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Author: Subject: Varying concentration of solution of salt without wasting said salt
Quince
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[*] posted on 17-10-2005 at 01:07
Varying concentration of solution of salt without wasting said salt


Say I have a large amount of potassium thiocyanate solution, and I want to vary its concentration over some range to match refractive indices of various glasses. Now, lowering the concentration by addition of water is simple, but increasing it without wasting the salt is a problem. How can I do it? Evaporation is not practical in my application for various reasons, including the size of the setup. What drying methods are likely to work? Would the use of something like diatomaceous earth molecular sieves work, or are the salt molecules not small enough for this to work? What other options do I have, where the drying agent can be recycled?



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azaleaemerson
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[*] posted on 17-10-2005 at 05:00


The idea that pops into my mind is making up a teabag of dried silica from the hobby store. The water should stick but not the salt.

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[*] posted on 17-10-2005 at 12:15


Stick the cold end of a peltier cooler in it and freeze out the water?
The trouble with drying agents like SiO2 is that they carry quite a lot of the salt solution trapped between the particles.
Depending on what you are doing you might be able to use a density gradient; high concn at the bottom- low concn at the top.
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mick
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[*] posted on 17-10-2005 at 13:28


I do not think you have many options. If you start of with a standard solution you can only dilute.
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Quince
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[*] posted on 17-10-2005 at 15:26


So a molecular sieve won't work here? They seem quite often used for drying ethanol.



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mick
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[*] posted on 17-10-2005 at 15:44


If you manage to concetrate a standard solution under guaranteed conditions let me know
mick

edit spelling

[Edited on 17-10-2005 by mick]
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Quince
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[*] posted on 17-10-2005 at 15:48


It's not standard, it's solution on the order of 65%-70%, which matches the refractive indices of most glasses. I need to be able to match different glasses, in any order, so I need to be able to have multiple dilutions and concentrations. I'm using about 3.5 L of the solution, and I need a way to minimize the loss of the salt.



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mick
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[*] posted on 17-10-2005 at 15:58


[Edited on 18-10-2005 by mick]
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epck
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[*] posted on 17-10-2005 at 18:36


Have you thought about reverse osmosis? You'd need a pump and a good membrane but that might be the fastest way of removing the water.
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Quince
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[*] posted on 17-10-2005 at 20:26


Interesting. Where to find a membrane? Do they last long?

Also, someone said rotary evaporator. But they are usually for smaller amounts of liquid.

I guess I could make up several different concentrations and switch between the solutions. But then I'd have to get several times the amount of potassium thiocyanate.

[Edited on 18-10-2005 by Quince]




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