughly 0.1 to 20 mg KOH/g.

Thermometric Titration — ASTM D8045

Thermometric titration determines the acid number by detecting the enthalpy change of the neutralization reaction using a thermistor probe instead of pH-sensitive electrodes. The technique offers fast analysis, rugged sensor handling, minimal electrode maintenance, and excellent reproducibility, and it is increasingly adopted in high-throughput used-oil laboratories for mineral oils, biodiesel blends, and ester-based lubricants.

Acidity of Insulating Liquids — IEC 62021 Series

For electrical insulating liquids, IEC 62021-1 specifies a potentiometric titration method applicable to both new and in-service mineral insulating oils, while IEC 62021-2 addresses low-acidity oils. These methods express results as acidity in mg KOH/g and are harmonized with transformer oil quality assessment programs alongside breakdown voltage, water content, and dissolved gas analysis. The corresponding Chinese national standard GB/T 7599 covers the water-soluble acid and acid value of transformer oil in service.

Related National and International Standards

• ASTM D664 — Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration.
• ASTM D974 — Standard Test Method for Acid and Base Number by Color-Indicator Titration.
• ASTM D3339 — Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration.
• ASTM D8045 — Standard Test Method for Acid Number by Thermometric Titration.
• ISO 6618 — Petroleum products and lubricants: determination of acid and base number by color-indicator titration.
• IEC 62021-1 / IEC 62021-2 — Insulating liquids: determination of acidity.
• GB/T 7304 — Determination of acid number of petroleum products by potentiometric titration.
• GB/T 4945 — Petroleum products and lubricants: determination of acid and base number (color indicator method).
• GB/T 264 — Determination of acid value of petroleum products.
• GB/T 7599 — Determination of water-soluble acid and acid value of transformer oil in service.

Applications

In-service oil acid number testing underpins a wide range of reliability engineering and asset management activities:

• Predictive and condition-based maintenance — trending acid number at scheduled intervals to detect oxidation before it manifests as sludge, varnish, or component wear, enabling planned intervention instead of reactive repair.
• Oil drain interval optimization — using measured acid number growth together with antioxidant depletion data to extend or shorten change-outs safely, reducing lubricant consumption and disposal costs while protecting equipment.
• Transformer life management — acidity measurement as part of routine dissolved gas and oil quality testing to schedule reclamation (fuller's earth treatment) or oil replacement, preserving solid insulation life.
• Corrosion and deposit risk control — identifying oils whose acidity has reached levels capable of attacking bearing metals, copper alloys, and coated surfaces, and triggering corrective actions such as polishing filtration or purification.
• New lubricant and supplier qualification — verifying that incoming oils meet the manufacturer's specified acid number range before they are charged into critical machines.
• Failure investigation and root cause analysis — acid number data used as evidence when diagnosing oxidation-related failures, overheating events, contamination incidents, or incorrect oil top-ups.
• Compliance and warranty documentation — providing objective test records from accredited third-party laboratories to satisfy OEM warranty requirements, insurance claims, and regulatory audits for fleet, marine, and power plant operators.

FAQ

Why does the acid number of oil increase during service? The acid number rises primarily due to oxidation of the base stock, which forms weak organic acids, and secondarily due to contamination with combustion products, acidic process gases, or water that hydrolyzes ester-based components. Depletion of antioxidant additives removes the oil's defense against these reactions, accelerating acid formation.

What acid number level indicates that oil should be changed? There is no universal limit; the correct criterion is the increase relative to the new oil value combined with the equipment manufacturer's guidance. Common practice flags turbine and hydraulic oils when the acid number rises by more than 0.5 to 1.0 mg KOH/g above baseline, while transformer oils are commonly actioned near 0.15 to 0.30 mg KOH/g.

Can the acid number of in-service oil decrease? Yes. Apparent decreases can occur when acidic species are depleted through additive neutralization, when acidic oxidation products are adsorbed onto varnish deposits or filter media, or after large oil additions. A falling acid number in a degrading system should therefore be interpreted together with other parameters such as varnish potential and remaining useful life testing.

What is the difference between total acid number and strong acid number? Total acid number measures all acidic constituents, mainly weak organic acids, whereas strong acid number isolates the strongly acidic fraction, usually of inorganic origin. A significant strong acid number strongly suggests external contamination such as blow-by gases or sulfur-bearing process contaminants.

How often should in-service oil be sampled for acid number testing? Typical intervals are monthly to quarterly for critical turbines and hydraulic systems, quarterly to semi-annually for general industrial equipment, and annually for distribution transformers, adjusted upward for severe duty, high operating temperatures, or after any abnormal event such as overheating or water ingress.

How should samples be collected and handled? Samples should be drawn from live, circulating zones or dedicated sampling valves using clean, sealed containers, labeled with equipment ID, run hours, and oil grade, and analyzed promptly. Because acids can segregate with sludge and water, homogeneity of the sample and consistent sampling location are essential for meaningful trend comparison.

Which test method should be selected for dark used oils? Potentiometric titration (ASTM D664 or GB/T 7304) or thermometric titration (ASTM D8045) is recommended for dark, opaque, or heavily degraded oils, since color-indicator methods such as ASTM D974 rely on a visual endpoint that is unreliable in such samples.