# Hygroscopic material absorption level

I live in a high humidity location and every winter, mold forms on one of my walls due to water sticking on it during the cold times. This also ruins the paint.

I found out that certain materials absorb water from the air, so I decided to make my own dehumidifier based on sea salt. The problem is alleviated slightly, but not fully - the salt becomes moist, but I think after a certain point it just can't absorb any more humidity.

So I was wondering how much salt I would need to bring the humidity in my room to a normal level.

I looked for things like "hygroscopic materials water absorption levels" on Google, but I did not have much luck.

What I am looking for is a formula like:

X kg of sea salt will absorb Y kg of water at Z% humidity

I have no background in chemistry, unfortunately, so I am not even sure if I am searching for this formula using the right terms.

• I'm looking into this problem as well. I'm considering it doing it through some sort of Gibbs energy/ equilibrium expression. Commented Nov 2, 2017 at 20:49

I tried to do something similar when I had a humid car.

In chemistry, we often use $\ce{CaSO4}$ or anhydrite to serve as a desiccant. You could probably get cost-effective plaster for similar purposes. You could also get gypsum and heat it to generate plaster of Paris yourself.

As you're probably aware, people also use silica gel in products (and food) to minimize water content.

Either material should be much better as a desiccant than regular salt.

One key criteria is to increase the surface area - make sure the material is a fine powder and enclose it in something porous. I've seen people use various cloth materials, paper towels, etc.

As to how much you need, you can work out how much it will eventually absorb. If I take plaster of Paris to gypsum, each mole of $\ce{CaSO4}$ will take ~1.5 moles of water. So for every 136.14 g of anhydrite or ~145.15 g of plaster, you can absorb ~27 g of water. (I use ~ here, since I'm not convinced that regular plaster of Paris will be completely $\ce{CaSO4*0.5H2O}$.. some will likely already have extra water in it.)

The trick is that humidity will continually increase in the room if there's exchange of outside air.

Personally... I'd go with an electric dehumidifier.

• I have the salt covered in cloth. The cloths gets very wet, but I can't tell if the salt continues to absorb water. Hence the need for the formula. Commented Dec 23, 2016 at 2:53
• Some of the gypsum desiccants include "indicator." For example, in a chemistry lab, the blue (dry) gypsum will change to pink when it's saturated and won't absorb further water. Commented Dec 23, 2016 at 3:03
• Salt is tricky, since it will eventually start to dissolve in the water, so I'm not sure there's a well-defined formula. Gypsum or silica gel, it's possible to work something out. Commented Dec 23, 2016 at 3:04