Distilling ethanol: what is the ethanol–water azeotrope?

Distillation is a way of separating liquids based on their volatility. At normal atmospheric pressure, ethanol and water form an azeotrope at approximately 95.6 percent ethanol by mass. At this composition, the liquid and vapor have the same proportions, so conventional distillation at the same pressure cannot further enrich the ethanol.

The cause? Ethanol forms an azeotropic mixture with water that maintains the same composition during boiling. This means you can’t further purify ethanol by simply distilling longer or at a higher temperature.

In this article, we explain what an azeotrope is, why conventional distillation cannot further separate ethanol and water at that composition, and which additional techniques are used to dry ethanol further.

What is an azeotrope?

An azeotrope is a mixture of two liquids that has a fixed boiling point and does not change composition during boiling. The vapor released has exactly the same ratio as the liquid.

In short:

• The mixture boils as if it were a single substance.
• You cannot separate it further by distillation.
• This is caused by strong interactions between the molecules, which means they no longer behave “ideally”.

When a mixture forms an azeotrope, normal distillation ceases to work, even with perfect equipment.

The ethanol–water azeotrope: the limit of conventional distillation

At normal atmospheric pressure, ethanol and water form a constant-boiling mixture containing approximately:

• 95.6 percent ethanol by mass
• 4.4 percent water by mass

This mixture boils at about 78.2°C, just below the boiling point of pure ethanol (78.37°C).

Mass percentage indicates what proportion of the mass consists of ethanol; volume percentage describes a proportion of the volume. These percentages are not interchangeable. A product designation such as “96%” therefore cannot be equated with the azeotropic limit without knowing the measurement basis.

Why is this important ?

Because this azeotrope has the lowest boiling point in the mixture, it behaves differently than a normal mixture:

• The composition of the vapor = composition of the liquid
• Both liquids evaporate together in a fixed ratio
• Further distillation does not change the purity.

At this composition, conventional distillation at the same pressure cannot further enrich the ethanol.

Why distillation stops at the azeotrope

Distillation relies on differences in volatility. Ethanol is normally more volatile than water, so in the early stages of distillation, the ethanol purity increases easily. However, once the azeotrope is reached, this separation power disappears.

The mixture has then reached vapor-liquid equilibrium at which:

• The boiling point is constant
• The composition is constant
• The vapor is no longer “richer” in ethanol

Heating for longer or distilling again does not overcome this azeotrope at the same pressure. At normal atmospheric pressure, the limit is around 95.6 percent ethanol by mass.

Why industry needs ethanol with a low water content

Some industrial processes require a very low water content. Examples include chemical synthesis, coatings and analytical work. Depending on the application, additional requirements apply to purity and maximum water content.

• Absolute ethanol (99–100%)
• Anhydrous ethanol
• Anhydrous ethanol for moisture-sensitive reactions

Because conventional distillation at the same pressure does not overcome this azeotrope, industry uses additional separation and drying methods, such as:

1. Molecular sieves
   Porous solids that adsorb water molecules while leaving ethanol behind.
   They can increase ethanol purity to over 99.9%.
2. Azeotropic or extractive distillation
   In this process, a third substance (an entrainer ) is added to alter the cooking behaviour and break the azeotrope.
3. Vacuum distillation
   Adjusting the pressure changes the cooking properties, which aids further separation.

Additional drying can achieve very low water contents. The achievable specification depends on the process used.

Commercially available standard purities of Ethanol

Ethanol is available in different concentrations and may also be denatured. The most common commercial concentrations are:

Ethanol 70%

This concentration is often used in environments where a diluted alcohol solution is required, such as for general cleaning or laboratory applications where a lower alcohol strength is sufficient. The water-to-ethanol ratio allows the liquid to reach certain surfaces better than higher concentrations.

Ethanol 96%

Ethanol 96% is a common commercial concentration. The product specification determines what the percentage means: mass and volume percentages must not be confused. This concentration therefore does not necessarily correspond to the composition of the ethanol–water azeotrope.

Summary

At normal atmospheric pressure, ethanol and water form an azeotrope at approximately 95.6 percent ethanol by mass. At this composition, vapor and liquid have the same proportions, so conventional distillation at the same pressure cannot further enrich the ethanol.

The azeotrope has a slightly lower boiling point than pure ethanol, causing the mixture to behave as a single substance when boiling.

Further removal of water from ethanol requires additional drying or separation techniques, such as molecular sieves. The stated azeotropic composition applies to the ethanol–water mixture at normal atmospheric pressure.