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The total acid number (TAN) is a crucial parameter to evaluate petroleum products. It provides information about the amount of acidic constituents that are present in the sample. This allows a conclusion to be drawn about the corrosiveness of the petroleum product.

Refined petroleum products such as engine oil can break down under operating conditions, forming acidic constituents. Over time, the concentration of these acidic compounds (and thus the acid number) increases. This means that the total acid number can serve as an indicator to change the used oil with fresh oil.

Various standard procedures highlight the importance of determining the acid number. They range from internal corporate specifications to national and international standards established by organizations like ASTM, DIN, and ISO.

To determine the acid number, the oil sample is dissolved in a solvent mixture of toluene, isopropyl alcohol, and water. The solution is then titrated with alcoholic potassium hydroxide (KOH). The result is then expressed as mg KOH per gram of sample. There are several methods available to detect the endpoint of the titration. 

The most often used test method is ASTM D664. It describes a potentiometric titration, and a pH electrode indicates the endpoint. The sample color does not affect this method. It is suitable for petroleum products, lubricants, biodiesel, and biodiesel blends.

ASTM D974 describes a photometric titration which uses an optical sensor for the indication. This test method is well-suited for light-colored petroleum products and lubricants.

The third ASTM test method for the determination of total acid number is ASTM D8045. In this method, a sensitive thermistor indicates the endpoint. As the titrant and analyte react, the temperature changes. When the titration is finished, a difference in the rate of temperature change indicates the endpoint. As this method uses xylene in the solvent mixture, it is suitable for crude oil and petroleum products.

Download the Application Bulletin to learn more about the three acid number test methods. The document includes detailed instructions and troubleshooting tips for all three methods. 

For more information about the automated analysis of the total acid number and total base number, watch the video:


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