asdercks Posted January 22, 2014 Posted January 22, 2014 Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment.Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent so I gues what you meant to say was that sodium nitrate is more hygroscopic than strontium nitrate. also, why do you compare everything to strontium nitrate?
Maserface Posted January 22, 2014 Posted January 22, 2014 I use strontium nitrate (for my own sake) as a sort of baseline for comparison. I have worked with it, and I know how to deal with it. I should have been more clear about my reasoning behind that. Wouldnt the more (water) soluble a chemical is directly affect the amount of water it will draw from the air?
FlaMtnBkr Posted January 22, 2014 Posted January 22, 2014 (edited) Nope. Unfortunately they don't really correlate with each other. They are separate physical properties and are unrelated. I think there is something that can predict water uptake based on relative humidity but not positive about that. I don't think there is any kind of number that allows things to be ranked in any way. Mumbles might be able to shed more light. Even things that don't dissolve in a solvent can be hygroscopic. For example paper and nylon webbing are considered hygroscopic. Edit: solubility might help determine if a hygroscopic chemical is deliquescent. But probably not. Edited January 22, 2014 by FlaMtnBkr
Mumbles Posted January 22, 2014 Posted January 22, 2014 Nope. There is only a vague correlation with solubility and hygroscopicity, but it's far from a steadfast rule. Some of the most hygroscopic things happen to be very soluble in water like potassium hydroxide, sodium nitrate, AN, magnesium perchlorate, calcium chloride, etc. However there are plenty of very soluble compounds like potassium nitrate, sodium chloride, many sugars are are only mildly hygroscopic to not at all. There is probably a little truth to compounds that want to associate water to dissolve also want to associate water in the solid state, but it's hardly a real phenomenon. There are some very hygroscopic chemicals that are basically insoluble too, like calcium sulfate, alumina, silica gel, molecular sieves. By the way, strontium nitrate really isn't all that hygroscopic. It's the strontium chloride impurity that gives it a bad reputation.
asdercks Posted January 22, 2014 Posted January 22, 2014 that is a really good question! I've read that sodium chloride is so hygroscopic that it readily dissolves in the water it absorbs but that is a property called deliquescence, and that is different from solubility. I've never seen my clumped barium nitrate dissolve
FlaMtnBkr Posted January 22, 2014 Posted January 22, 2014 I don't think barium nitrate is very hygroscopic. When it's a powder it just locks together under its own weight into hard 'rocks'. Potassium chlorate will do the same thing, as well as quite a few other chemicals when a powder and not coarser granular form. Maybe someone else will know the phenomenon that causes it.
asdercks Posted January 22, 2014 Posted January 22, 2014 (edited) Interesting, I always thought that it was moisture causing my Ba(NO3)2 to be clumpy Edited January 22, 2014 by asdercks
Maserface Posted January 22, 2014 Posted January 22, 2014 (edited) thanks for the clarification. Im still having trouble wrapping my head around it. Does the tendency of something to be hygroscopic have to do with its physical structure? In the case of molecular sieves, they act as a sponge, likewise with silica gels I THINK.. Much to study Edited January 22, 2014 by Maserface
FlaMtnBkr Posted January 22, 2014 Posted January 22, 2014 When (most?) chemicals dissolve in water they are breaking apart into positively and negatively charged ions. So KCl becomes K+ and Cl- so that the bond is broken. I think when something is hygroscopic it isn't breaking the bonds (usually) but attracting a molecule of water for various reasons. Two different things going on. It has been a long time since chemistry but I'm sure there are some websites that will better explain it.
psyco_1322 Posted January 22, 2014 Posted January 22, 2014 that is a really good question! I've read that sodium chloride is so hygroscopic that it readily dissolves in the water it absorbs but that is a property called deliquescence, and that is different from solubility. I've never seen my clumped barium nitrate dissolve I think you are a bit off here. Sodium chloride (NaCl, table salt) is not really that hygroscopic, although it is quite soluble in water. Solubility refers to the substance dissolving into a solvent, while hygroscopicity refers to the substances property of absorbing moisture from the surrounding atmosphere. If you dump salt out on the table, it's not going to turn into a puddle of water, but if you dump some into a glass of water, it will go into solution readily. 1
schroedinger Posted January 23, 2014 Posted January 23, 2014 Just one small hjnt, NaCl isn't hygroscopic at all. Tabke salt is, because off the anti-cake
LambentPyro Posted January 23, 2014 Posted January 23, 2014 (edited) By the way, strontium nitrate really isn't all that hygroscopic. It's the strontium chloride impurity that gives it a bad reputation.Chris, is there any way to get rid of the impurity? I have DuPont SrNO3, don't know if that is pure enough to be not so hygroscopic. There are SrNO3's that are high purity grade Japanese made. I don't use it that often, I have the product and it's crystallized, never got around to milling it to see if it clumps up. Edited January 23, 2014 by LambentPyro
asdercks Posted January 23, 2014 Posted January 23, 2014 I think you are a bit off here. Sodium chloride (NaCl, table salt) is not really that hygroscopic, although it is quite soluble in water. Solubility refers to the substance dissolving into a solvent, while hygroscopicity refers to the substances property of absorbing moisture from the surrounding atmosphere. If you dump salt out on the table, it's not going to turn into a puddle of water, but if you dump some into a glass of water, it will go into solution readily. ooops! I meant to say calcium chloride sorry and thanks for pointing out my mistake.
psyco_1322 Posted January 23, 2014 Posted January 23, 2014 Chris, is there any way to get rid of the impurity? I have DuPont SrNO3, don't know if that is pure enough to be not so hygroscopic. There are SrNO3's that are high purity grade Japanese made. I don't use it that often, I have the product and it's crystallized, never got around to milling it to see if it clumps up. A lot of the time the clumping of ground chems is not from moisture. I have been using the "high purity" strontium nitrate and it will still absorb a good amount of moisture. I would just dry the chems out that you need for your comp, that way you can weigh up the correct amount.
Recommended Posts