Introductory Guide to Acid Number Lubricant Analysis

Lubricants are an essential part of any industrial operation, but understanding their properties and how to properly maintain them is critical for keeping machines running smoothly. One important property of lubricants is their acid number (AN), which you can determine through simple laboratory analysis. In this blog post, we’ll provide an introduction to the AN test and explain its importance for maintaining effective lubrication programs. We’ll also discuss some of the factors that can affect AN values and offer tips for ensuring accurate results.

What is the Acid Number Test?

An acid number (AN) test is a common lubricant analysis test for measuring oxidation levels, additive depletion, and acidic contamination.


Acid number testing measures oxidation, but not by the oxidation rate. Instead, AN tests measure the byproduct of oxidation. As oil oxidates and antioxidants are depleted, acidic byproducts form in your lubricant. That makes acid number testing useful for monitoring acid buildup in oils.

Additive Depletion

In addition to oxidation, high acid levels are also an indicator of additive depletion. This often leads to corrosion of equipment components. However, you can prevent this damage by monitoring the acid level. Furthermore, trending AN can help determine the rate of additive depletion.

In conclusion, the acid number serves as a gauge for the serviceability of your lubricant.

How Does the Acid Number Test Work?

A part of your oil sample is diluted before being titrated to an endpoint with a basic solution. The resultant acid number indicates how much titrant is required for the given sample volume. Higher numbers denote a higher level of acidity.

Why is this an Essential Lubricant Analysis Test?

As antioxidants are used up, acids are not neutralized and begin to build up in concentration. You gain valuable insight into your lubricant health by keeping track of acid number results.

What Acid Types Does it Detect?

It tests cover several common acids found in lubricants, such as:

  • Organic Acids – These indicate oil oxidation in all common lubricants.
  • Sulfuric Acids – Typically from sulfur in anti-wear hydraulic oils, extreme pressure gear oils, diesel fuel, and natural gas engines and compressors.
  • Nitric Acids – Generally found in diesel, gasoline, and natural gas engines and compressors.
  • Hydrochloric and Hydrofluoric Acids – These may be present in refrigeration compressors as a result of the refrigerant breakdown.

What Applications are Suitable for Acid Number Testing?

You should not test high water content fluids like water glycols and oil/water emulsions using this method. However, all industrial lubricating oils and some combustion engine oils can benefit from acid number testing, including:

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

Acid Number Field Testing

Acid number testing is so useful that field test kits are readily available and used by many organizations. They frequently have premeasured reagents that make field acid number testing simple. Many of the field kits include a pass/fail test that entails diluting the solution with a predetermined quantity of KOH (potassium hydroxide). While this wouldn’t provide you with a specific acid number, it does help you determine if your lubricant is within the appropriate threshold.

However, there are field test kits that report actual acid number results. These kits generally involve a volume-sampling syringe to guarantee that the oil samples are of the same precise size. Then, you titrate the KOH using a single-use burette. The burette has a scale to represent the AN according to the specific size of the oil sample.

How Does Acid Number Compare to pH Testing?

While both pH and acid number testing measure acidity or alkalinity, the two tests measure different characteristics. The pH test technique determines your lubricant’s apparent pH. Although it does not reflect the proportion of acidic or alkaline elements, the perceived pH is a reflection of how potentially corrosive the lubricant may be. In applications where corrosive lubricant could cause significant harm or with a high risk of strong acid formation or contamination, the pH test method is helpful.

Alternatively, weak acids, which do not easily dissociate in water, are easier to detect and monitor with AN than with pH. Due to this, it is impossible to use the pH test method to get a reliable indication of weak acid concentration in your lubricant.

Using Acid Number to Improve Lubrication

Lubricant degradation frequently occurs through additive depletion, contamination, and oxidation. If it exceeds acceptable limits, it is best to resample for confirmation before changing the lubricant, especially if it is a large-capacity reservoir. But, you can usually resolve the situation with a simple oil change. Furthermore, using lubrication management software to analyze your acid number trends is a useful technique for precisely estimating the serviceability and remaining life of industrial lubricants.

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