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Ethanol/Water Azeotropic Mixture

Updated on October 22, 2011

Ethanol obtained by Distillation do not exist in its pure form, but in combination due to the existence of common hydroxyl group in both Ethanol and Water. As such, the Ethanol must be further purified by advanced separation Techniques if the Inclination is on absolutely pure Ethanol. Here, we will discuss those Techniques, Terms associated with Ethanol Water, and some related Issues.

Ethanol Water Azeotropes

A unique characteristic of Ethanol is its ability to form a homogeneous binary mixture of Ethanol/Water Azeotropic Mixture. Here, Ethanol cannot only be separated from water by simple Distillation, but by further Purification that targets the Water constituent of the Mixture.

The binary mixture of Ethanol and Water shares a common Boiling Point (B.P) that deviates widely from their Individual Boiling Points. Water at a boiling point of 100°C and Ethanol 78.4°C, should have had (in an Ideal case) an average boiling point that is indicative of their individual fractional contributions to the mixture. But the real case is that the Azeotropic Mixture of Ethanol/Water has a minimum boiling point - at room Temperature - of 78.2°C, lower than the respective B.P of Ethanol and Water, thus forming a minimum boiling mixture or Positive Azeotropes.

Dissolved Salt Distillation

The Ethanol/Water Bond can be decoupled by the addition of Salt to the mixture. This Principle capitalises on the fact that the salt could not dissolve in the Ethanol. Potassium acetate, for example, dissolves in the water to reduce the Solubility of the Water component of the Mixture. This helps to create a wider difference in Boiling Point so that the Ethanol is easily separated by Distillation.

Azeotropic Distillation

The Idea is to convert the Binary Azeotropic mixture of Ethanol and Water to a separable ternary Mixture by the addition of a third chemical called an Entrainer. Cyclohexane (in some cases Benzene) can absorb all the water molecules into its structure when combined in the following Percentage: Cyclohexane 76%, Ethanol 17%, Water 7%, so that the water can evaporate with the Cyclohexane when heated.

Separation by Chemical Action

A chemical that has an affinity to only the water (and not the Ethanol) can be added and shaken with the Mixture to separate the Water from the Binary Mixture. Calcium Oxide for example can react with water to form Calcium Hydroxide, but will not react with the Ethanol content of the Mixture. The Calcium hydroxide so formed is non-volatile, and can be filtered off so that pure Ethanol can be obtained from the Filtrate by Distillation.

Terms Associated with Azeotropic Mixtures

It is essential to state that the this Concept goes beyond Water/Ethanol to a wider Range of Azeotropic Applications that involves a mixture of other Chemicals. Hence, it is essential that an introductory summary of terms associated with Azeotropy be included as follows:

Azeotropes - A mixture of two or more solvent (such as Ethanol and Water) that cannot be separated by simple distillation.

Zeotropes - Other Solvent that do not form Azeotropes when mixed

Positive Azeotropes - This describes a situation where the Boiling point of each of the component in the mixture (eg. Water/Ethanol Mixture) is higher than that of their Azeotropic mixture.

Negative Azeotropes - This is when the Boiling Point of each of the Constituents is less than that of the Azeotropes.

Binary Azeotropes - When there are only two constituents in the Azeotropes.

Ternary Azeotropes - When three Constituents makes up the Azeotropes.

Homogenous Azeotropes - When the Azeotropic Constituent are completely miscible.

Heterogenous Azeotropes - When the Azeotropic Constituents are not completely miscible.

Pressure Maxim Azeotropes - This is another name for positive Azeotropes or minimum boiling mixture.


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