Working with hygroscopic materials

A hygroscopic material takes up water from the air around it. Some do this slowly and modestly; others pull in enough water to change visibly within minutes, and the most extreme, called deliquescent, absorb so much that they dissolve in it. For anyone who weighs, stores or certifies these materials, the water is not a side issue. It changes the mass on the balance, the composition in the container, and in some cases the material itself.

What the water does

The first effect is on mass. A sample that has absorbed water weighs more than the dry material it contains, so a solution prepared by weight is more dilute than intended. The error grows with every minute the container stays open and with every step up in the relative humidity of the room. On a balance, it shows as a reading that drifts upward while the operator waits for it to settle.

The second effect is on handling. Powders that take up water clump, cake and stick to spatulas and weighing boats, which makes transfers less complete and less reproducible.

The third effect is chemical. Water is a reactant. Materials that hydrolyze, that change crystal form when hydrated, or that react with water-borne impurities degrade with the water they absorb. A container that is opened repeatedly in humid air can hold material that no longer matches its certificate, even within its stated shelf life.

What the certificate says about water

For a hygroscopic material, the certificate of analysis usually reports a water content, measured by Karl Fischer titration or by loss on drying. Karl Fischer titration measures water specifically; loss on drying measures everything that leaves the sample under the drying conditions, which may include other volatile components. The certificate should say which was used.

Assay results may then be expressed in two ways. An “as is” result describes the material including its water. A result on the anhydrous or dried basis describes the material with the water mathematically removed. The two numbers differ by the water content, and using one where the method expects the other introduces an error of exactly that size. The certificate should state the basis, and the method should state which one it needs.

The water content on the certificate describes the material when it was tested. It does not describe the material after it has been opened. For quantitative work, laboratories often measure water content again at the time of use, or dry the material by a stated procedure before weighing.

Handling practice

• Let a container that has been stored cold reach room temperature before opening it, still sealed. Opening a cold container draws moisture from the air onto the cold material as condensation.
• Keep the open time short. Open, take what is needed, close, and return the container to storage.
• Weigh by difference from a closed container where the method allows, rather than leaving material on an open balance pan.
• Divide a large container into smaller sealed portions when it arrives, so that each use exposes only one portion.
• Store opened containers in a desiccator or with a desiccant suited to the material, and replace or regenerate the desiccant on a schedule.
• For the most sensitive materials, work in a glove box or a controlled-humidity enclosure.

What the record should show

A procedure for a hygroscopic material should say that the material is hygroscopic. It should name the handling precautions, the basis on which results are calculated, and whether water content is measured at the time of use. The material’s log should record when the container was first opened and how it has been stored since, because that history is the only way to judge whether the certificate’s water figure still applies. Where a material is dried before use, the drying conditions and the dried mass belong in the record of the work.

Room conditions belong in the record too. A humidity reading logged at the balance takes no effort once a sensor is in place, and it is the one piece of evidence that explains a balance reading that would not settle on a particular afternoon.

When a result for a hygroscopic material is questioned, the first questions are about water: how long was the container open, what was the humidity in the room, and on which basis was the result calculated? A record that can answer those questions turns a disputed result into a checkable one.