Question

(a) What are the disadvantages of using too little drying agent? (b) What are the disadvantages of using too much drying agent?

Answer 1

listed below are a few drying agents that are commonly used by organic chemists. Organic liquids are considered to be wet if they contain water. Realize that the organic liquid will still be a liquid after it is dried.

Magnesium Sulfate: Magnesium sulfate is a great drying agent. It has a high capacity, is complete in its drying and is rapid. Capacity refers to how much water per gram the drying agent holds and complete means that drying equilibrium favors the hydrate. The only disadvantages to using magnesium sulfate is that it is normally available in a powder form and must be filtered out. More importantly, magnesium is a very strong Lewis acid and as such, is not inert to all functional groups. For example, epoxides are sensitive to magnesium.

Sodium Sulfate: Sodium Sulfate is the most widely used drying agent. It is very similar to magnesium sulfate in its capacity, but it is less complete (will leave more water in solution) and it is slower in terms of its rate. Sodium sulfate has the advantage in that it is less reactive and in granular form, is very easy to remove from liquids. The liquid can often be decanted off the drying agent without filtration.

Calcium Sulfate: Calcium sulfate is known by the trade name drierite. Calcium sulfate has a low capacity, but it is very complete and rapid. This means that you will have to use more of it to dry a solution. Calcium sulfate can be purchased in a chunky form. This form is very convenient to work with. It is also used as a drying agent in dessicators and to dry the air entering a water sensitive reaction. The drierite can be obtained in a dyed form. The dye functions as an indicator of how much water has been absorbed. The dyed form is not suitable for use as a drying agent for liquids. Why?

Calcium Chloride: Calcium chloride is very much like calcium sulfate except that it will also absorb methanol and ethanol.

Potassium Carbonate: Potassium carbonate is basic and as such, is generally used in basic media. It is of average capacity, completeness and rate.

At this point it is important to reiterate that when you add a drying agent to a solution, it will just sit on the bottom of the container because it is an insoluble solid. The problem with drying agents is there is a tendency to add to much. So how much is enough? It is best to start conservatively. Add a spatula tip and then swirl. After the drying agent has settled, observe the solution. If the solution is transparent that is a good sign that you are close to the end point of addition. If the solution is cloudy, you probably need to add more in small increments. Once the solution begins to appear to be clear, study the drying agent itself. Does it look all clumpy or is it freely flowing? In other words, does it look the same as when you first added it? Drying agents clump and stick to the bottom of the vessel as they pick up water, so if the agent looks really clumpy and sticky, you need to add more. Continue to add drying agent until the newly added material appears unchanged and free flowing. Recognize the originally clumped material will never unclump.

If you use copious quantities of drying agent, you will lose large amounts of the desired compound.