Acid Number (AN)

Oxygen in the air constantly reacts with oils, producing organic oxidation products that are acidic in nature.


CMT offers a manual titration test and an electronic device for analyzing the acid number.


The acidification of the oil leads to increased viscosity and material attack, which must be prevented.


The test kits from CMT can be used for a wide variety of oil types such as gearboxes or turbines.

Every oil reacts permanently with the oxygen in the air when it comes into contact with it, producing substances with acidic properties. At ambient temperature, this process is slow and has little impact on oil condition.
However, at the high temperatures that prevail, for example, inside a running engine, the willingness to react is much higher. This applies in particular to sliding surfaces, where the locally prevailing temperature is often significantly higher than that of the surrounding metal parts. Combustion products and wear particles dissolved in the oil can also promote the reaction of the oil with the oxygen in the air. The decomposition products from these reactions are organic acids, which react only slowly with the alkaline BN additives and therefore often coexist. They increase the viscosity of the oil and can form a film on hot surfaces.
The acid number or AN (Acid Number) is a measure of the total of weak organic and strong inorganic acids in the oil. It can be used for gear oils, gas turbine oils, gas engine oils and hydraulic systems. Circulating oils for internal combustion engines are usually not tested for their AN unless they are particularly heavily loaded. The only exception are gas engines, in which the BN can drop quickly and the high operating temperatures of the engines can lead to high levels of weak organic acids in the oil.

Test devices for AN measurement:
Acid Number Titrations Test Kit - CMT's Acid Number Titrations Test Kit comes complete with all necessary reagents and equipment as a kit.

When performing tests, a titration is first carried out using chemicals. A titrant from a bottle is dosed drop by drop into a red carrier liquid until a clear color change from red to green can be seen. Adding a certain amount of the oil to be tested to this solution causes the color to change back to red. Repeated addition of individual drops of the titrant causes a noticeable color change to green. The number of drops required until this color change is then multiplied by a factor to give the AN of the oil.

Electronic Acid Number Test Kit - The Electronic Acid Number Test Kit measures with the highest accuracy in the same measuring range as the Acid Number Titrations Test Kit.

Replacement reagents can be delivered at short notice. Reagents are supplied in kits complete for 50 tests. The AN reagents are classified as dangerous goods UN 1903 or 1170.


Product info

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Your Benefits

  • Great accuracy
  • Simple test procedure
  • Electric test cell or spot test available
  • Increases lifetime of engine parts

Ordering Information


Electronic Acid Number Test Kit
Range: 0-6 AN
Accuracy: +/- 0.1 AN
Test Time: 2 minutes
No. of Tests: 25
Reagents: hazardous components

Acid Number Reagent Pack
No. of Tests: 25
Reagents: hazardous components

Spare Acid Number Console

Glass Vial Consumables Pack (100)

Acid Number Drop Test Kit
Range: 0-6 AN
Accuracy: +/- 0.3 AN
Test Time: 2 minutes
No. of Tests: 25
Reagents: hazardous components