The aniline point of an oil is defined as the temperature at which equal volumes of aniline

Aniline

Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...

  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

Aromaticity

In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

 which is dissolve

Solvation

Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...

d on heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

ing 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

Paraffin

In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...

.

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.

Determination of aniline point

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.