crystal moisture absorber - how do I know when it's 'spent'

[jon]

Something crystal that's not woo, for once, but does have crap instructions. I use widgets where you put a bag of crystals in the top compartment and water drips into a 'reservoir' underneath. Water seems to be absorbed by the crystals, and drips into the container underneath. Seems to help prevent condensation.

Anyone know if there's a way to figure out when the bag is 'spent' and needs replacing?

Thanks.

 

[AK-Dave]

When the crystals are gone, it is used up. It's just a type of salt, and as it pulls moisture from the air, it dissolves in that moisture and the resulting salt solution drips to the bottom.

 

[AK-Dave]

For more, see the DampRid web site: http://www.damprid.dsiwebbuilder.com/index.asp?cat=173010

The crystals are most likely anhydrous calcuim chloride.
http://en.wikipedia.org/wiki/Calcium_chloride

 

[Oldpossum]

Neat!

Now I know what I can use that 200 Kg of anhydrous CaCl2 for, that I just happen to have taking up shelf space, in my chemical store!
(It was obtained for packing Reflux drying tubes, but at the rate it gets used, 200 Kg makes for a 200 year supply!)

 

[jon]

Great - there's still plenty of crystals left, so there must be a while left in it yet

 

[Schneibster]

Actually, there are some that are silica- and they have a capacity, and then they're done. But you can supposedly heat them up and they will lose the moisture. The cadillac ones change color when they're full.

 

[John Hewitt]

Actually, there are some that are silica- and they have a capacity, and then they're done. But you can supposedly heat them up and they will lose the moisture. The cadillac ones change color when they're full.

I'll stand correction but I recall that silica gel crystals are often doped with a cobalt (II) salt as an indicator. The cobalt ions change colour from blue to pink when it hydrates, indicating less of dessicating power. This is reversible and you can regenerate the dessicant power by drying them out in the oven, when they should go back to blue.