Azeotropes are mixtures of chemicals that cannot be separated by distillation due to a common boiling and vaporization point. They are difficult to make and are formed when chemicals have similar boiling points or different electrical polarities. Azeotropes are useful in industry for their non-flammable properties and are commonly used in solvent recycling and laboratory processes. Examples include the binary ethanol-water azeotrope and the ternary methanol azeotrope.
An azeotrope is a mixture of two or more chemicals in solution that cannot be separated by basic distillation processes because they share a common boiling point and vaporization point. When an azeotrope is heated or boiled, the chemicals all convert to a gaseous state in equal amounts and are sometimes called constant boiling mixtures. Azeotropes can be difficult to make because the concentrations of each chemical must be precisely balanced and their density, initial boiling point, and surface tension must fall within a range that can be brought to an equilibrium level in the solution.
The formation of azeotropic mixtures is more likely to occur if the precursor chemicals have boiling points close to each other. Chemicals with different electrical polarities or ionic charges are also more likely to be attracted to each other to form this type of mixture. Different types of solvents will naturally combine in azeotropes due to these conditions and, therefore, must be kept carefully separated during industrial processes.
In industry, the azeotrope has value because it is a mixture of chemicals in solution and not a compound where those chemicals exhibit strong molecular bonds that are not easily broken. This, therefore, makes it possible to design an azeotrope in which the content consists of a flammable and a non-flammable chemical which overall demonstrates non-flammable characteristics. Since the chemicals do not separate during evaporation, spills will not create a dangerous fire condition.
Flammable solvents are often part of azeotropic blends because they can be used and then the distilled liquid to recover the solvent and leave the contaminants. The process avoids the risk of fire and air pollution from the solvent as it is related to the non-flammable component of the azeotrope. The most common example of this type of solvent azeotrope is the binary ethanol-water azeotrope. Normally, water boils at 212° Fahrenheit (100° Celsius) and ethanol at 172.94° Fahrenheit (78.3° Celsius), but in a concentration of 95% ethanol and 5% water, both boil at 172.76° Fahrenheit (78.2° Celsius) . This type of mixture is also called a minimum boiling point azeotrope, due to the fact that the new boiling point for both constituents is lower than the original one.
A common methanol azeotrope which is a mixture of three chemicals, known as a ternary azeotrope, contains methanol, acetonitrite and water in concentrations of 52%, 44% and 4%, respectively. Combined, they have a boiling point between 149° and 158° Fahrenheit (65° to 70° Celsius), which is closest to methanol’s natural boiling point at 148.1° Fahrenheit (64.5° Celsius). This particular blend is used in solvent recycling and the high performance liquid chromatography (HPLC) laboratory process.
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