Total acid number (TAN)

What is total acid number?

Total acid number (TAN) is a key analytical measurement used to determine the acidity of a substance, particularly in oils and fuels. It is defined as the amount of potassium hydroxide (KOH) in milligrams required to neutralize the acidic components in one gram of a sample. TAN is an essential parameter in monitoring the condition of lubricants, fuels, and other petroleum-based products, as it helps identify degradation, contamination, and corrosion risks.

TAN is also an important parameter to measure for maintenance, specifically for machines and equipment that rely on oil and lubricant. By monitoring TAN, maintenance teams can proactively manage equipment health, optimize performance, and prevent costly failures.

Where is TAN used?

TAN is widely used in industries where lubricants, fuels, and oils play a crucial role in machinery and operations. Some of the key sectors where TAN analysis is employed include:

• Petroleum, lubricants and refining: To monitor the acidity of crude oil and prevent corrosion in pipelines and refining equipment.
• Automotive and transportation: To assess lubricant and fuel degradation in engines.
• Power generation: To ensure turbine and generator oils remain effective over time.
• Manufacturing: To evaluate the quality of industrial lubricants and hydraulic fluids.
• Food and beverage: To determine the acidity of edible oils and fats, ensuring product stability and compliance with regulations.

How is TAN used in different industries?

• Oil and gas: TAN is used to assess the corrosive potential of crude oil and refine it accordingly. High TAN values indicate the presence of naphthenic acids, which can corrode pipelines and processing units.
• Automotive: Monitoring TAN in engine oils helps determine oil degradation, preventing wear and tear in engines.
• Aviation: TAN testing ensures jet fuel remains free from corrosive contaminants that could impact aircraft performance.
• Power plants: TAN analysis in turbine oils helps in preventive maintenance, ensuring smooth operations and reducing downtime.

How is TAN measured?

TAN is typically measured using titration methods, where a base solution (usually potassium hydroxide) is added to an oil or fuel sample. The acid content is determined by the amount of base required to neutralize it. This measurement can be performed manually through colorimetric titration or automated using potentiometric titration.

The formula for calculating TAN

The formula for calculating TAN is the following:

The formula is broken down by the following:

• V is the volume of titrant in milliliters (mL) used for the sample
• B is the volume of titrant in milliliters (mL) used for the blank
• N is the normality of the titrant (in mol/L)
• W is the weight of the sample in grams
• 56.1 is the molecular weight of potassium hydroxide (KOH)

Here's an example of TAN being calculated for a lubricant sample:

• Volume of titrant used for the sample (V): 10 mL
• Volume of titrant used for the blank (B): 0.5 mL
• Normality of the titrant (N): 0.1 mol/L
• Weight of the sample (W): 2 grams

Using the TAN formula:

Here is the step-by-step:

• Subtract the blank volume from the sample volume: 10 − 0.5 = 9.5 mL
• Multiply by the normality of the titrant: 9.5 × 0.1 = 0.95
• Multiply by the molecular weight of KOH: 0.95 × 56.1 = 53.295
• Divide by the weight of the sample: 53.295 ÷ 2 = 26.6475

So, the total acid number (TAN) for this lubricant sample is approximately 26.65 mg KOH/g.

What is the TAN testing process?

The TAN testing process consists of four core steps:

1. Sample preparation: The sample is prepared by dissolving it in a solvent mixture (such as toluene, isopropanol, and water) to ensure even titration.
2. Titration: A known concentration of KOH solution is added to neutralize the acids in the sample.
3. Endpoint detection: The neutralization endpoint is detected using either a color indicator or an electrochemical sensor.
4. Calculation: The TAN value is calculated based on the volume of titrant used to reach neutrality.

What factors affect TAN values?

Several factors can influence the total acid number, including:

• Oxidation: Exposure to oxygen leads to the breakdown of oil molecules, increasing acidity.
• Contamination: Water, dirt, or other foreign substances can alter acidity levels.
• Thermal degradation: High temperatures accelerate oil breakdown, raising the TAN value.
• Additive depletion: Some oils contain anti-acid additives that diminish over time, affecting TAN measurements.

TAN vs. Total base numbers (TBN)

While TAN measures the acidity of a lubricant, total base number (TBN) assesses its alkalinity. TBN is crucial for monitoring the effectiveness of engine oils in neutralizing acidic byproducts formed during combustion. Oils with high TBN values are often used in diesel engines to counteract acid buildup. In contrast, TAN is more relevant for assessing oil degradation and contamination. Together, both measurements provide a comprehensive view of an oil's condition.

Tribology and TAN in relation to maintenance

Tribology is the study of friction, wear, and lubrication, and is closely related to the total acid number (TAN) in lubricants. Here's how the two connect concerning maintenance: By regularly monitoring the TAN, tribologists can assess the condition of the lubricant and predict potential issues. This helps in planning maintenance activities to prevent excessive wear and extend the life of machinery.

Techniques in TAN analysis?

Several techniques are used for accurate TAN measurement:

• Potentiometric titration: An electrochemical method (opens in new tab) providing precise endpoint detection.
• Colorimetric titration: Uses an indicator dye that changes color at the endpoint.
• Infrared spectroscopy: Analyzes molecular vibrations to detect acidic compounds.
• Fourier transform infrared (FTIR) spectroscopy: Provides real-time monitoring of oil degradation and acidity.

What are the industry standards for TAN?

Standardized methods ensure consistent and accurate TAN measurement across industries. Two primary standards are widely followed:

ASTM D664

ASTM D664 is a widely recognized method for determining TAN in petroleum products, lubricants, and biodiesel. It employs potentiometric titration for precise acidity measurement. This method is preferred in industries requiring high accuracy and repeatability.

ISO standards

The international organization for standardization (ISO) has established guidelines for TAN measurement, ensuring compliance across different industries. ISO 6618 is one such standard that defines the procedures for titrimetric TAN analysis in petroleum products.