Azeotropes or constant boiling mixtures is a mixture of two or more liquid which cannot be separated by simple distillation. This happens because when azeotrope is boiled, vapor has same proportion of constituents as unboiled mixture.
Maximum boiling azeotropes:
These are obtained from solution that shows negative deviation from ideal solution. We know, vapor pressure is inversely proportional to boiling point. Since first type of mixture showing negative deviation have lowest vapor pressure, they form maximum boiling azeotropes. Here the mixtures will boil at the temperature above boiling temperature of components from which they are made and the mixture will boil undistilled.
Let us take example of solution obtained by mixing HCl and H2O. HCl has boiling point of 188K and H2O is 373k. As distillation continues, we will reach at such point where composition of solution will become equal (20.2 % HCl, 79.8% H2O) both in liquid and vapor phase and further distillation is impossible. At this point boiling temperature of solution will reach maximum (381.5K) which is greater than boiling temperature of HCl and H2O alone.
Minimum boiling azeotropes:
These are obtained from the solution that shows positive deviation from ideal solution. These have highest vapor pressure and minimum boiling point. At certain point, the mixture will boil at constant temperature without being distilled. Azeotropes in such condition will have minimum boiling point than that of components from which they are made.Let us take example of solution obtained by mixing H2O and C2H5OH. Water has boiling point of 373K and C2H5OH is 351.5k. As distillation continues, we will reach at such point where composition of solution will become equal (95 % C2H5OH, 5% H2O) both in liquid and vapor phase and further distillation is impossible. At this point, boiling temperature of solution will reach minimum (351.5K) which is less than boiling temperature of HCl and H2O.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.