jsc
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How can I separate a fine precipitate from water?
Hi, chemistry beginner here. So, I decided to make CaCO3 (chalk) from washing soda (NaCO3) and calcium chloride (CaCl2) and it worked great... sort
of. The problem is that when the reaction occurs the precipitate is so fine it makes sort of slimy gel that is suspended in the water. To try to get
the goods out I tried a couple of different strategies.
First I tried a coffee filter. That did not work. The precipitate goes through the filter. Is there some kind of system for filter pore sizes? How can
I find out what filters will work with what precipitates? I do have a flask with a nozzle and a filtration funnel so I could do the hand vacuum pump
thing.
Another thought I had is centrifuge it. My centrifuge takes pretty small vials though so it would be a pain to divide up 1 liter of gunk into little
25 ml centrifuge tubes. Is there an easy way to centrifuge 1 liter containers? After I centrifuge I guess I need to decant and then fill with more
water to wash, right?
Thanks for any assistance.
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bfesser
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Boil the mixture.
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jsc
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I want to have a pure outcome. Since the supernatant has dissolved NaCl, boiling would result in the chalk being contaminated with salt.
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Magic Muzzlet
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I believe he means that after boiling for some time the precipitate clumps together enough to where it is able to be filtered. Not boiling to dryness.
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UnintentionalChaos
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Second the motion to boil it. Add some water so it's not a pot of glop first, though.
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Sedit
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Freeze it.
It will force the particals to clump into vains in the ice and if you leave it alone while it melts you will be able to decant the mixture if you
wish.
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Chordate
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I would try diluting it in two parts water, heating the mixture to just shy of boiling with reflux, then very slowly adding HCl dropwise until the
material dissolves completely.
Once its dissolved, cool the mixture slowly in a vacuum flask or by reducing the heat gradually over the course of a day or two. Hopefully you'd get
large easily filtered calcite crystals.
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aonomus
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If you want to filter a fine precipitate, surface area is your friend. Big filter paper, big funnels.
I think that your best bet overall would be to take a large volume of water to triturate the suspension to remove residual chlorides, 10-20 parts wrt.
weight perhaps, with several washes, then you can just decant most of the water and reduce the volume before filtration.
In general another trick you can use to increase the crystal size would be to heat a suspension in solvent until it dissolves, then slowly cool to
crystallize the product. However since calcium carbonate (and most carbonates in general) is fairly insoluble, you won't be able to recrystallize it
very well.
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Saerynide
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Quote: Originally posted by jsc  | | Another thought I had is centrifuge it. My centrifuge takes pretty small vials though so it would be a pain to divide up 1 liter of gunk into little
25 ml centrifuge tubes. Is there an easy way to centrifuge 1 liter containers? After I centrifuge I guess I need to decant and then fill with more
water to wash, right? |
You have a centrifuge at home?  Even if you divided it into 25 ml tubes, it
would only take 7 runs, if say your centrifuge could fit 6 tubes at once.
If you really want to filter it, whatman makes every type of filter paper imaginable. If you only have coffee filters and still wanna filter, you can
try more coffee filters. Keep adding filters until the filtrate runs clear - it does work, from experience before I had real filter papers. You can
also try to freeze it like Sedit mentioned.
But if you think about it, by the time youve tried all these other options, you would have finished centrifuging it all already 
For things that really like to stay in suspension, I stick it in icewater, then set the centrifuge to -5C, and spin for 10 min at 12,000 rpm. That
gets pretty much everything out. It is the most efficient and least time consuming method. So if you have a centrifuge, use it 
And then theres the 160,000 rpm ultracentrifuge down the hall, which I'm too afraid of to even be in the same room with I might have to start using it though... *gulp*
[Edited on 3/17/2011 by Saerynide]
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UnintentionalChaos
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| Quote: Originally posted by Saerynide |
You have a centrifuge at home? Even if you divided it into 25 ml tubes, it
would only take 7 runs, if say your centrifuge could fit 6 tubes at once. |
This reminds me of when I made 6,6'-dibromoindigo in dilute solution according to a particular paper instead of the small volume typically used for
Baeyer-Drewson indigo synthesis. The product was such a fine powder that it completely plugged frits immediately. I didn't even have the luxury of
25ml tubes, I only had one that took 10ml tubes (6 at a time)...and a good 500ml of solution.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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ScienceSquirrel
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Boiling can be good, as it will cause the particles to agglomerate.
Diluting the solution by a lot and giving it a good boil and then allowing it to settle over night could have a good effect.
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jsc
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Follow up to this post.
I ended up just allowing the product to settle. I would add water in the morning. When I got back from work it would be settled. I then decant the
supernatant using a siphon. The process is then repeated.
I found the problem with this is that I had to wash the product many times, about 10-12 times actually, before obtaining a purity greater than that of
tap water at which time I switched to distilled water and washed two more times. All told it was a LOT of washing.
I think what is needed is some kind of specialized centrifuge that allows this process to be accelerated.
Note that the quantities I making, 3-5 kilograms, are too large for little mini setups.
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jsc
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I have found out that there are several different technologies to do this. In general these devices are called "dewatering centrifuges" and there are
various types.
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ldanielrosa
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I'm curious, what are you working on that needs three to five kilograms of extremely clean calcium carbonate? I get some as a byproduct of a few of
my experiments and I wash it for later use, but I haven't found many applications for it.
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RAzium
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fine precipitate of ni(oh)2 is to be seperated from water
hey gus i am working on nanoparticles and i have made nickel hydroxide precursers for nickel oxide particles so i need to seperate them from wate they
are very fine and even appear to be turbid so can u suggest me something plz
contact : razium_ali@yahoo.com
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LanthanumK
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Is not this supposed to be started at a different thread?
Above your current location is a discussion on how to separate fine particles from water.
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