What Is the Aniline Point?

The aniline point is a physical characteristic of hydrocarbon compounds, such as oils, and refers to the minimum temperature at which the hydrocarbon and the same amount of the compound aniline (C₅H₅NH₂) are perfectly miscible. At temperatures below this point, a mixture of the compound and an equal volume of aniline will not dissolve together. The result of a test, called the aniline test, gives chemists this information about a hydrocarbon, including certain details about its composition, such as the relative content of aromatic chemicals and various paraffins.

The test for finding the aniline point of a given oil or other hydrocarbon, such as a petroleum distillate, is relatively simple. A volume of the test liquid and an equal volume of aniline are placed together in a container, such as a test tube or other laboratory vessel, and slowly heated while being gently agitated. When two liquids completely dissolve into one another, the mixture turns from cloudy to clear. The sample is removed from the heat and stirred until it becomes cloudy again. The temperature at that point is the aniline point of that compound.

The aniline point of a liquid will vary, depending upon the relative concentration of aromatic compounds dissolved in it. Generally, a higher aniline point means a relatively low level of dissolved aromatics. By using reference materials for a pure sample of a given substance and comparing the aniline point of the pure sample to that of the test sample, a chemist can calculate the amount of aromatics in the test sample with a high degree of accuracy. Similar techniques and calculations can be used to determine the level of paraffin compounds that are present.

This data is important primarily for oils that may be used as lubricants or that may come into contact with natural rubber. Oils with a high concentration of aromatics are less suitable for use in systems or machinery that have rubber parts because the aromatics gradually break down natural rubber, potentially causing failures in gaskets, seals, and other components. An oil with a very high aniline point will be very low in aromatics, making such an oil more suitable for use with parts, systems, or machinery that are made from or use natural rubber or rubber products.