What is the acid number AN of lubricating oil?
author: Leila
2024-08-13
Testing the quality of lubricating oil in use is part of equipment operation monitoring. Lubricating oil includes many categories, including turbine oil, engine oil, compressor oil, hydraulic oil, etc., but the main reasons for the deterioration of lubricating oil quality include the following aspects: exhaustion of additives, entry of other substances into the lubricating oil, and oxidation of the lubricating oil. The quality of lubricating oil can be preliminarily checked on site by the color, transparency, viscosity and odor of the oil, but the most accurate method is to test the oil, among which the acid number (AN) test is an important basis for oil quality monitoring.
The acid number test indirectly determines the degree of oil deterioration by measuring the oxidation byproducts in the lubricating oil, which can help us understand the degree of oxidation of the lubricating oil and the consumption of additives.
What is acid number:
In new and used oils, the acid components include organic acids, inorganic acids, esters, phenolic compounds, resins and metal salts, acid salts of polyacids, and certain additives such as antioxidants and detergents. Acid number (AN) is a method of determining the concentration of acidic substances in non-aqueous solutions. In the field of lubricating oil applications, it can be used to determine the amount of acidic substances in the oil. The acid value refers to the amount of alkali (using potassium hydroxide KOH) required to neutralize all acidic components in 1 gram of lubricating oil (mg), expressed in mgKOH/g. Obviously, the more acidic substances in the lubricating oil, the more potassium hydroxide is needed to neutralize it, so the acid value is greater. For the oil in use, the acid value indicates the greater the degree of oxidation and deterioration of the oil. During the use of the oil, at a certain temperature, the oil reacts with oxygen in the air to generate organic acids, or due to the consumption of additives, the acid value of the oil will change. When the oil acid value is greater than 2, you should consider changing the oil.
In new and used oils, the acid components include organic acids, inorganic acids, esters, phenolic compounds, resins and metal salts, acid salts of polyacids, and certain additives such as antioxidants and detergents. Acid number (AN) is a method of determining the concentration of acidic substances in non-aqueous solutions. In the field of lubricating oil applications, it can be used to determine the amount of acidic substances in the oil. The acid value refers to the amount of alkali (using potassium hydroxide KOH) required to neutralize all acidic components in 1 gram of lubricating oil (mg), expressed in mgKOH/g. Obviously, the more acidic substances in the lubricating oil, the more potassium hydroxide is needed to neutralize it, so the acid value is greater. For the oil in use, the acid value indicates the greater the degree of oxidation and deterioration of the oil. During the use of the oil, at a certain temperature, the oil reacts with oxygen in the air to generate organic acids, or due to the consumption of additives, the acid value of the oil will change. When the oil acid value is greater than 2, you should consider changing the oil.
The difference between total acid number and acid value:
Sometimes we see the total acid number (TAN), and sometimes the acid number (AN). What is the difference between the two? The acid concentration of lubricating oil includes both strong acid and weak acid. Although the acid value AN includes strong acid and weak acid components, the extremely weak acid with an ionization constant less than 10~9 has almost no effect on AN. Therefore, the determination of the AN value does not reflect all the acidic substances (total acid value) in the lubricating oil. At present, TAN has gradually been replaced by AN in North America and other regions.
Sometimes we see the total acid number (TAN), and sometimes the acid number (AN). What is the difference between the two? The acid concentration of lubricating oil includes both strong acid and weak acid. Although the acid value AN includes strong acid and weak acid components, the extremely weak acid with an ionization constant less than 10~9 has almost no effect on AN. Therefore, the determination of the AN value does not reflect all the acidic substances (total acid value) in the lubricating oil. At present, TAN has gradually been replaced by AN in North America and other regions.
The difference between pH value and acid value AN:
The pH value of lubricating oil determines the acidity and alkalinity of the oil, which helps us to qualitatively understand the corrosiveness of the lubricating oil. Lubricating oil with higher acidity has stronger corrosiveness, but the pH value determination method cannot help us quantitatively determine the concentration of acidic and alkaline substances in the lubricating oil. Therefore, testing the pH value of lubricating oil can help us qualitatively judge the corrosiveness of the oil. Some systems are more sensitive to acid corrosion of the oil. In some cases, if it is suspected that strong acidic substances are mixed in the oil, it can be reflected through pH detection. For occasions that are easily contaminated by strong acidic substances, pH value determination can be used to help us monitor whether acidic substances have invaded the oil. However, pH testing cannot measure unionized acidic substances, nor can it reflect changes in weakly acidic substances contained in lubricating oils.
The pH value of lubricating oil determines the acidity and alkalinity of the oil, which helps us to qualitatively understand the corrosiveness of the lubricating oil. Lubricating oil with higher acidity has stronger corrosiveness, but the pH value determination method cannot help us quantitatively determine the concentration of acidic and alkaline substances in the lubricating oil. Therefore, testing the pH value of lubricating oil can help us qualitatively judge the corrosiveness of the oil. Some systems are more sensitive to acid corrosion of the oil. In some cases, if it is suspected that strong acidic substances are mixed in the oil, it can be reflected through pH detection. For occasions that are easily contaminated by strong acidic substances, pH value determination can be used to help us monitor whether acidic substances have invaded the oil. However, pH testing cannot measure unionized acidic substances, nor can it reflect changes in weakly acidic substances contained in lubricating oils.
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