dangerous amateur
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Hygroscopy vs. purity
Hi,
can you explain, why hygroscopy in certain chemicals, especially salts, seems to depend on their purity grade?
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ScienceSquirrel
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In the case of sodium chloride, very pure sea salt is only slightly hygroscopic. I have some from the Carmargue that is pure white and free running
but I also have some sel gris 'grey salt' from Guérande and that is always slightly damp like moist sand. It is also slightly bitter and more salt
tasting and I think this is due to things like magnesium and potassium salts, some of which may be more hygroscopic than sodium chloride.
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dangerous amateur
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Yes, there can be contaminations that are more hygroscopic. But can it be that hygroscopy is affected by the simple fact that several different salts
are mixed?
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unionised
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Yes.
It's possible for a pair of non reacting, non hygroscopic salts to make a mixture that is hygroscopic.
A salt will be hygroscopic if the vapour pressure of water above a saturated solution is higher then the vapour pressure of water in the air. (so,
whether or not something is hygroscopic depends on the ambient humidity)
Adding another salt will reduce the vapour pressure of water above the solution so the amount of water in the air needed to make the mixture
hygroscopic will be smaller.
So the mixture will be hygroscopic at lower humidities than either of the salts it's made from.
If that change brings the cut-off below the ambient humidity then the mixture will be hygroscopic, even if the components aren't.
On a practical basis a mixture of salt and sugar is a lot more hygroscopic than either component.
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woelen
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Another nice example of a mix which is very hygroscopic while its constituents are not is KNO3 + NH4ClO4. KNO3 is not hygroscopic at all. NH4ClO4 is
very weakly hygroscopic. When these chemicals are mixed as fine powder, then the mix attracts water easily, due to formation of NH4NO3, which is very
hygroscopic.
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