Sciencemadness Discussion Board

Hygroscopicity

ahill - 6-12-2016 at 15:55

I've just made some cobalt nitrate and cobalt chloride from pottery shop cobalt carbonate, and have been trying to dry the samples using my conventional method of placing the chemical in a smaller beaker inside a larger beaker containing calcium chloride with a little sodium hydroxide and covering with a few layers of clingwrap.

It doesnt seem to be working (much) - I suspect the cobalt salts are quite hydroscopic themselves - and there is a tug of war going on between them and the calcium chloride for the free water. (the sodium hydroxide is there mainly just to deal with any of the fumes from the excess acid)

Is there some kind of scale / measure for quantifying the power of desiccants ?

Edit: Fixed spelling of title

[Edited on 12-7-2016 by zts16]

careysub - 6-12-2016 at 17:00

Quote: Originally posted by ahill  
I've just made some cobalt nitrate and cobalt chloride from pottery shop cobalt carbonate, and have been trying to dry the samples using my conventional method of placing the chemical in a smaller beaker inside a larger beaker containing calcium chloride with a little sodium hydroxide and covering with a few layers of clingwrap.

It doesnt seem to be working (much) - I suspect the cobalt salts are quite hydroscopic themselves - and there is a tug of war going on between them and the calcium chloride for the free water. (the sodium hydroxide is there mainly just to deal with any of the fumes from the excess acid)

Is there some kind of scale / measure for quantifying the power of desiccants ?


The usual practice has been to measure them by the amount of residual water vapor left after desiccating. Numerous papers have been published using this as the standard. mg/L or g/M^3 (which are the same) are the usual measures used.

Another body of literature looks at the residual moisture in solvents, but unavoidably is solvent specific.

These are measures of efficiency.

Desiccant capacity (g H2O/g desiccant, or a percentage of the desiccant mass) and speed are also relevant.

Here is a commonly seen chart (in different forms, with the same data):
https://erowid.org/archive/rhodium/chemistry/equipment/dryin...

And this gives an efficiency ranking sequence:
https://erowid.org/archive/rhodium/chemistry/equipment/dryin...

These three factors are more or less independent of each other.

[Edited on 7-12-2016 by careysub]