Glycerine can be purified by (a) vacuum distillation (b) simple distillation (c) steam distillation (d) fractional distillatlon.

Vacuum distillation is a technique adapted to purify compounds that have high boiling points, such as glycerine. Under normal atmospheric conditions, glycerine boils at 290°C, which could lead to thermal degradation or oxidation. By reducing the pressure inside the distillation apparatus, the boiling point of the liquid is also lowered. This means that glycerine can be distilled at lower temperatures, thus preserving its integrity and resulting in a purer product.

This method is particularly significant in the pharmaceutical and food industries where gentle handling of sensitive compounds is crucial. Using vacuum distillation, one can ensure that the glycerine's quality remains unaltered, and its efficacy intact.

Steam distillation is employed for substances that are immiscible with water, allowing them to vaporize at temperatures lower than their boiling points. During this process, steam is passed through the liquid containing the desired compound. The steam effectively carries the volatile compounds with it as it passes through the mixture and is later condensed back into a liquid.

While steam distillation is quite useful for extracting essential oils and perfumes, it is not the most appropriate method for glycerine. This is because glycerine is miscible with water and does not require the addition of steam to lower its boiling point for distillation.

Fractional distillation is a refined version of simple distillation, designed to separate mixtures of liquids with close boiling points. The process involves using a fractionating column, which increases the surface area and allows for a series of continuous vaporization-condensation cycles.

This method is particularly beneficial for industrial processes where separating chemical components with precision is necessary. However, when dealing with a single, high-boiling substance like glycerine, fractional distillation offers no significant advantage over other distillation methods, such as vacuum distillation.

The boiling point is a key property of a compound, indicating the temperature at which it transitions from a liquid to a gas. For glycerine, with a boiling point of 290°C, it's vital to choose a distillation method that will not expose it to such high temperatures, risking decomposition.

In purification processes, understanding the boiling point is crucial when selecting the appropriate distillation technique. It helps determine whether to apply vacuum distillation, which exploits reduced pressure to lower boiling points safely, or another method better suited for different physical properties or boiling point ranges.