FINA Lubricants Neutralization Number (TAN)

Total Acid Number (TAN)

Glossary description: A neutralization number intended for measuring all acidic and acid-acting materials in the lube, including strong and weak acids.

How is the test performed? The sample is diluted with solvent and base (alkaline), in the form of potassium hydroxide (KOH), which is added in controlled increments from a burette (a calibrated tube with a stopcock valve at the bottom to allow gravity feed) until the sample is neutralized. This process is called a titration. The KOH is the titrant. Neutralization is determined with a volt or pH meter, and the neutralization point usually occurs around pH 11. Acid number is calculated from the amount of KOH consumed.

Reporting units: Milligrams (mg) of potassium hydroxide/milliliter (ml) of lube.

Repeatability: +/- 10% Range: 0.01 – 20 (practical)

Technical overview: Many standard lube additives yield a non-harmful TAN even as new, thus it is important to establish baseline values. Phosphorus/Zinc compounds may contribute as much as 2.0 TAN dependent upon concentrations. Magnesium, Calcium, and Barium (even though they are alkaline contributors), can still yield a TAN value of as much as 2-3, again dependent upon concentration, so a ‘fully compounded’ lube might easily have a starting acid number of 2-5. This is a difficult concept to find comfort with, but it stems from the tendency to regard all acids as corrosive, rather than acids in the sense of their electrochemical behavior, i.e., there are degrees of acid behavior. Such degrees are expressed as pH, which represents acid strength (lower = greater strength), but not necessarily the quality. PH, therefore, does not relate to AN in any sense.

Interpretation: Acid number is comprised of both strong and weak acids. Strong acids tend to be corrosive, and are accordingly much more necessary to control. Examples of strong acids are:

¨ Hydrochloric Acid (HCL) breakdown of Freon in refrigeration compressors; certain work environments.

¨ Hydrofluoric Acid (HF) breakdown of Freon.

¨ Sulfuric Acid (H2SO4) Diesel combustion by-product from sulfur in fuel; certain work environments.

¨ Nitric Acid (HNO3) certain work environments (e.g., a phosphate mine).

All of the above will etch or corrode various metals in varying degrees. A low pH (<4.5) probably indicates the presence of one or more of the above acid types in at least the beginning stages of concern, particularly if water is present, since the acid can then achieve its fullest chemical potential.

Weak acids do not usually cause corrosion problems. However weak acids can attack certain leaded bearings and coatings. Often the development of weak acids may indicate lube oxidation and, as such, represents an effect, not a cause. Once this occurs to excess, however, it can be self-sustaining, thus a lube might need to be drained (or “sweetened” {a partial drain}, if the sump capacity is too large to drain economically).

Crude rule of thumb: Increases of 2.0 or 50% (whichever is greater) against baseline (new lube reference) = drain or sweeten (drain partially and top with fresh oil).

Suggested application: Plant equipment; engines using high sulfur fuel, most non-engines, extended drain studies for engines.

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