Absolute alcohol represents the highest degree of purity achievable for ethanol (ethyl alcohol). This highly specialized chemical is distinct from the alcohol found in beverages or household cleaners because it contains virtually no water. It is a necessary grade for technical applications where even trace amounts of water would interfere with a process or reaction.
Defining Purity and Concentration
Absolute alcohol is defined as ethanol that is at least 99.5% pure by volume, often 99.9% or higher, with the remainder being trace water content. It is frequently referred to as anhydrous alcohol. For perspective, common distilled spirits rarely exceed 50% alcohol by volume, and even laboratory-grade ethanol is typically 95% pure.
The presence of water can significantly alter the properties of a solvent, which is why anhydrous conditions are required for certain scientific procedures. Water interferes with many organic synthesis reactions and can degrade sensitive chemical reagents. The high purity of absolute alcohol ensures it functions as a reliable and consistent solvent.
Overcoming the Distillation Limit
Achieving this extreme level of purity is challenging because ethanol and water form an azeotrope, a mixture with a constant boiling point. Standard distillation methods can only concentrate ethanol to about 95.6% purity because the liquid and vapor phases have the same composition at this point, preventing further separation through boiling. This azeotropic barrier prevents 100% pure ethanol from being produced using traditional distillation techniques.
To remove the remaining water, specialized industrial processes are employed to “break” the azeotrope. One common method is azeotropic distillation, where a third substance, called an entrainer (such as cyclohexane), is added. The entrainer forms a new, lower-boiling mixture with the water, which is then distilled away, leaving behind the anhydrous ethanol.
Molecular sieves are also used; these porous materials selectively adsorb and trap water molecules while allowing ethanol molecules to pass through. Other methods include specialized membrane separation processes or pressure-swing distillation, which exploits the fact that the azeotrope composition shifts under different pressures. These complex methods make absolute alcohol significantly more expensive than its diluted counterparts.
Essential Uses in Science and Industry
The virtually water-free nature of absolute alcohol makes it indispensable across various scientific and industrial fields. In chemical laboratories, it functions as a solvent in organic synthesis reactions, preparing sensitive reagents, and high-precision analytical techniques like chromatography. In the pharmaceutical sector, it is used as a solvent in the creation of medicines, tinctures, and vaccines where purity is paramount. Its ability to dissolve both polar and non-polar compounds makes it a versatile carrier. Industrially, it is used as a high-purity cleaning solvent for circuit boards and precision electronic equipment.
Absolute alcohol also plays a role in the energy sector as an anhydrous fuel additive (gasohol) to prevent phase separation when blended with gasoline. Finally, in molecular biology, 99.5% ethanol is used for the precipitation and purification of DNA and RNA samples.
Handling Highly Concentrated Alcohol
Due to its high purity, absolute alcohol requires specific handling and storage protocols that differ from those for lower-concentration alcohols. Its flammability is significantly increased, possessing a low flash point of approximately 13°C (55°F), meaning it can easily ignite at room temperature. This necessitates storing it in cool, well-ventilated areas, away from any heat source, spark, or open flame.
The substance is also highly hygroscopic, meaning it readily absorbs moisture from the air, which can quickly compromise its anhydrous purity. Therefore, containers must be kept tightly sealed to maintain the integrity of the product. When handling absolute alcohol, personal protective equipment, including safety goggles and gloves, is necessary to prevent skin and eye irritation from the dehydrating effect of the pure alcohol.