Liquid - Vapor Equilibrium/Distillation

Background The volatility of a compound is its ability to change into a vapor, or to evaporate at a particular temperature and pressure. The normal boiling point of a liquid usually allows comparison of its volatility to other materials, with lower boiling points indicating greater volatility, but this may be affected somewhat by the presence of other materials. Comparison of the concentration ratios in the vapor and liquid states gives a more precise measure of the relative volatility of the components of a mixture. One liter of an aqueous solution (the boiling point of water is 100 °C) containing 1.0 M ammonia (NH3, boiling point 33.9°C) and 1.0 M sodium chloride (NaCl, boiling point 1413 °C) has a molar ratio water:ammonia:salt = 55:1:1 . If the equilibrium vapor above this mixture is collected and analyzed, no sodium chloride is found, and the molar ratio of water to ammonia is less than 4:1. The vapor is depleted in sodium chloride and enriched in ammonia relative to the liquid or solution state, as one might expect from the boiling points. The vapor still has a greater percentage of water than ammonia, but because of the enrichment of ammonia in the vapor relative to the liquid we recognize the greater relative volatility of ammonia in the solution. The presence of an inorganic salt such as sodium chloride usually decreases the volatility of water and increases the volatility of less polar compounds like ammonia, a type of salting-out. If this solution is heated at atmospheric pressure, it will begin to boil at a temperature slightly less than 100 °C. If a small fraction of the liquid is vaporized, collected, and cooled it will return to the liquid state (condensation) with essentially the same molar ratio as the vapor had. This new liquid will have a much greater concentration of ammonia (about 9 M) than the original solution. The remaining liquid in the boiler (also called the reboiler) which was not evaporated will have a lower concentration of ammonia than the original solution, and a greater concentration of sodium chloride. There is a fairly constant ratio of the concentration in the collected liquid (the condensate or distillate) to the concentration in the liquid from which it was distilled, which we will call a partitioning constant: