How to weigh a higroscopic substance

[inge]

What is the best way to weigh a higroscopic substance?
Like, are we supposed to maintain the humidity in the weighing room? And how can we do that, because I heard that there can't be any strong air flow in the weighing room area cause it would make the balance unstable.

[JGK]

We used to place a container with dessicant in the balance in irder to remove moisture from the local area.

If the material is not thermally labile, you could dry in an oven and transfer to a dessicator to cool before weighing.

[Einar Ponten]

In the past I used to work with a calcium salt that absorbed a lot of moisture from ambient air ("hygroscopic").

Then I used a TGA, thermogravimetric analyzer, in which you usually measure the dry weight to estimate the content of water (solvent). After getting the dry weight you can take the sample for further analysis. If you need to keep the sample dry then it becomes even more difficult.

At that time Perkin Elmer instruments were used, but less expensive are (were) available from Mettler.

[Bruce Hamilton]

For Karl Fischer water determinations, I usually preweigh a sealable container ( eg screw cap vial ) that has been purged with N2 or Ar. I then add the hydroscopic substance quickly, repurge, seal and reweigh.
I then quickly pour the substance into the titrator, repurge the container, seal and reweigh the container to accurately know what was dispensed.

Given that even air-conditioned labs are usually around 40 - 50% relative humidity, trying to dry balance chambers in an open lab would be a mission.

The problem is more about " what is in the vial?" after accurate weighing because, unless you know the water content of the weighed hydroscopic substance, what use is an accurate weight?. I usually, when possible, prefer to dry substances, and then use the above procedure to obtain accurately-weighed samples for analysis.

Not many analytical procedures recommend weighing partially-hydrated materials, they usually specify an initial drying step, or using a relatively- stable hydrated form.

Please keep having fun.

Bruce Hamilton

[inge]

Thank you for the replies.
I'm gonna try and find out which method is the best and the simplest.

[Flappytango]

a temperture and humidity controlled glove box is best...


this has worked pretty well for me in the past, run a good dehumidifier in the balance room overnight and then wiegh first thing in the morning.