The aniline point is the minimum temperature at which a hydrocarbon and aniline compound are perfectly miscible. The aniline test is used to determine the aniline point, which can provide information about the composition of the hydrocarbon, including the level of aromatics and paraffins. This data is relevant for oils used as lubricants or in contact with natural rubber, as high levels of aromatics can degrade rubber components.
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 aniline compound (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 an aniline test, gives chemists this information about a hydrocarbon, including some details about its composition, such as the relative content of aromatic chemicals and various paraffins.
The test to find the aniline point of a given oil or other hydrocarbon, such as a petroleum distillate, is relatively straightforward. 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 by gentle swirling. When two liquids completely dissolve in each other, the mixture changes 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 on the relative concentration of dissolved aromatic compounds in it. Generally, a higher aniline point means a relatively lower 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 present.
This data is mainly relevant for oils which can be used as lubricants or which can 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 aromatics gradually degrade natural rubber, potentially causing seals, gaskets and other components to fail. An oil with a very high aniline point will be very low in aromatics, making it more suitable for use with parts, systems or machinery made or using natural rubber or rubber products.
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