What is The Purpose of Absolute Ethanol?

by Kate Onissiphorou

Absolute ethanol, also known as anhydrous ethanol, is used mainly as a reagent in synthetic organic chemistry and chromatography. It has a purity level of almost 100% alcohol. 

As ethanol is highly miscible with water, it can dissolve many types of organic compounds. This includes both polar and non-polar compounds such as hexane. Thanks to its versatility, absolute ethanol has a wide range of applications across various industries.

How is Absolute Ethanol Used in Chemistry?

The high purity of absolute ethanol makes it very useful as an organic solvent for industrial applications and laboratory purposes. Other uses of absolute ethanol include the manufacture of various alcohol-based products, such as denatured alcohol, rubbing alcohol, hand sanitiser, lotions, colognes, and perfumes.

As a solvent, absolute ethanol can dissolve non-polar organic compounds, such as the scents found in perfumes. It also acts as a reagent (reactant) in analytical experiments and plays a key role in the manufacture of new substances or chemical derivatives.

The ethanol-based products commonly used in laboratories are 95% pure. In this environment, ethanol is typically used to fuel alcohol lamps, preserve biological specimens, disinfect laboratory equipment, and dissolve organic compounds.

Absolute ethanol, on the other hand, has a purity level of between 99% and 100%. It’s also highly hygroscopic or, in other words, readily absorbs water. As a result, it makes an effective antiseptic and disinfectant because it can inactivate many species of microbes through dehydration. 

Although absolute ethanol doesn’t fully permeate the cell membrane, it forces the water inside the cells to leave. Not only does this destroy the structural integrity of the cell, but it also stops its biochemical activities, eventually leading to cell death.

What is the difference between ethanol and absolute ethanol?

Ethanol and absolute ethanol are chemically identical. They both have the chemical formula C2H5OH. The main difference between them is the concentration or purity level.

Molecular formula of ethanoll

Ethanol is found at low concentrations in alcoholic beverages, such as beer and wine, where it’s dissolved in water with other organic compounds. By comparison, absolute ethanol is highly concentrated at almost 100%, although it may still contain trace impurities of additives like benzene.

How is absolute ethanol produced?

The hygroscopic nature of ethanol makes it difficult to distil into absolute ethanol. Not only does the process involve multiple distillations and the use of separation agents, but its tendency to absorb moisture also means it’s hard to maintain its high purity once it’s exposed to air.

One industrial method of distilling ethanol into absolute ethanol is to use benzene as a separation agent. How does it work? A small amount of benzene is added to a 95.6% solution of ethanol (the upper limit of ordinary distillation). Absolute ethanol is then produced in the third fractional distillation by maintaining a temperature of 78.2 °C.

Chemical distillation plant
A typical chemical distillation and manufacturing plant

While ethanol can be consumed by humans at a relatively high concentration without becoming lethal, absolute ethanol is not safe to drink. That’s because traces of benzene, which is both poisonous and carcinogenic, are still present after the distillation process.

Another method of obtaining absolute ethanol is through the desiccation process using glycerol. Currently, the most popular and effective method of producing absolute alcohol through desiccation is to use adsorbents like starch or zeolites. Known as spectroscopic alcohol, the absolute ethanol that’s produced in this way is commonly used as a solvent in spectroscopic analysis. 

What is the concentration of absolute ethanol?

Absolute alcohol contains no more than 1% water. This level of purity is impossible to achieve through simple fractional distillation because a mixture of 95.6% alcohol and 4.4% water has a constant boiling point.

As the alcohol evaporates at the same rate as water, it can’t be separated any further. This is called an azeotropic mixture, which means it has the same concentration of fluid and vapour phases.

What are common uses of absolute ethanol? 

Absolute ethanol has a variety of applications and is commonly used as a reagent, solvent, disinfectant, antiseptic, feedstock, fuel, and low-temperature liquid.

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