Removing trace amounts of water from organic solutions

Although water is immiscible with many organic solvents, all solvents will typically contain some traces of water, unless special precautions have been taken to remove them.

After work-up, the desired compound is typically present in an organic solution. The water content of the solution can be reduced by washing with saturated brine (see above). After this, some traces of water will likely remain, and so the organic solution is treated with a drying agent. Drying agents are typically anhydrous inorganic solids, that take up water from the organic solution, becoming hydrated in the process. Drying agents differ from one another in terms of their capacity (how much water they can absorb), their speed (how quickly they absorb water), and their reactivity (some may react with acidic or basic functional groups). Anhydrous magnesium sulfate is very commonly used.

The organic solution is placed in a conical flask, and an amount of anhydrous magnesium sulfate is added. The flask is then swirled, and observed. Over several minutes, the magnesium sulfate should absorb any residual water, converting into hydrated magnesium sulfate. If the solution is dry, then some magnesium sulfate will remain anhydrous. By swirling the flask, a “snowstorm” effect can be observed, as the anhydrous magnesium sulfate forms a slow-settling cloudy suspension. If the magnesium sulfate has all been hydrated, then the suspension will settle more quickly; more anhydrous magnesium sulfate should be added.

Once the solution is dry, the spent drying agent can be filtered off, leaving the organic solution. The solution should not be left for long, since all solvents gradually redissolve water from the atmosphere over time.