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In an article written by W. Van Dam of Chevron Oronite Technology and published in Practicing Oil Analysis magazine’s July-August 2002 issue, a number of different ASTM test procedures were outlined for determining acid number (AN) and base number (BN).
There are numerous test methods prescribed by ASTM for determining both BN and AN (Table 1). Although these methods use the same basic idea, a wet chemistry titration to neutralize either acidic or basic (alkalinic) components in an oil sample, they differ either in the titrant or the method used to determine the end point of the neutralization reagent.
To clear up some of the ambiguities surrounding the different test methods, we asked Janet Barker, research scientist in the Petroleum Products Research Department at Southwest Research Institute, to answer some questions about acid number and base number testing.
Strictly speaking, the pH of a solution is a measure of hydrogen ion concentration. In aqueous solutions, a pH of 0 to 6 is acidic, pH 7 is neutral, and a pH of 8 to 14 is alkaline. Acidic and alkaline character of fluids cannot be fully described by the pH alone because the pH describes the strength of an acid or alkali, not its concentration.
Acid or base concentrations, not strengths, are actually being investigated when we talk about acid and base numbers. In fact, running either of these methods tests the fluid’s resistance to being neutralized. Acid and base number are used to measure the concentration of acidic components (organic acids formed by oil oxidation, certain additives, etc.) and basic components, for example, overbase detergent additives in engine oils, as a means of performing condition-based oil changes.
ASTM does not specifically recommend any one procedure over another. ASTM is an entity that provides written instructions about how to perform reliable testing and monitoring. It provides several applicable methods from which to choose. Some testing may be more reliable or applicable to the given circumstances, but the final decision ultimately rests with the end user.
ASTM currently does not prescribe AN or BN test procedures specifically for used oils, and care must be exercised to select the most appropriate procedure for a specific application.
Colorimetric titrations use an indicator with a color change at a specific pH to detect the titration endpoint, while potentiometric titration uses an electrical activity reading to detect the same reading. Some indicators used for colorimetric titrations provide indistinct endpoints that are difficult to detect, even for trained individuals.
In addition, some oils, especially highly colored or heavily contaminated oils such as diesel engine oils, are more suited to potentiometric endpoint titration than a colorimetric endpoint method. Oil darkens with age and this darkening can also contribute to indistinct endpoints.
Potentiometric titrations will distinguish titration endpoints on samples otherwise obscured due to the oil’s colored nature. It is important to be consistent. Pick a test procedure that applies to your specific circumstance and run all samples to this method.
Any oil can have an acid and base number at the same time because they measure different constituents with the two different titration procedures. In fact, some additives, particularly in engine oils, are amphoteric - they can behave as either an acid or a base.
Both values are valid and it is wise to track both in some cases. In fact, the numeric crossing point of acid number and base number is sometimes used as an indicator of a condition-based oil change point for in-service natural gas engine oils. Acid and base numbers are not reciprocal in nature, although acid numbers do trend up as base numbers trend down through an oil’s life span.
ASTM covers the determination of two parameters, strong acid number and acid number, sometimes referred to as weak acid number. When using ASTM D664 to determine strong acid number, the sample is titrated to a freshly prepared nonaqueous acidic buffer solution’s meter equivalent or well-defined inflection point.
By contrast, a sample run for acid number is titrated to the freshly prepared basic buffer solution’s meter equivalent or well-defined inflection point. In simpler terms, this means that the strong acid number titration reaches the end-point sooner than the conventional acid number reported by most oil analysis labs. In general, strong acid number measures only strong acids and hence would provide a lower result than weak acid number.
Conventional (weak) acid number measurement reflect total acidic components including, according to ASTM, organic and inorganic acids, esters, phenolic compounds, lactones, resins, salts of heavy metals, salts of ammonia with other weak bases, acid salts of polybasic acids, and additional agents such as inhibitors and detergents.
Because used oils provide ill-defined or no inflection points, it is standard practice to run all used oils to the basic, buffer endpoint and report this as an oil’s acid number. Strong acid numbers are rarely requested or reported, especially for used oils, although they can be useful in determining the presence of strongly acidic contaminants such as hydrochloric and hydrofluoric acids, formed by freon decomposition in refrigeration compressors, and other sources of strong acids.
High over-based oils are specially formulated for these engines to provide the longer drain intervals that long-haul vehicles require, and to counteract the effects of higher fuel sulfur levels used in locomotive diesel fuel and bunker fuels used in marine applications.
While a higher base number oil can potentially extend the useable life of the oil, it is important to remember that higher base numbers are provided by higher levels of metallic detergent additives. When these burn, they form ash, which can deposit on valve stems and other engine components. Such deposits can be detrimental to engine life. Most manufacturers set ash level limits for their engines.