Base Number: ASTM D2896 vs. D4739

Acidic contamination can happen from various sources including oxidative and thermal stress. This can result in undesirable by-products that increase machine wear and decrease tribological performance. Acids are some of the most impactful actors in the degradation of a lubricant, impacting the base oil and its additives.

Reserve alkalinity is the term used in the measurement of an engine oil’s ability to tolerate contamination by acids.  Various lubricating oil additives, such as detergents, dispersants and antioxidants, impact on reserve alkalinity by neutralizing acidic contaminants in the engine oil.

The most common method of evaluating an engine oil’s ability to neutralize acids is to measure Base Number (BN). Two methods are generally accepted for measuring reserve alkalinity are ASTM D2896 and ASTM D4739.

D2896 versus D4739: Which test do you use?

Similarities

The two methods are similar in that they involve adding a measured amount of acid to the oil until all of the reserve alkalinity has been neutralized. The BN is calculated from the amount of acid required to titrate the sample to a specified endpoint.

Differences

The difference between the two methods comes in the choice of acid used as the reagent, and solvent in which the oil is dissolved to run the test. D2896 uses a stronger acid (perchloric acid) and more polar solvent system than D4739  which uses hydrochloric acid. The combination of a stronger acid and a more polar solvent results in a more repeatable method that measures the presence of both strong and weaker base. For some lubricant additive types, D4739 does not measure all the base that is present. In general, D4739 gives a lower result because only the stronger basic species are titrated.

Why are there two test methodologies?

A common concern heard from customers is that when they look at an engine oil spec sheet, they may see a BN = 10. However, when they review their oil analysis results from the lab, the BN = 8. This discrepancy can be explained easily once they realize that ASTM D2896 is usually specified for monitoring NEW OILS (you likely will see this test method noted on the certificate of analysis). ASTM D4739, on the other hand, is reserved for USED / IN-SERVICE oils and is most commonly used by oil testing laboratories.

Generally, the Base Number test is supposed to determine how much reserve alkalinity is left and how fast is it being depleted. For performing a test on fresh new oils to confirm oil quality, ASTM D2896 is the specified test as it can confirm that all additives have been included in the blend. The stronger perchloric acid can neutralize both strong and weak alkaline components providing a more complete picture of the quality of the new oil.

However, in a used oil sample, not ALL the base in the sample is functionally neutralizing acids. If you’re using ASTM D2896 to measure USED oils, it gives you a rosier picture than the D4739 which only measures the active bases in the sample. This is an important distinction if you are looking to extend oil drain intervals. The stronger perchloric acid used in D2896 reacts with both the strong and weak alkaline components in the used oil giving you a higher BN result. This could result in an overextended drain interval. The D4739 test only monitors the amount of the acid neutralizing additive package is left. Hence, the D4739 is  appropriate for used oil analysis to monitor the depletion of the additive package over time.

Additional Considerations

The ASTM D2896 method and D4739 methods are both offered by Fluid Life. However, the D4739 is the preferred methodology because:

• The lab is most often monitoring the condition of in-service/used oils
• ASTM D2896 can provide misleading results for in-service lubricants and may impact on maintenance decisions
• Health & safety concerns: Perchloric acid used in D2896 is an extremely strong acid – it’s corrosive, and oxidizer and can cause health issues. Additionally, the solvent, Chlorobenzene, is harmful to the environment

If you are only requesting D2896 testing based on the original certificate of analysis from your oil supplier, you may not be getting the right data to support your maintenance decision-making needs. Talk to us today about reserve alkalinity and choosing the right BN test to support your oil analysis programs.