When oil turns acidic, it loses its protective properties and becomes actively destructive. It eats away at seals, corrodes metal surfaces, and breaks down the very components it's supposed to protect. To avoid this, the Total Acid Number (TAN) was developed as an early warning system.
Most maintenance teams are aware that oil degrades over time. But they don't realize how much damage acidic oil causes before it shows obvious signs of failure. By the time you see discolored oil or feel the rough operating conditions, acid corrosion has already started compromising your equipment's internal components.
This guide will walk you through everything you need to know about total acid number testing, from understanding what drives acid formation to building TAN monitoring into a maintenance strategy that catches problems before they become expensive failures.
What Is Total Acid Number In Maintenance
Total Acid Number is a quantitative measurement of acidity in oil, expressed as the amount of potassium hydroxide (KOH) in milligrams needed to neutralize the acids in one gram of oil sample. This measurement tells you exactly how acidic your lubricant has become. This matters because acidic oil can eat your equipment from the inside out.
The chemical basis for this phenomenon is straightforward. As oil degrades through oxidation, heat, and contamination, it forms acidic compounds. TAN evaluates these acids by determining how much KOH (a strong base) is required to neutralize them. The result is expressed in mg KOH/g, which gives you a precise acid value that you can track over time.
The difference between TAN and pH measurement is that pH only works in water-based solutions, while TAN works specifically with oils and other non-aqueous fluids. That's why maintenance teams rely on TAN instead of pH strips when they need to understand what's happening inside their gearboxes, hydraulic systems, and turbines.
When you're measuring acid in industrial lubricants, these key principles guide accurate results:
- Acid value: The mg KOH/g reading that quantifies total acidity
- KOH/g meaning: Milligrams of potassium hydroxide needed per gram of oil
- Neutralization point: Where acids and bases balance out during titration
- Trending capability: How TAN changes reveal oil degradation patterns
When you measure TAN, you're essentially asking: "How much chemical damage is building up in this oil?" The answer helps you decide whether to keep running, add treatment, or change the oil entirely. Understanding what acid is measured in becomes crucial for interpreting these results properly.
Why TAN Matters For Equipment Reliability
Equipment failures don't announce themselves. They build up slowly through processes like acid corrosion, which TAN monitoring can detect early. When acidic compounds accumulate in your oil, they attack metal surfaces, degrade seals, and reduce the oil's ability to protect moving parts.
Corrosion prevention starts with understanding that monitoring TAN helps you stay ahead of acid-induced corrosion before it damages bearings, shafts, and internal components. Acids in oil create an electrochemical environment that accelerates metal degradation, especially in the presence of water or high temperatures.
There's a direct relationship between rising TAN values and lubricant breakdown. As TAN increases, the oil's protective properties decrease, meaning you're getting less lubrication effectiveness even though the oil still looks fine.
TAN changes signal maintenance needs in advance of catastrophic failures. A gradual TAN increase might indicate normal aging, but a sudden spike often points to contamination, overheating, or additive depletion that needs immediate attention.
The cost implications are significant. Catching problems through TAN monitoring costs far less than replacing damaged equipment or dealing with unplanned downtime.
Consider what happened at a paper mill where TAN monitoring revealed rising acidity in their main turbine oil. Instead of waiting for the next scheduled change, they acted on the TAN data and discovered water contamination that would have caused bearing failure within weeks.
High TAN values create these specific problems in your equipment:
- Bearing corrosion and premature failure
- Seal deterioration and fluid leaks
- Reduced oil film strength and increased metal-to-metal contact
- Increased friction and wear throughout the system
- Deposit formation in oil passages and filters
For maintenance teams, though, the real question isn't whether or not oil will become acidic over time. Instead, it’s a matter of whether you catch the problem before it catches you. Understanding whether oil is acidic helps you determine the urgency and priority of required maintenance actions.
Essential TAN Testing Methods
Accurate TAN measurement depends on using proven testing methods that deliver consistent, reliable results. The two primary approaches (potentiometric and color indicator titration) each have their place in maintenance programs, depending on your accuracy needs and testing environment.
Potentiometric Vs Color Indicator
These methods differ in how they determine the neutralization endpoint. Potentiometric titration uses electronic sensors to detect the exact neutralization point, while color indicator methods rely on visual color changes to signal when acids have been neutralized. Both methods add KOH solution gradually until all acids are neutralized, but they determine the endpoint differently.
The potentiometric method measures electrical potential changes as the titration progresses, giving you a precise, objective endpoint that doesn't depend on human interpretation. Color indicator methods use chemical dyes that change color at the neutralization point, requiring the operator to judge when the color shift occurs.
Equipment requirements vary significantly between methods. Potentiometric testing requires pH meters, magnetic stirrers, and calibrated electrodes, while color indicator testing uses basic glassware and visual comparison charts.
Overview Of ASTM D664 And ASTM D974
Understanding these standardized approaches ensures consistent results across different operators and locations. ASTM D664 defines the potentiometric titration method for determining TAN in petroleum products.
This standard specifies using an automatic titrator with glass and reference electrodes to detect the neutralization endpoint electronically. The method requires dissolving the oil sample in a mixture of toluene and isopropanol, then titrating with standardized KOH solution while monitoring voltage changes.
ASTM D974 covers the color-indicator titration method, which uses p-naphtholbenzene indicator to signal the neutralization point through a color change from orange to green-brown. This method also uses a toluene-isopropanol solvent system but relies on visual endpoint detection rather than electronic measurement.
Both standards matter because they ensure consistent results regardless of who performs the test or where it's conducted. Equipment calibration requirements include regular standardization of KOH solutions, electrode maintenance for potentiometric methods, and proper solvent handling for both approaches.
Key steps in ASTM D664 include these critical phases:
- Sample dissolution in specified solvents
- Electrode calibration and setup
- Automated titration with endpoint detection
- KOH-based consumption calculations
Key steps in ASTM D974 follow this sequence:
- Sample preparation with indicator addition
- Manual titration with visual monitoring
- Color change recognition at the endpoint
- Manual TAN value calculations
These standardized methods eliminate guesswork and provide the consistency needed for reliable trend analysis across different operators and testing locations. The acid number titration process becomes repeatable and defensible when following these established protocols.
Steps For Accurate TAN Measurement In Oils
Getting reliable TAN results requires attention to detail at every step, from sample collection through final calculation. Small mistakes in any phase can throw off your results and lead to poor maintenance decisions.
1. Prepare The Sample
Proper sampling starts with getting a representative TAN sample from your equipment while it's at operating temperature. Cold oil doesn't mix well, so contaminants and additives may not be evenly distributed, giving you a sample that doesn't reflect the true oil condition.
Sample size typically requires 2-5 grams of oil for most TAN tests, but check your specific method requirements. Handle samples with clean glassware and avoid contamination from water, dirt, or other oils that could affect the acid content.
Storage considerations become important if you can't test immediately. Keep samples in clean, sealed containers away from heat and light to prevent further oxidation that would artificially increase TAN values.
Common sampling mistakes that compromise results include these critical errors:
- Taking samples from drain ports that contain settled contaminants
- Using contaminated sampling equipment
- Allowing samples to sit exposed to air for extended periods
- Mixing samples from different systems or time periods
- Sampling when equipment is shut down and oil isn't circulating
2. Perform The Titration
Equipment setup begins with calibrating your titration system using fresh reagents and clean glassware. For potentiometric methods, this includes electrode calibration and stirrer speed adjustment. Color indicator methods need proper lighting and clean indicator solutions.
Reagent preparation involves making standardized KOH solutions at the correct concentration, typically 0.1 N (normal) for most applications. The KOH must be fresh because it absorbs moisture and carbon dioxide from air, which changes its strength over time.
The actual titration procedure adds KOH solution gradually while monitoring for the neutralization endpoint. Potentiometric methods show this as a sharp voltage change, while color indicator methods show it as a distinct color shift from orange to green-brown.
Endpoint determination requires recognizing when all acids have been neutralized without over-titrating. This takes practice with color indicator methods, but happens automatically with properly calibrated potentiometric equipment.
3. Record The Results
Calculating TAN values from titration data uses this formula: TAN = (mL KOH used × Normality of KOH × 56.1) / grams of sample. The 56.1 factor converts the result to mg KOH/g units, establishing the tan unit measurement that maintenance teams track over time.
Document results properly by recording the sample source, test date, method used, and any observations about sample condition. This documentation becomes crucial for trend analysis and troubleshooting.
Interpreting the numbers requires understanding what constitutes normal versus concerning values for your specific oil and application. New oils generally have low TAN values, and a significant increase in TAN can signal that an oil change or treatment may be necessary.
Tracking changes over time reveals more than individual readings. A steady increase suggests normal aging, while sudden spikes indicate contamination or other problems requiring immediate investigation.
Factors That Affect TAN Values
Understanding what drives TAN changes helps you distinguish between normal oil aging and problems that need immediate attention. Multiple factors influence acid formation in oils, and recognizing their effects prevents unnecessary oil changes while catching real problems early.
Oil oxidation represents the primary cause of TAN increase in most applications. When oil molecules react with oxygen at elevated temperatures, they form acidic compounds that raise TAN values. This process accelerates significantly as temperature increases, making oil more susceptible to oxidation under higher heat conditions.
Also, contamination sources introduce acids directly into the oil system, and water contamination promotes oxidation and can carry acidic compounds from external sources. Process materials like chemicals or combustion byproducts add their own acidic components. Even seemingly clean systems can develop contamination through seal leaks or improper maintenance practices.
Base oil type affects natural acidity levels and oxidation resistance. Synthetic oils typically start with lower TAN values and resist acid formation better than conventional mineral oils. However, they're not immune to contamination or extreme operating conditions. Understanding whether the oil is basic or acidic helps determine appropriate monitoring frequencies.
Additive depletion changes the oil's ability to resist acid formation. Antioxidants consume themselves while protecting the base oil, and once depleted, acid formation accelerates rapidly. This often shows up as a sudden TAN increase after months of stable readings.
Operating conditions impact TAN through temperature, pressure, and duty cycle effects. High-temperature applications naturally develop higher TAN values, while systems with frequent start-stop cycles may show different patterns than continuous-duty equipment.
Common contaminants that affect TAN readings include these troublesome sources:
- Water from seal leaks or condensation
- Process chemicals from system cross-contamination
- Combustion byproducts in engine oils
- Cleaning solvents from maintenance activities
- Metal particles that catalyze oxidation
- Atmospheric acids in coastal or industrial environments
The key is establishing baseline TAN values for your specific equipment and operating conditions, then watching for deviations that signal problems rather than normal aging. This approach helps answer questions like “Is oil acidic?” by providing equipment-specific context rather than generic limits.
Integrating TAN Into Maintenance Strategy
Building TAN monitoring into your maintenance program transforms oil analysis from a periodic check into a predictive maintenance tool that drives equipment reliability decisions. The key lies in establishing systematic testing protocols that align with your equipment criticality and operating conditions.
Recommended testing frequencies vary by equipment type and importance to operations. More frequent TAN testing is generally recommended for highly critical equipment, while less critical systems may require testing at longer intervals. The goal is to catch problems early enough to take corrective action before equipment damage occurs.
Also, trend limits work better than absolute values for triggering maintenance actions. A gradual TAN increase from 0.5 to 1.5 mg KOH/g over six months might be normal aging, while the same increase over two weeks indicates a serious problem requiring immediate attention.
Integration with other oil analysis parameters provides a complete picture of oil and equipment condition. TAN combines with viscosity, particle counts, and wear metals to reveal the full story of what's happening inside your equipment.
6 Steps to develop a TAN-based maintenance strategy follow this logical progression:
Using TAN data to drive maintenance decisions means acting on trends rather than waiting for absolute limits. A sudden TAN increase, even within normal ranges, often signals problems that need investigation before they cause equipment damage.
CP Kelco saved $483K by implementing comprehensive condition monitoring. "I'm really impressed by the reliability metrics that TRACTIAN is able to calculate in real time, and the level of detail when it comes to the failure modes and the insight generation. TRACTIAN has really improved our asset availability," says Gautam Sane, CP Kelco Senior Reliability Engineer.
This success demonstrates how integrating multiple condition monitoring techniques, including oil analysis parameters like TAN, creates a comprehensive view of equipment health that enables proactive maintenance decisions.
Advancing Maintenance With TAN Insights and a CMMS
TAN monitoring reveals itself as more than a data point, and instead, an early warning signal that something’s off inside your most critical equipment. But its real value is demonstrated when these signals drive action.
And this is where most teams get stuck. Often, data sits in spreadsheets or lab reports, disconnected from the work that needs to happen. However, teams that connect oil condition data to actual maintenance workflows quickly find themselves on the winning side of the equation.
For these teams, Tractian CMMS turns TAN insights into task-level execution. From trend alerts to work order creation, everything flows in one system. No more chasing lab results or guessing when to intervene.
Combined with other predictive tools—like vibration, thermography, or runtime analytics—TAN gives you another lens on asset health. And when it’s all integrated, your team stops reacting and starts getting ahead of failures.
Tractian CMMS transforms maintenance operations by providing the structure and automation needed to act on condition monitoring data effectively. With features like automated scheduling, real-time alerts, and comprehensive reporting, maintenance teams can focus on solving problems rather than managing paperwork and tracking spreadsheets.
Want to make that shift? Start a free trial and see how Tractian CMMS helps maintenance teams turn data into uptime, prevent unexpected failures, and extend equipment life.
