Sciencemadness Discussion Board

Vacuum Filtration Woes...

smaerd - 30-1-2011 at 10:02

This is pretty embarrassing. Figure I'd cut my pride though before I implode my filtration flask.

I have a water aspirator as a vacuum source. The solution I am trying to filter is an aqueous solution with an organic precipitate(not crystalline). Nothing special.

So here's what I've been trying. I turn on the water aspirator, connect it to the filtration flask with the rubber filtration aid securely placed on top. I test suction with my palm, it always has section. I attach the buchner funnel lay down the filter paper(properly). Wet the filter paper with water. Test suction with my palm, there is suction. So now I've tried this three ways...

1) Adding the solution immediately to the buchner funnel in large quantities. Then trying to cut the vacuum but before I can get to the faucet *pffffewwww* filter paper rips. I've tried this with the faucet on full mast, then tried it at about 3/4's then gave up.

2)Cutting the vacuum then hoping over to the buchner funnel to add the solution. No vacuum is present and gravity filtration slowly takes place.

3) Leaving the faucet on, and adding small splashes ~0.5ml of the solution at a time, About half-way through each time the paper rips, and this takes a very long-time. Gravity filtration would be more practical.

So I'm wondering should I invest in a needle valve to control the vacuum better? What am I doing wrong. It has to be obvious.

smuv - 30-1-2011 at 10:17

So even if you just poured plain water into the filter, the force of it going through the paper would rip it? Or is it this specific solution that rips the filter paper?

If its only that particular solution, keep in mind that highly acidic and oxidizing solutions can weaken filter paper.

That being said, I have never had a filter paper (commercial or coffee filter) rip from the force of suction (though I have ripped tons by nicking them with a spatula...but thats another story).

P.S. is the filter paper in a bucher funnel? Or are you trying to vac filter with filter paper pressed into a normal funnel?

smaerd - 30-1-2011 at 10:23

The solution is pretty basic not acidic though. Would it be a bad idea to use two sheets of filter paper? The stuff is qualitative and came with the flask, buchner, and filtration aid.

Yes the filter paper is inside of a 70mm buchner funnel.

I'll have to try a water solution next but not right now my confidence is pretty low lol.:P

I'm kind of mystified too, I've gone through maybe 10 discs failing at this.



[Edited on 30-1-2011 by smaerd]

Magic Muzzlet - 30-1-2011 at 10:27

It shouldn't be ripping, you need to lower the vacuum, or maybe the papers are just really shitty. I've vac filtered plenty basic/acidic solutions and haven't had the paper rip, just get clogged sometimes, still it does not rip. Then again I stop vacuum when such things happen.

DJF90 - 30-1-2011 at 10:36

It could be the holes in the perforated plate of your buchner funnel are too large. They should ideally be slightly larger than pinholes (~1mm?). If they're too large this can encourage the paper to rip, as its not "supported" enough under the strain of the vacuum.

smaerd - 30-1-2011 at 10:44

They appear to be 2-3mm in diameter. So is my buchner funnel bunk? Should I buy a new one?

not_important - 30-1-2011 at 11:12

Put a T connector between the pump and the flask, connect a short section of flexible tubing onto the arm of the T, and an adjustable clamp on that to control how much air leaks in through it. A C clamp with a couple of small blocks of wood or bits of metal sheet works OK.

Alternatively, use a three-hole stopper in your anti-suckback bottle (you do have one between the Buchner and the filter, don't you?), stick a short bit of glass tubing in the 3rd hole and a short section of flex tubing on the end of that, add adjustable clamp as above.

The hole size is typical for that big of Buchner, nothing wrong there. Small holes mean more holes needed to maintain flow rate, on a large Buchner this increased hole density reduces the strength of the plate at the same time there's more force from the pressure difference; large Buchners often have fewer and larger holes. (I've a few 30 cm or more in diameter)



The adjustable leak allows you to control the degree of vacuum applied to the filter. Start off with the clamp open enough that there is no or very little suction on the filter, moisten the paper with the solution being filter, close the clamp enough to get a bit of suck-down on the paper, add more solution and then increase vacuum to speed up filtration. Note that not all filter paper is strong enough to take much vacuum. when wet, there are varieties intended for vacuum filtration that have higher wet strengths.

If you can find it, a thin sheet of small pore open cell polyolefin foam can 'spread' the vacuum so that there isn't the full stress of the Buchner's holes. Not useful for solutions above 50 to 60 C, as the foam weakens and flattens down.




[Edited on 30-1-2011 by not_important]

Magpie - 30-1-2011 at 12:01

I've never had a filter paper rip on a Buchner funnel, except maybe once when the solution was chemically weakening the paper.

If you are using a proper filtration flask you will not implode it with a full vacuum unless it is a defective flask.

I control the vacuum by the amount of water I send through
the aspirator.

I pour the slurry onto the wetted paper first with no vacuum. Then slowly bring up the vacuum as needed. Bring to full aspirator capability if necessary.

You will not get a full vacuum with a water driven aspirator anyway. The best I ever get is maybe 27-28"Hg, even in the dead of winter when the tap water is coldest.

smaerd - 30-1-2011 at 12:10

Thanks so much for the advice guys. I think I'm going to try that air bleeding technique first.

Magpie, thanks for the sense of security over the flask implosion :). I tried controlling with the tap however, my basement faucet is pretty dodgey to say the least.

Thanks again I'll try the recommended suggestions and get back to you :).

SWilkin676 - 31-1-2011 at 02:03
Quote:

turn on the water aspirator, connect it to the filtration flask with the rubber filtration aid securely placed on top. I test suction with my palm, it always has section. I attach the buchner funnel lay down the filter paper(properly). Wet the filter paper with water. Test suction with my palm, there is suction. So now I've tried this three ways...



The order you are performing these operations may be impacting your results. Put the material to be filtered into the funnel first. See how much comes through in gravity, then attach the vacuum. Otherwise the material hits the paper in an uneven manner causing spotted areas of vacuum rather than uniform pull, so yes that will cause the paper to rip.


SmashGlass - 31-1-2011 at 03:36

yeah. I've seen this kind of thing happen before.... Not fun
when it happens to you. We actually have a name for this type
of fine precipitate that take forever to filter. (But it's an internal
lab joke.)

Here are a few additional ways to approach this.

1) try two papers. wet up papers with water before adding the
stuff you are trying to filter. Use a glass rod to decant as much
liquid as possible before getting to the fine precipitate. If 2
paper blow out under suction, your Buchner funnel holes are
definitely too big.

2) allow the precipitate to stir overnight. Some of the water will
evaporate and possibly more of the precipitate will clump
together. But this is compound specific and may not work for
you.

Good luck.

peach - 31-1-2011 at 05:02

How basic is fairly basic?

Strong bases can digest cellulose. I have watched it turning dry, solid plant material to mush.

Your paper failing may be less related to the filter funnel and more to what is going through the paper. You may need to get rid of some of the base prior to using cellulose papers.

I would advise caution testing ceramic filter funnels by putting your hand over the top. Only the stem of the funnel is designed to go under vacuum. The walls will likely be fine but, should they go, you could end up with bits of ceramic in your palm. It's also more likely the flask and funnel will end up broken from being knocked over.

Squirt some water over the paper. You should be able to see the liquid going through and gurgling. If you spray a little volatile solvent on the paper, you'll see it disappear as the air current evaporates it.

You can't really use your skin to sense vacuums. I've seen people put their hands over inch wide ports on rotary pumps with a blower on top of that, then pull it back off as you can with a hoover's tube. Yet the hoover is dropping the pressure from 1013.5mbar to about 700->800mBar, and the rotary will be going out the other side of 0. ---> down ---> mBar.

[Edited on 31-1-2011 by peach]

smaerd - 31-1-2011 at 19:17

ph 10

I had a little bit better of a success with it today. I did the T adapter thing the problem is the tubing i am using is pvc so I can't exactly clamp it fully shut. I need two inches of flexible tubing :(. So I was using my finger to plug the hole on and off and on and off.

I'm beginning to suspect the culprit is the base. I only had one tear using a lightly acidic solution. Though after a successful run using the above finger plugging technique I removed the filter paper and it was ripping with GREAT ease. Hmmm....

The problem with removing the base is if I do then the compounds I am precipitating go back into solution. I am considering a different method for removal(solvent based) instead. Wish it didn't come down to this because solvents are a pain to redistill and the compounds are pretty finicky for solvent selection. Guess we'll see.


Thanks again for all the advice its greatly appreciated.

[Edited on 1-2-2011 by smaerd]

crazyboy - 31-1-2011 at 19:52

I only have high vacuum sources. When I do vacuum filtration I set everything up, use TWO pieces of filter paper, wet them then apply vacuum. I pour the solution in a gradual but steady stream over the paper. If it sucks right through I increase the speed. If the filter gets clogged I go slowly and switch out the papers. If I use one paper it almost always tears, two pieces it rarely tears.

peach - 31-1-2011 at 23:40

Before going any further, and mainly for the sake of knowing but possibly ease as well, try ripping a new filter paper in half.

Dip one half in the thing you're trying to filter and the other in whatever your solvent is (fresh). Take them out and let them sit.

Then observe them for any difference in strength.

That will then qualify or rule out the base issue, which may mean you can or can't vacuum filter it through cellulose.

This is where you may begin considering glass sinters instead. Unfortunately, the problem still persists, as strong bases will gradually ruin the frits as well.

Depending on how large your precipitate particles are, you may be able to use a coarser grade of paper, to get the filtrate through quicker and put less stress on the paper. Another possibility would be to purposefully set about increasing the size of the particulate.

In water treatment, hydroxides are used to precipitate heavy metals. To encourage them out of the pseudo-suspension (to make them fall faster), flocculating agents are added. These attract the tiny particles of materials towards themselves then fall to the bottom of the sedimentation tanks.

I believe bentonite is used in a number of instances.

I have also had to filter particulates that have been insanely annoying to work with - clogging the sinters almost immediately and then not budging over hours of sucking.

Whilst dealing with this, I discovered it became more and more worthwhile to simply let the precipitate fall out by it's self when standing (even if that meant hours or days), and to then decant the supernatant to waste before filtering. I also discovered touching an electric toothbrush against the side of the glass could be used to encourage the solid to fall - but results may differ depending on your solid and toothbrush.

By, removing some of the base, this is more along the lines of what I meant, as opposed to neutralisation, as I am guessing you will be rinsing the filter cake with something other than the original strength base before using it for anything else.

[Edited on 1-2-2011 by peach]

anotheronebitesthedust - 1-2-2011 at 20:01

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smaerd - 2-2-2011 at 17:01

Ah yes peach excellent idea. I will perform that experiment to see :). I see what you mean by removing some of the base now. Makes perfect sense another excellent suggestion.

Thanks anotheronebitesthedust I was actually trying to figure out what type of filter papers are base/acid resistant.

Excellent thank you all again :). I'll get back on this in the near future and report back with results. Though I believe the double filter paper will solves this issue quite fine :).

[Edited on 3-2-2011 by smaerd]

Magpie - 2-2-2011 at 18:38

I'm assuming that you have a good quality filter paper here. If it is "qualitative" it should be giving you sufficient porosity also.

I'm having a hard time believing that a relatively short exposure to a relatively mild base (pH=10) is going to weaken cellulose appreciably. Heres why: Cellulose fibers, from which paper is made, often starts its life as a tree. Wood chips are then cooked under pressure with an alkaline cooking liquor composed of Na2S and NaOH for a couple hours to separate the cellulose from the lignin. The 1st of several stages of bleaching is often a wash with a hot solution of NaOH. If the cellulose can take those conditions your Buchner filtering at room temperature for a few minutes should be insignificant.

Luftwaffe - 20-11-2011 at 20:30

I encountered something strange again today when I was filtering out a solution with a fine precipitate. I tried vacuum filtering and everything went smoothly, I dampened the filter paper over the holes in the buchner funnel and everything was flush, I turned on the vacuum and pulled the solution through it. The sludge was easily filtered but there was a much finer contaminant that seems to go right through the filter paper, multiple passes didnt help either. So I tried paper towels and coffee filters to see if it was my filter paper that was the issue, it wasnt as it went through all of them.

Grasping at straws I decided to line my glass funnel with the lab filter paper I used for vacuum filtering by carefully folding it into a funnel shape and filtered the cloudy solution through it and to my great suprise it came through crystal clear! This baffles me entirely, the same filter paper that failed me in vacuum filtration worked using a different technique. Unfortunately what was once a large capacity in my buchner funnel for filtering is now less than optimal as the volume of filtrate that can be put through at a time is greatly reduced.

My attempt at explaining this is that when the filter paper is at an angle, the filtrate must pass through more paper, or that due to less surface area, the solution is filtered slower(which it is much slower) allowing more particles to be caught.

watson.fawkes - 21-11-2011 at 05:50

Quote: Originally posted by Luftwaffe  
Grasping at straws I decided to line my glass funnel with the lab filter paper I used for vacuum filtering by carefully folding it into a funnel shape and filtered the cloudy solution through it and to my great suprise it came through crystal clear!
The difference is the ΔP across the filter paper. With gravity filtering, all you have is gravitation and the ΔP arises entirely from gravitational potential. With vacuum filtering, you have an additional term from the pressure difference arising from the atmosphere. Extra pressure just means extra force on the solution. I would guess that the filter paper is deforming under the extra force, albeit at a microscopic scale, but enough to allow very fine particles to pass.

Luftwaffe - 21-11-2011 at 13:56

I tried filtering without the vacuum on and just using gravity and the solution comes out as dirty as it went in. Perhaps the rigidity of the cone shape makes it more resistance to deformities than being exposed across it's entire surface area.