In petroleum product testing, the aniline point is defined as the minimum temperature at which equal volumes of aniline and the test petroleum sample (e.g., diesel or kerosene fraction) become completely miscible in all proportions without phase separation.

Introduction:
The aniline point is a classic solubility-based property used in petroleum product characterization, especially for middle distillates such as kerosene and diesel. It gives a quick indication of the aromatic content and solvency of a fuel. A correct, operational definition helps technicians run the test consistently and interpret results in specifications and blending decisions.

Given Data / Assumptions:

• Two liquids are involved: aniline (an aromatic amine) and a petroleum test sample.
• The test uses equal volumes of aniline and sample, not equal weights.
• The measurement seeks the minimum temperature for complete miscibility.

Concept / Approach:
Aniline is highly aromatic; its miscibility with a petroleum fraction depends on the fraction’s aromaticity. Aromatic-rich fuels have stronger interactions with aniline and therefore show a lower aniline point, while paraffinic fuels exhibit weaker interactions and hence a higher aniline point. The standardized definition relies on equal volumes so that the temperature reflects composition effects rather than density differences between the liquids.

Step-by-Step Solution:
1) Identify the definition element: “minimum temperature of complete miscibility.”2) Confirm the volumetric basis: equal volumes of aniline and test sample are used.3) Recognize the practical outcome: record the temperature at which the two phases merge into a single clear phase and remain so on slight cooling.

Verification / Alternative check:
Standard lab protocols and refinery manuals phrase the aniline point as the temperature for complete miscibility of equal volumes of aniline and the petroleum sample. This volumetric criterion is repeated across specifications and handbooks.

Why Other Options Are Wrong:
Equal weight statements (options a and b) misstate the test basis, which is volumetric.“Aniline vaporises” (option d) is unrelated to the definition.Azeotrope formation (option e) is irrelevant to the standard definition.

Common Pitfalls:
Confusing equal volume with equal weight; overlooking that aniline point trends inversely with aromatic content; and assuming it measures volatility rather than solvency/aromaticity relationships.

Final Answer:
Equal volume of aniline and the test sample are completely miscible.