Drying: Capacity and Intensity Are Not the Same Thing

**Add the agent to the liquid, swirl, and leave it.** Fifteen minutes is often enough; overnight is better. Then filter or decant it off. **Anhydrous magnesium sulfate** is the general-purpose first choice: high capacity, moderate intensity, fast, cheap, and it works with almost everything. It is a fine powder, so it needs a proper filtration. **Anhydrous sodium sulfate** has higher capacity still but only works below about 32 C and is slower. Coarser, so easier to filter off. **Calcium chloride** is cheap and high capacity, and it reacts with alcohols, amines and some other classes — so it dries hydrocarbons well and ruins other things. **Molecular sieves** are the high-intensity answer: they take a solvent to genuinely low water, they are regenerable in an oven, and they are slow. This is what dries a solvent for a moisture-sensitive reaction. **The visual test.** A wet organic liquid is cloudy; a dry one is clear. Add agent until fresh agent stops clumping and swirls freely, and the liquid has gone clear. That is the endpoint, and it needs no instrument. **Every agent holds some product.** Use the least that does the job, and rinse the agent with a little fresh solvent if the preparation is small.

**Air drying** on a watch glass. Free, slow, and it only reaches the humidity of the room — which for a hygroscopic solid means it never dries at all. **An oven**, well below the melting point and below any decomposition temperature. Fast, and it drives off solvent as well as water. Not for anything that sublimes, melts low, or oxidises in air. **A desiccator** — a sealed vessel with a drying agent in the bottom and the sample on a shelf above it. The agent lowers the humidity in the enclosed air and the sample equilibrates with it. This is what reaches a genuinely low water content at room temperature. **A vacuum desiccator** does the same far faster, and also pulls out solvent. Let it down slowly, or the incoming air blows the sample around the inside. **Drying to constant weight** is the only way to know it is done: weigh, dry an hour, weigh again, repeat until two successive weights agree. Anything else is a guess, and the difference between the first and last weight is a free measurement of how much water there was. **Cool in the desiccator before weighing.** A warm sample sets up convection currents on the balance and reads light, and it takes up water as it cools on the bench.

The two properties are independent, and the published figures make the point better than any description. **Residual water left in air at 25 C** — this is intensity, in milligrams of water per litre, lower being drier: * calcium chloride — about 0.14 to 0.25 * silica gel — about 0.03 * potassium hydroxide — about 0.002 * magnesium perchlorate — about 0.0005 * phosphorus pentoxide — about 0.00002 **Capacity**, roughly how much water an agent holds as a fraction of its own weight, runs in nearly the opposite direction: calcium chloride and magnesium sulfate are high, phosphorus pentoxide is low and is spent quickly. So the sensible arrangement is two stages: **a cheap high-capacity agent to take out the bulk, then a high-intensity one to finish.** That is exactly how a drying train is built, and why a desiccator charged with silica gel will never reach what one charged with phosphorus pentoxide does — nor need to, for most work. **Silica gel with a colour indicator** shows when it is spent, and regenerates in an oven. For almost all practical purposes it is the right desiccant.

A dried substance does not stay dry, and the rate depends entirely on what it is. **Hygroscopic** substances take water from the air — most anhydrous salts, many organic acids. They must be stored sealed, and weighed quickly. **Deliquescent** substances take up so much that they dissolve in it. Calcium chloride and sodium hydroxide left open become puddles. This is dramatic and it also means a sample left on a balance is changing while it is weighed. **Efflorescent** substances do the opposite: a hydrate loses its water to dry air and crumbles. Washing soda crystals turning to powder is this. Three habits follow: **Weigh by difference and weigh fast**, which is the weighing rung's rule and this is why it exists. **Store in a sealed container with a desiccant**, not merely a lid. **Re-dry before use** if a hygroscopic reagent has been open. The bottle that was dried last year is not dry now, and the fraction of water in it is exactly the fraction by which every weighing from it is wrong.