The aniline point of an oil is defined as the temperature at which equal volumes of aniline (C6H5NH2) and the desired oil are completely miscible.
The value gives an indication of the aromatic content of the oil, since aniline is an aromatic compound which is dissolved on heating by the aromatics in the oil. The greater the aniline point, the lower the aromatics in the oil. A higher aniline point also indicates a higher proportion of paraffin.
It is an indication of the possible deterioration of the oil in contact with rubber seals since aromatic hydrocarbons present in oil have a tendency to dissolve natural rubber and some synthetic rubber.
Equal volumes of aniline and oil are stirred continuously in a test tube and heated until the two merge into a homogeneous solution. Heating is stopped and the tube is allowed to cool. The temperature at which the two phases separate out is recorded as aniline point.
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