[macca202]

Hey guys.

Yes that's right, im havving trouble making salt water.

Im doing a thesis, and it's due bloody soon and i havn't gotten any experiments on paper and im pretty much screwed and can't work out how to fix this problem.

NaCl is theoretically soluble in water up to apprx 25% of the total volume of the salt water.

For my experiment, i am just trying to mix 2kg in a bin to make a total volume of 63L. That's a mass of 3.1%.
And guess what, even with a stirer turned on overnight, the solution remains cloudy!
I can't work it out. Im using home brand cooking salt. the packet says ingredients: sea salt
iv'e tried working in smaller containers, and i've been able to dissolve close to 20% salt in water, yet i just can't seem to do it in this green bin.
Also, when i leave a small amount in the tank for a week, a milky non solid residual cloud sort of thing sits at the bottom.
i have no idea what it is.

i've got a bad feeling it has something to do with the rust from the fitting on the bottom of the tank




this is the experiment.



 the milky solution is the salt water (and the red liquid is freshwater)
the milky solution should be clear, but isn't.
i don't know why.
the only important thing to me is density, but i still need the salt water to be clear so that i can take measurements of the coloured flow

Im pretty sure there isn't anti caking agent in it, because on the table salt packet, it mentions anti caking agent, but it doesnt mention it on the cooking salt packet.
I've tried pool salt, and it doesn't seem to make a white cloud, but it takes absolutely forever to dissolve in the water!!!


thanks for your help, im deep in it

[Borek]

Sea salt (or any cooking salt) is not pure NaCl, but mixture containing many other salts in small quantities. No wonder you have troubles.

[macca202]

that answer seems so obvious.
so im gonna try and ring up chemical suppliers and ask for pure NaCl

[Ψ×Ψ]

Are you trying to dissolve it at room temperature or are you heating it?

[Yggdrasil]

I'm with ?*?.  The easiest way to dissolve the salt would be to dissolve it in hot water first.  Here's how I would probably go about it.  Split your 2kg into two 1 kg batches.  Heat up 5L of water to near boiling, then dissolve 1kg of salt in it (making a ~20% solution).  After everything is dissolved, let it cool, then add it to your larger container of water.  Repeat with your second batch of NaCl then top off the tank to 63L, and mix.  This method should get everything dissolved more quickly.

[Custos]

I don't think heating will help. Sea salt is a mixture of salts - the negative ions are mostly chloride with about 7% sulfate. The positive ions are mostly sodium but with small amounts of magnesium, calcium and potassium. All of these combinations are pretty water soluble (some more than sodium chloride) with the exception of calcium sulfate (hydrates known as gypsum or plaster of Paris), which is quite insoluble in water. I think it's the calcium sulfate you are seeing.

By the way, pure sodium chloride is interesting in that it's solubility in hot water is about the same as it's solubility in cold water - ie. heating the solution might speed up the dissolution but will not make it more soluble.

[Borek]

Heating will speed dissolution up, even if it will not change solubility.

Precipitation can be removed by filtration, but still I will just start with pure NaCl to avoid additional work.

[macca202]

wahoo!

thanks for the replies guys.

i rung around for hours this morn and found a place selling 25kg bags of "superfine salt" which he said was minimum 99.4%pure NaCl
he said it was something like $2.40 a kilo, and ended up charging me AU$60 for it, which i thought was a bit expensive.

i went to this chemical supplier, and the security was insane. two stage opening and closing security buzzer gates and the whole place electrified. lol
"one bag of salt please" haha

anyway, this stuff is great, it's even finer than table/cooking salt (quicker disolving!), and it dissolves clear!

the only thing im dirty about, is that when i saw what brand it was, "mermaid superfine salt", i remembered seeing that stuff online elsewhere for only $12.30! http://www.jbdistributors.com.au/Grocery+Foods/Grocery+Foods/Peppers+And+Salts/Productlist.phtml
im gonna try get some other quotes and then politely ring him up and tell him he must have gotten the details wrong, even though he was probably just another dodgy salesman trying to take advantage of a student.

I did speak to another chemical supplier who had AR and Technical grade salt, but they were about AU$90 per bag.

Thanks again