Measuring TAN and TBN in Oil

A lubricant containing a high concentration of acidic compounds can lead to clogged oil filters and corrosion of machine parts due to the formation of sludge and varnish. When a lubricant breaks down, acidic by-products will be formed from the chemical decomposition of the base stock and additives in the presence of air and heat.

Total Acid Number (TAN) is a measure of acid concentration present in a lubricant. The presence of acidic contamination, additive package, and oxidation by-products govern the acid concentration of a lubricant.

Occasionally, the depletion of an additive package may lead to an initial decrease in TAN of fresh oil, however the accumulation of acidic contaminants and oxidation by-products in an oil over time will always lead to an increase in TAN. This is the most meaningful test in industrial machinery applications, but sometimes it is also recommended in engine applications along with Total Base Number (TBN).

Total Base Number

TBN is a measure of alkaline concentration present in a lubricant. Alkaline additives are used to formulate engine oils to prevent the build-up of acids in a lubricant as it breaks down.

In a lubricant, the TBN level is targeted for its application. Generally, gasoline engine oils are formulated with starting TBN of about 5 - 10 mg KOH/g, while diesel engine oils are likely to be higher (15 - 30 mg KOH/g) due to the more severe operating conditions.

Marine engines and other similar specialized applications may need more than 30 mg KOH/g. As the oil remains in service, this BN additive is depleted. However, the lubricant will not perform its function after the alkaline additives are depleted beyond a certain limit. This can lead to varnish, sludge, and corrosion of the engine. At this point, the oil should be changed or topped-off.

Techniques for Measuring TAN and TBN

Potentiometric Titration

Potentiometric titration methods are the most widely accepted techniques to measure TBN or TAN. Potentiometric titration is very accurate, and can measure a variety of sample types, regardless of color or contaminants present. It does involve solvents and a careful technique, so it is best performed by a well-trained chemist.

The equipment cost can be reasonable, but even the most easy to use and easy to maintain systems can be quite expensive. The need to purchase solvents and dispose waste can also make it more costly compared to other techniques.

Metrohm Fully Automatic TAN and TBN Titration System

TAN

ASTM D664 is the commonly accepted test method for TAN. In this technique, the sample is dissolved in isopropanol and toluene with a small amount of water, and the solution is titrated with alcoholic potassium hydroxide.

A reference electrode and a glass electrode are then placed in the solution and connected to a potentiometer/voltmeter. The titration endpoint is reached when the meter reading in millivolts corresponds to a buffer solution, or a well-defined inflection point is found.

TBN

ASTM D2896 is the accepted method for TBN for used and new oils. This involves dissolving the sample in glacial acetic acid and chlorobenzene and titrating with perchloric acid in glacial acetic acid. Potentiometric titration is used to determine the endpoint with a glass indicating electrode within the solution and a reference electrode connected to the sample solution through a salt bridge.

Potentiometric determination of TAN (blue = titration curve, pink = ERC)

ASTM D4739 is another method that is also used to measure TBN in used oils. The titrant in D4739 is a milder acid than in D2896, hydrochloric versus perchloric. This can lead to lower TBN results if there are weak bases present in the sample that are not neutralized by the hydrochloric acid. The BN additive is a fairly strong base, so including weaker bases in the measurement is not significant when looking at the remaining lifetime of the lubricant.

The following are pros and cons of potentiometric titration:

Pros:

• Accurate
• The industry standard
• Tolerant of difficult samples (contaminants, color)

Cons:

• Requires expensive equipment
• Relatively high cost per test
• Can require up to 4 g of sample
• Requires solvents
• Requires a skilled operator
• Must be carried out in a lab

Colorimetric Titration

As long as the oil is not dark, colorimetric titration methods can be used. In these methods, the titration endpoint is detected by a visible change in color through the use of an indicator that reacts to a pH change. For TBH measurement, it can be difficult due to the dark nature of crankcase oils, especially when soot may be present.

In the ASTM D974 method, a sample is dissolved in p-naptholbenzene, toluene, and isopropyl alcohol containing water, and the solution is treated with HCl or KOH until a color change indicates the titration endpoint. The TAN or TBN is determined by the amount of HCl or KOH added to reach the endpoint. Although similar to the ASTM D974 method, ASTM D3339 is designed for a smaller sample size (2 g vs 20 g).

The following are pros and cons of colorimetric titration:

Pros:

• Accurate
• Does not need expensive electrodes

Cons:

• Cannot be used in contaminated or dark oils
• Relatively high cost per test
• Must be carried out in the lab
• Requires solvents
• Requires a skilled operator

Field Test Kits

For TAN or TBN measurement, test kits are available as a convenient first line test. These kits include premeasured reagents, and the result obtained is measured as a color change by eye. In certain cases, the kits include a predesignated amount of HCl or KOH titrant, which allows them to provide a qualitative pass or fail response.

Other kits contain titrant syringes that are marked with graduated increments already converted to TAN or TBN units. The user begins with a known amount of sample in a test tube, adds aliquots of HCl or KOH reagent, and observes for a change in color. Once the solution changes color, the number on the syringe placed next to the plunger is checked and that indicates its TAN or TBN.

The following are pros and cons of the field test kits:

Pros:

• Fast results
• Can be performed in the field
• Premeasured materials

Cons:

• Cannot be used in contaminated or dark oils
• Moderate cost per test
• Requires solvents
• Requires a skilled operator

The Fluidscan – Infrared Spectroscopy

Infrared spectroscopy employs a detector, radiative source, and a computer to study the interaction between matter and light. Oil degradation and oxidation lead to build-up of acids in a lubricant, which can be detected by changes in the infrared spectrum.

Nitration and oxidation products emerge as peaks in the infrared spectrum between 1600 and 1800 cm^-1. However, due to the mixture of acids produced during the breakdown of a lubricant, the spectrum does not have a single peak that can be correlated to TAN.

Changes in TBN are seen in the infrared spectra as decreases in absorbance peaks related to the basic additives present in the engine oils, as well as changes to standard degradation peaks. Calcium or magnesium sulphonates, salicylates, and phenates are the most typically used BN additives.

While a specific BN additive package of a lubricant may include any mixture of these, they all have peaks in the 1000 and 1900 cm^-1 region of the infrared spectrum.

The following are pros and cons of the Fluidscan – infrared spectroscopy:

Pros:

• Fast results
• No solvents
• Very low cost per sample
• Can be performed in the field
• Any operator can obtain good results
• Only needs 2 drops of oil
• Precalibrated for more than 90% of oils

Cons:

• Very sooty samples (>3%) cannot be measured
• Oil must fit into a precalibrated family

The FluidScan spectra show an increase in the oxidation and sulfation byproducts with increasing TAN for gear oils.

The FluidScan spectra show depletion of the BN additive and an increase in the sulfate by-products with decreasing TBN for heavy duty engine oils.

Comparison of FluidScan TAN measurements with Titration TAN measurements shows excellent correlation.

Comparison of FluidScan TBN measurements with Titration TBN measurements shows excellent correlation.

The portable handheld spectrometer, FluidScan is used to measure oil condition and chemistry. Hundreds of used and new lubricants of different types and levels of degradation have been gathered into a sample library on the FluidScan.

Using a standard ASTM titration method (D664 for TAN and D4739 for TBN), the infrared spectrum of these lubricants was recorded together with their TAN and/or TBN value. Multivariate data analysis techniques are then employed to relate the known TBN or TAN to the infrared spectrum. The end-result is quantitative readings of TBN and TAN using infrared spectroscopy.

Conclusion

Lubricant condition can be measured by monitoring TBN and TAN. Several methods are available, ranging from quick field tests to expensive laboratory methods. In a laboratory setting, methods are selected based on the highest repeatability and accuracy that can be obtained with a decent throughput.

Out in the field, a trustworthy result must be obtained quickly, so that corrective or preventative maintenance action can be taken before any major equipment failure. Tthe best method is one that depends on the application requirement.

References:

1. http://www.machinerylubrication.com/Read/1052/ acid-number-test

2. “Using Infrared Spectroscopy for the Determination of TAN and TBN in Machinery Lubrication Oils,” Christy L. DiCologero, Thomas G. Barraclough, and Patrick F. Henning. Spectro, Inc., QinetiQ North America

3. “Overview of FluidScan Handheld Infrared Oil Analyzer,” White Paper, spectrosci.com

4. “Determination of the total acid number in petroleum products,” Metrohm, Application Bulletin AB-404_1_EN

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.