Answer :
The capillary tube is heated before introducing the oil plug to remove air and fill it with vapor; this procedure is critical for precise boiling point measurements. Controlled heating minimizes Joule heating's effects on viscosity and solute migration rates. The boiling point is noted when the oil starts entering the cooled capillary tube.
The capillary tube must be heated before the oil plug is inserted due to a process intended to expunge the air from within the tube and to ensure its replacement with the sample's vapor. This step is crucial for the subsequent accurate determination of the boiling point of the compound. When heating is done gently and continuously, it prevents the starting and stopping that can negatively affect the results. Additionally, the capillary tube is designed such that the controlled heating process helps in minimizing the effect of Joule heating, which impacts the viscosity of the buffer inside the tube and thereby the migration rate of solute species during electrophoresis.
During boiling point determination, the sample is uniformly heated by the mineral oil in the Thiele tube, which creates an oil current, ensuring indirect and slow heating. Once the correct amount of bubbling indicates that the tube is free of air and filled with vapor, the heat is removed, and as the oil cools, the boiling point is recorded when the oil just begins to enter the capillary tube.
