rates of water absorption of hygroscopic substances

[Adze]

Is there a standard, commonly used measurement for the rate of absorption for hygroscopic solids and liquids?.

 

I'm looking for a comparison of the hygroscopicity of various substances such as:

Calcium nitrate

Potassium Hydroxide

Ethanol

Sulfuric acid

 

 

Eg, if I have 1L of Ethanol, with a given surface area exposed, for a given temperature and humidity, can I calculate a quantity of water absorbed per unit of time?

 

How would I compare the rate of water absorption between KOH and NaCl?

[Enthalpy]

Welcome, Adze!

 

I measured it for glycerine, and my opinion is that the flow of air above the liquid surface determines the rate - unless the absorbent is very inefficient or saturated. And even if the air had a flow fast and turbulent enough, then the spreading of water molecules in the liquid would be the limiting factor, which itself depends on flow and turbulence of the liquid rather than on its hygroscopic properties.

 

For a powder, the grain size and the way air flows (Lifting the powder? Gently through the powder? Over it?) would be determining.

 

So the rate of absorption would not be an intrinsic property of a good hygroscopic substance.

 

NaCl is inefficient as a hygroscopic substance, while KOH is very efficient (but has drawbacks). With ethanol, you'll get more vapour in air than humidity in alcohol. Why shouldn't you choose normal compounds like glycerine, PEG, calcium chloride?

[Adze]

Thanks for the welcome!

 

Your reply explains a few things for me, I was thinking it would be an instrinsic property, but was wondering why I couldn't find data of that kind for specific substances.

 

Can you talk about the reasons for the differences in efficiency of NaCl vs KOH?

 

I'd chosen a few compounds that I have some interaction with, thinking I might be able to use the data. Eg. If I leave a container of ethanol fuel with the lid off for a while, could I get a very rough approximation of how much water it will absorb in a certain time. But it sounds like it would do more evaporating than absorbing of water - which means I shouldn't be as worried about water getting into my fuel as I used to be.

[Enthalpy]

NaCl vs KOH is just experimental knowledge. If you leave a packet of table salt open, it takes weeks to get damp. Do the same with NaOH (and KOH does supposedly the same) and soon you get a paste.

 

I did the experiment of moisture absorption. It was glycerine (known to be hygroscopic), impregnating a cloth, left in open air in a workshop. I measured the mass every day. It took weeks to add 10% to its mass. So with just an open canister there's nothing to worry about.

 

And, yes, ethanol will evaporate.