TOTAL ACID NUMBER
A high concentration of acidic compounds in a lubricant can lead to corrosion of machine parts and clogged oil filters due to the formation of varnish and sludge. 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 acid concentration of a lubricant depends on the presence of additive package, acidic contamination, and oxidation by-products. Occasionally, the depletion of an additive package may cause an initial decrease in TAN of fresh oil. However, the accumulation of oxidation by-products and acidic contaminants in an oil over time will always lead to an increase in TAN. This test is most meaningful in industrial machinery applications although sometimes it is recommended in engine applications along with Total Base Number (TBN).
TOTAL BASE NUMBER
Total Base Number (TBN) is a measure of alkaline concentration present in a lubricant. Engine oils are formulated with alkaline additives in order to combat the build-up of acids in a lubricant as it breaks down. The TBN level in a lubricant is targeted for its application. Gasoline engine oils are typically formulated with starting TBN around 5-10 mg KOH/g whereas diesel engine oils tend to be higher (15-30 mg KOH/g) due to the more severe operating conditions. Specialized applications, such as marine engines, may require >30 mg KOH/g. As the oil remains in service, this BN additive is depleted. Once the alkaline additives are depleted beyond a certain limit the lubricant no longer performs its function, and the engine is at risk of corrosion, sludge, and varnish. At this point it is necessary to top-off or change the oil.
Techniques for measuring TAN and TBN
► THE FLUIDSCAN – INFRARED SPECTROSCOPY
Infrared spectroscopy uses a radiative source, a detector, and a computer to study the interaction of matter and light. The build-up of acids in a lubricant caused by oil degradation and oxidation can be detected by changes in the infrared spectrum. Oxidation and nitration products appear as peaks in the IR spectrum between 1600-1800 cm-1. Because of the mixture of acids generated during the breakdown of a lubricant, there is not a single peak in the spectrum which can be correlated to TAN.
Changes in TBN are observed in the IR spectra as decreases in absorbance peaks related to the basic additives that are present in the engine oils, as well as changes to standard degradation peaks. The BN additives most typically used are calcium or magnesium sulphonates, phenates and salicylates. The specific BN additive package of a lubricant may contain any mixture of these, but they all have peaks in the 1000 and 1900 cm-1 region of the infrared spectrum.
The FluidScan is a portable handheld spectrometer used to measure oil condition and chemistry. On the FluidScan, hundreds of new and used lubricants of widely varying type and level of degradation have been gathered into a sample library. Their infrared spectrum was recorded along with their TAN and/or TBN value using a standard ASTM titration technique (D4739 for TBN and D664 for TAN). Next, multivariate data analysis techniques are used to relate the known TAN or TBN to the infrared spectrum. The end-result is quantitative readings of TAN and TBN using infrared spectroscopy.
► POTENTIOMETRIC TITRATION
The most widely accepted techniques for measuring TAN or TBN are the potentiometric titration methods. Potentiometric titration is very accurate and can measure many different sample types without regard to color or the presence of contaminants. It does involve solvents and careful technique and is best performed by a well-trained chemist. The cost of equipment can be reasonable, but the most user-friendly and easy to maintain systems tend to be fairly expensive. The need to purchase solvents and to dispose of waste can also make it more costly compared to other methods.
For TAN, the commonly accepted test method is ASTM D664 which involves dissolving the sample in toluene and isopropanol with a bit of water and titrating that solution with alcoholic potassium hydroxide. A glass electrode and reference electrode are placed in the solution and connected to a voltmeter/potentiometer. The titration endpoint is reached when a well-defined inflection point is found or the meter reading in millivolts corresponds to a buffer solution.
ASTM D2896 is the accepted method for TBN for new and used oils. The sample is dissolved in chlorobenzene and glacial acetic acid and titrated with perchloric acid in glacial acetic acid. The endpoint is determined by potentiometric titration with a glass indicating electrode inside the solution and a reference electrode connected with the sample solution by a salt bridge.
Another method ASTM D4739 is also accepted for measuring TBN in used oils. The titrant in D4739 is a milder acid than in D2896, hydrochloric vs perchloric, which can result in lower TBN results if there are weak bases present in the sample that the hydrochloric acid will not neutralize. The BN additive is a relatively strong base, so the inclusion of weaker bases in the measurement is not important when looking at the lifetime left in the lubricant.
► COLORIMETRIC TITRATION
Colorimetric titration methods can be used as long as the oil is not dark. For these methods, the endpoint is detected by a visible color change through the use of an indicator which reacts to a change in pH. For TBN measurement, it can be challenging due to the dark nature of crankcase oils, especially when soot may be present.
In ASTM D974, a sample is dissolved in toluene, p-naptholbenzene, and isopropyl alcohol containing water. Then the solution is treated with KOH or HCl until a color change occurs to indicate the endpoint. The amount of KOH or HCl added to reach the endpoint determines the TAN or TBN. ASTM D3339 is a similar method to ASTM D974, except that it is designed for a smaller sample size (2 g vs 20 g).
► FIELD TEST KITS
Test kits are available as a convenient first line test for TAN or TBN. The kits contain premeasured reagents and the result is measured as a color change by eye. In some cases the kits contain a predesignated amount of KOH or HCl titrant so that they provide a qualitative pass or fail response. In other kits, titrant syringes will come marked with graduated increments already converted to TBN or TAN units. The user starts with a known volume of sample in a test tube, and then adds aliquots of KOH/ HCl reagent and observe for color change. When the solution changes color, they check the number on the syringe next to the plunger and that is their TBN or TAN.
Monitoring TAN and TBN is an important test for measuring lubricant condition. There are several methods available ranging from expensive laboratory methods to quick field tests. In a laboratory setting, methods are chosen based on the highest accuracy and repeatability that can be achieved with a decent throughput. Out in the field, it is most important to get a trustworthy result quickly enough that preventative or corrective maintenance action can be taken before major equipment failure. The best method to use depends on the application need.