Acid Number (AN)

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Testing the acid number (AN) of lubricants is important to ensure that they are functioning properly and not corroding metal parts. The AN measures the amount of acidic material present in a lubricant. This information can help you choose the right lubricant for your equipment and minimize maintenance costs. In this post, we will discuss what acid number testing is, how it is performed, and what the results mean.

What is Acid Number Testing?

In lubricant analysis, an acid number test measures oxidation levels, additive depletion, and acid contamination. Basically, acid number helps you understand the remaining life of your lubricant. Oil samples with higher acid numbers have a higher acidity level.

As far as lubricant oxidation, acid number testing measures it a bit differently. Rather than measuring the oxidation rate, AN tests the byproduct of oxidation. Your lubricant develops acidic byproducts as oil oxidizes and antioxidants get depleted. So, acid number tests are incredibly helpful for detecting acid accumulation in oils. High quantities of acid also signal additive depletion in addition to oxidation. Corrosion of equipment parts is frequently the result of this. By monitoring the acid level, you can, however, stop this damage. You can also calculate the rate of additive depletion by monitoring your AN trends.

Acid Number vs. pH

Acid number and pH testing both measure acidity or alkalinity, but they measure different characteristics. By applying a pH meter to your lubricant, you can determine how corrosive your lubricant is. Although the pH meter does not measure alkalinity or acidity, its value indicates the lubricant’s apparent pH. In applications where corrosive lubricants could cause significant harm or with a high risk of strong acid formation or contamination, the pH test method is helpful.

AN is a more effective or reliable method for detecting weak acids, which don’t easily dissociate in water. Due to this, pH is not a reliable method to determine the weak acid concentration in lubricants.

Is the Acid Number Test Vital to Lubricant Analysis?

When antioxidants become exhausted, acids aren’t neutralized and begin to build up. The results of your acid number test can provide valuable insights into the health of your lubricant.

The Types of Acids You Can Detect

There are several common acids found in lubricants that you can detect with acid number tests, such as:

  • Hydrochloric and Hydrofluoric Acids – These acids are the result of refrigerant breakdown in refrigeration compressors.
  • Nitric Acids – These acids are commonly found in diesel, gasoline, and natural gas engines, as well as in compressors.
  • Organic Acids – Found in all common lubricants, these indicate oil oxidation.
  • Sulfuric Acids – These are typically found in hydraulic oil, gear oil, diesel fuel, and natural gas engines and compressors.

Acid Number Testing Applications

Acid number testing is not necessary for fluids with high water content, like water glycols and oil/water emulsions. However, acid number testing can benefit all industrial lubricating oils and some combustion engine oils, such as:

  • Mineral Oils
  • Gear Oils
  • Power Train Oils
  • Hydraulic Anti-Wear Oils
  • Engine Oils
  • Automatic Transmission Oils
  • Refrigeration and Air Compressor Lubricants
  • Alkylbenzenes and other compressor lubricants
  • Phosphate Esters
  • Polyolesters
  • Polyglycols

Acid Number Field Test Kits

A field test kit that measures acid number is readily available. Acid number testing in field environments is made easier by using premeasured reagents. Most of these kits provide a pass/fail test, which involves diluting the solution with KOH (potassium hydroxide). Although you wouldn’t get an exact acid number from this method, you would be able to determine if the lubricant is within the proper range.

Additionally, there are some acid number field test kits that will provide an exact result. To ensure that the oil samples are precisely the same size, these kits provide a volume-sampling syringe. Then, the single-use burette of KOH has a scale that represents your sample’s acid number.

A Key Component of Lubricant Analysis

Understanding lubricant characteristics and how to properly manage them is crucial for maintaining machinery. Lubricants are a necessary component of any industrial process. The acid number of lubricants is a crucial part of your lubricant analysis because additive depletion, contamination, and oxidation are common causes of lubricant degradation. It is also a useful method for properly determining the serviceability and remaining life of your lubricants.

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