The next lab in this book was for purifying copper sulfate through crystalization using it's solubility in boiling vs near freezing water.
The only real hitch was how long the gravity filtration part takes. Sheeeesh. I have a vacuum pump coming in the mail, though I've no vacuum
filtration flask yet. I do, however, have this elbow joint that I think is used in vacuum distilling. Could I just use a distillation setup, minus
the distillation tube, for filtration, or would this be a bad idea for any reason? UnintentionalChaos - 17-7-2009 at 00:55
If you let the solution cool slowly and undisturbed, you should get large crystals. Just pour off the supernatant, crush the crystals, and let them
dry. The large crystals trap very little liquid between them so there isn't much point in filtering. A swirl with distilled water to remove adhering
solution may be in order though.
The elbow joint? You mean a 105 degree bend vacuum adapter? You always need some sort of opening (usually supplied by a vacuum barb like the one
you're talking about) on on a setup even without vacuum as it provides pressure relief.
How would you filter using a distillation setup? I don't get it. Distillation tube? Do you mean condenser? Why do you own this stuff if you have no
idea what half of it's called?
A vacuum pump is overkill for many applications. A water aspirator is enough for most lab work, especially basics.
[Edited on 7-17-09 by UnintentionalChaos]kclo4 - 17-7-2009 at 01:05
Two hole stopper on a flask, one with the funnel, and the other with a tube to suck on... just really pay attention not to suck in any stuff.. maybe
make a trap
How the heck are you going to filter with a distillation setup? lmfao stainedflask - 17-7-2009 at 02:48
Why do I need to know what it's called to own it? And this is the beginnings area. I'm posting in here for good reason. Ah right, condenser, I
knew that, the word was just escaping me at the moment, and distillation tube seemed an adequate replacement.
But to answer your query, I got damn lucky one day and got a whole mess of *stuff* for cheap.
I've gone ahead and taken a picture so that you see what I mean without my bad explanations getting in the way. Yes, I know it lacks a proper
buchner funnel and the apparatus itself isn't proper. I'm just wanting to know if there's a reason why this is a bad idea. (Such as say, is this
thing likely to implode on me?)
You won't be able to vacuum filter using that funnel. I don't see any reason that the rest won't work though, assuming the joints are greased so you
can pull a good vacuum.
If the wet filter paper clings to the walls, all of the suction is going to be focused on that tiny spot where the stem of the funnel is and it will
tear the filter paper and suck your product down. A buchner is designed to distribute that suction across a wide area for effective liquid removal.
I stand by my original suggestion of growing large crystals. It's rather pretty as well. Just don't move the container at all once you start cooling.
The solution lays down the larger crystals slower than it cools (it's constantly supersaturated) and if you agitate it, you'll get "snow" of tiny
crystals that you're trying to avoid. I purified about 2 kilos of CuSO4*5H2O when I was first starting by this method and I'm still using that same
stock. Except, it started off with some dirt in it, so all of it was dissolved in boiling water and filtered through a cotton plug in a wider neck
funnel to give a clear solution.not_important - 17-7-2009 at 07:37
UC is right - decanting (pouring a liquid off a solid) is too often ignored. Even when filtering, decanting much of the liquid through the filter and
then pushing the solids into the filter can speed things up. This is particularly true when you don't have a Buchner or similar funnel, and/or are
filtering a large amount of liquid. When filtering insoluble precipitates that are going to be washed, allowing the solids to settle well, decanting
off as much liquid as possible through the filter, then adding the wash solution, stirring, settling, decanting, repeated as needed, can save time
even though you do wait for the solids to settle.
If you are going to be vacuuming filter much, get a Buchner to match the size of filter paper you have on hand. It'll save a lot of frustration.
A side note - recrystallising copper sulfate does not do a good job of removing iron, the sulfates form mixed crystals. A quick
check for iron in CuSO4 is to dissolve a small crystal in as little warm (body temperature) water as possible, add a couple of drops of hydrogen
peroxide solution, put a drop onto some filter paper or paper towel, then slowly drop a few drops of strong aqueous ammonia onto the spot of copper
sulfate. The copper should dissolve as the deep blue ammonia complex which is spread and thinned out as the ammonia is dropped on it, iron
contamination is left behind as a rusty spot. To remove iron from copper sulfate make a strong hot solution of the sulfate, drop in ammonia until you
start seeing a persistent precipitate (not a lot, just a bit forming from adding a few drops of ammonia), then add some H2O2. Boil briefly, test for
iron, if positive repeat adding H2O2 and boiling until the test for iron is negative. Then let cool just a bit, so the solution isn't boiling, and
filter while hot. The precipitate is ferric hydroxide, a bit of CuO, and junk in the raw CuSO4 that didn't dissolve. UnintentionalChaos - 17-7-2009 at 08:36
Sidenote- the "filter paper" that tore at the time was actually coffee filters. I don't think real filter paper would do this, it just wouldn't filter
properly.
I have both fritted glass and standard porcelain buchners. Both have their uses.chemrox - 17-7-2009 at 12:45
Buy the f**king sidearm flask; what are you kidding us?stainedflask - 17-7-2009 at 13:52
This book(Illustrated guide to home chemistry experiments) was saying I want lots of small crystals, rather than a few big ones. Also to clarify what
I'm doing, I was saturating boiling water with the copper sulfate, then pouring this saturated solution through a filter to remove any insoluble
impurities, then crystalizing the filtrate in an ice bath. I'm not thinking that decanting had much application here. It certainly helped in
separating the copper carbonate I used the remaining copper sulfate to make though, thanks for mentioning it. I'll give the test for iron impurities
a try too, thanks.
One of of the things that slowed the filtering down so much was the copper sulfate crystalizing as it hit the cooler filter paper. This slowed it
down, and gave the rest even more time to cool, and so on.
I noted also that wikipedia shows KNO3 as has similarly high solubility in boiling water vs freezing. I decided to repeat the experiment today using
some technical grade KNO3. Yikes. I ran into even worse filtering problems. 10 seconds and funnel and paper was quite clogged. I tried slowly
pouring, letting just a little filter at a time, as well as seeing if completely filling the funnel would work better. It got clogged fast either
way. I also didn't succeed in using anything finer than paper towel as a filter, and even that wasn't so great and clogged badly as well (there was a
bit of brown stuff that stayed in the filters tho, so they obviously removed something). Preheating the funnel and receiving flask didn't help very
much either, though perhaps I didn't heat them enough. -- I hadn't done this with the copper sulfate.
This is why I'm wanting to try vacuum filtration, to get this done as quickly as possible to avoid excessive cooling and premature crystalization. I
apologize if I wasn't clear earlier.
I'll likely get a real vacuum flask sooner or later. I just also happen to find amusement in cobbling things together with what I do have. I only
wanted to know if I was setting myself up for a disaster by doing so -- this stuff is new to me. I'll also look into getting a proper buchner funnel
soon. Or would using gooch crucible filters be sensible? I have about 2 dozen medium and coarse crucible filters. I could modify one of the large
rubber stoppers to hold one. entropy51 - 17-7-2009 at 14:58
SF, hot filtrations are difficult, messy, and often not successful. The only real solution is to use a funnel heater, which most people don't have.
Applying a vacuum won't make a hot filtration any easier.
One trick is to preheat the funnel and paper by resting it on a flask of boiling solvent, in your case water. Transfer the hot funnel to a dry flask
and try again. As you've noted, that doesn't always help. If the impurities are visible, like the dirt found in OTC chemicals, use a fast filter
paper. I would use a paper towel or coffee filter, or even something as coarse as glass wool or cotton. Some use cloth, like an old tee-shirt.
For inorganic salts like CuSO4 and KNO3 my preference is to make a saturated solution at room temperature and filter that. Then boil the filtrate
down until a scum of crystals forms on the surface. Then cool and watch the crystals form! This avoids a hot filtration altogether and I recommend
it.
Don't be discouraged. If chemistry was easy there would be a lot more chemists than there are now.
But do get a filter flask and Buchner funnel. But neither will help with hot filtrations; just avoid them if possible.
[Edited on 17-7-2009 by entropy51]smuv - 17-7-2009 at 15:46
Fluted filter paper. Also cotton plugs work well, though it might be hard to get your product off the cotton. A lot can be done without a filter
flask.
A side note I have few filter flasks, but I have many rb flasks and I have a straight vacuum adapter, this way any round bottom flask is a filter
flask.stainedflask - 17-7-2009 at 16:29
ah HA! A funnel heater! Thanks! I just cobbled one together with some magnet wire and a 12v power supply. And what luck, the temp of the tube of
the funnel stabilizes at a little over 100C, and the cone is in the 90's. Not bad for a random guess on the wire length. And if it should end up
needing more heat, I can move the contacts closer together.
I'll let you know how this works out. Thanks again for making that click.entropy51 - 17-7-2009 at 16:36
