How to Dry Isopropanol

An ancient distillation apparatus. Heated vapors cooled by snake-like tube.

Almost all isopropanol contains at least a small amount of water. Isopropanol can be dried in either one or two steps. If the water level is much greater than 12 percent, simple distillation can reduce it. Use of a drying agent followed by filtration reduces it further. These steps are necessary because isopropanol forms a constant boiling mixture with water --- an azeotrope --- consisting of 88 percent alcohol and 12 percent water. It is not possible to separate these by simple distillation.

Things You'll Need

• 2 Erlenmeyer flasks
• Calcium oxide (lime)
• Filter funnel
• Filter paper
• Optional distillation apparatus

Instructions

1. Ascertain the amount of water in the isopropanol. If it is greater than 12 percent water --- say 30 percent --- it can be distilled using a distillation apparatus to bring it down closer to 12 percent. It is possible, if clumsier, to use a drying agent alone (usually lime). Assuming the level is now near 12 percent maximum water, the drying agent may be used to reduce it further.

2. Place a magnetic stir bar into an Erlenmeyer flask of at least twice the volume of the alcohol (e.g., for 100 ml., use a 250 ml. flask), then pour the isopropanol and turn on the stirring mechanism.

3. Gradually add drying agent. Depending upon the anticipated moisture content, slowly add up to ten grams drying agent per 100 ml. isopropanol. Continue stirring for at least 15 minutes. Turn off the stirring and allow settling.

4. Place the filter funnel inside an Erlenmeyer equal in size to the first one used and fold a filter paper. Slowly decant the liquid out of the flask --- carefully keeping as much of the dregs as possible in the original flask in that flask --- into the filter funnel, not filling the funnel too full. The clear liquid collected into the second flask should be dry, clean isopropanol.

5. Distill the isopropanol. Absolutely dry isopropanol distills at 82.4 degrees Celsius. In place of this, purity may be determined by use of a hydrometer. Pure isopropanol has a specific gravity of 0.78556. Impure isopropanol has a higher specific gravity.

Tips & Warnings

• More advanced technological methods are used commercially to prepare water-free isopropanol, including the use of drying by molecular sieves and "pervaporation" techniques. It may be easier in some instances to purchase dry isopropanol.

• Carefully avoid open flame and electric sparks. Sparks can trigger explosion in a mixture of air and alcohol fumes, so carry out the process in a chemical fume hood. Also be cautious of lime, which can attack mucous membranes and eyes. Wear appropriate protective gear such as gloves and goggles.

References

• Separation Processes: Azeotrope/Azeotropic Mixture
• University of Massachusetts: Molecular Sieve Information
• EPA: Pervaporation