Acid Number – The Indicator of Serviceability

Oil Acidity is an Indicator of Serviceability. Oil acidity increases with oxidation and the introduction of combustion by-products.

This test measures the total acids present so is often referred to as the Total Acid Number (TAN). This includes the strong mineral acids as well as strong and weak organic acids.

The strong mineral acids can be formed from Halogens (refrigerant gases, landfill gas) and Sulphur found in fuel. These can produce corrosion, oil thickening, deposit formation and accelerated wear.

The weak organic acids are from oil oxidation and additives and are usually non-corrosive.

The units used for the acid number (TAN) test are milligrams of Potassium Hydroxide per gram of oil (mgKOH/gm), which is the same as the Base Number test.

If you have a low TAN, this could be caused by:

  • Admixture with lower TAN oil
  • Depletion of Additives Contributing to TAN
  • Contamination with alkaline oils or materials e.g. cleaning agents

If you have a high TAN, this could be caused by:

  • Admixture with higher TAN Oil
  • Oil oxidation
  • Contamination with Acid Combustion Products
  • Thermal Breakdown of Certain Additives

There are two test methods used by laboratories, ASTM D664 and ASTM D974. ASTM D974 is a colour-indicator titration test (look for a colour change), whereas ASTM D664 is a potentiometric titration test and is more widely used for the TAN test, especially for dark or black oils where you cannot see the ASTM D974 colour change.

Although ASTM D664 and D974 both measure the TAN they do give slightly different results. Also ASTM D664 can be reported in two ways, either from the Buffer Inflection Point (BEP) or from the Inflection End Point (IEP). The BEP is often used where the IEP cannot be determined or the oil exhibits multiple IEPs. As the oil ages in service the IEP will give a slightly higher result to the BEP. This difference between the two increases with oil age and can be as high as 3 units.

As mentioned in our early article on Total Base Number (TBN) these differences in reported results, depending on the test method used and the way in which its result is determined (TAN by IEP or BEP), is particularly important to understand in the case of gas engine oils where the cross over point is often used to determine when the oil should be changed.

As the chart below shows, the test used and how it is reported can mean that the cross-over point can be as low as 1200 hours or as high as 1900 hours. Therefore you must ask the laboratory doing the testing to advise the method used, which is usually shown on the report and how the result is being determined for the TAN.


So when should you change the oil? Bearing all this in mind you should still consult your oil supplier and/or testing laboratory to ascertain their alert levels, as these would have been determined from their technical experience and historical data.

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