Introduction to Acetonitrile and Its Azeotrope with Water
Acetonitrile is a versatile organic solvent widely used in chemical synthesis, pharmaceutical manufacturing, and chromatography. When mixed with water, it can form an azeotrope—a mixture with a constant boiling point and fixed composition during distillation. Understanding the acetonitrile-water azeotrope is essential in separation processes and industrial applications.
Physical Properties of the Acetonitrile-Water Azeotrope
An azeotrope occurs when the vapor composition equals the liquid composition, making separation by simple distillation difficult. The acetonitrile-water azeotrope typically has a boiling point lower than pure acetonitrile (81.6 °C) and pure water (100 °C). According to NIST Chemistry WebBook, the azeotropic composition is well documented and used for precise chemical processing.
| Property | Acetonitrile | Water | Acetonitrile-Water Azeotrope |
|---|---|---|---|
| Chemical Formula | C₂H₃N | H₂O | — |
| Molecular Weight | 41.05 g/mol | 18.02 g/mol | — |
| Boiling Point (°C) | 81.6 | 100 | ~76 |
| Common Industrial Use | Solvent in HPLC, synthesis | Universal solvent | Purification, separation |
The azeotropic composition is often around 84–85% acetonitrile by weight, depending on pressure (PubChem Data Source).
Industrial Relevance and Separation Challenges
In pharmaceutical and analytical laboratories, acetonitrile-water mixtures are common in High-Performance Liquid Chromatography (HPLC) mobile phases. However, when solvent recovery is needed, the azeotropic behavior complicates purification. Standard distillation cannot fully separate the two, requiring advanced techniques like azeotropic distillation with entrainers or molecular sieves for dehydration.
Applications in Chemical and Analytical Processes
The acetonitrile-water azeotrope is useful in certain extraction processes, as its constant boiling composition ensures reproducible solvent behavior. In organic synthesis, controlled azeotrope removal can be used for dehydration reactions. In environmental analysis, azeotropic distillation with acetonitrile helps concentrate analytes for detection.
