NEMO-Chemistry
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Glass frit filter funnels and filter paper numbers
Hi
I have 2 small funnels with glass filter frits in them, they are very small and i have decided to splash out and buy some filtration stuff.
The funnels i have seem to be a 'number 4' pore size, this is written on the side. I have tried to filter calcium chloride solution and calcium
carbonate with them and they are not very effective!
My set up was a standard conical filter flask and vacuum applied. I have also tried putting a couple of layers of coffee filter paper on the top and
filtering under vacuum, eventually a precipitate did form on the paper however the solution was cloudy.
So this has me wondering about the number system and relative pore size of glass frits and filter paper.
I have seen whatman number 1 filter paper, but the price was staggering! So while i assume the numbering system indicate the pore size i dont actually
know what the numbers equate to.
What size particles are the different numbers intended to filter? And as a general rule what size filter (pore size?) do you use most often in the
lab?
I intend to buy a decent size glass filter funnel and maybe a buchener funnel and some filter papers, i am a bit lost as to which papers would be best
to get.
I ended up have most success filtering the calcium chloride/carbonate with a real mash up system of various sponges and wipes pushed inside a large
plastic funnel!
But i would prefer to get the real thing, considering the funnels seem pretty expensive i would like to get one/maybe two of the most common sized
used.
Thanks in advance.
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Sulaiman
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I use a porcelain buchner funnel for 90mm dia. papers and vacuum,
I use cheap Chinese papers, 'slow' or 'medium' (crappy versions of #1 and #3 respectively)
many things filter well with 'medium' paper, very quick with vacuum
if the 'slow' paper does not do a good job then run the filtrate through the same paper,
the bigger pores get 'filled up' on the first pass so the second pass is like a really fine pore size
Why 90mm dia ? ... because they are the cheapest on eBay 
you only need one funnel but an endless supply of papers.
I have very little experience with fritted filters (I have two types) so my observations are worthless here.
for very small/valuable solid/liquid separation I use gravity, patience and decanting.
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NEMO-Chemistry
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Ok thanks for the info. I like the porcelain ones
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Sulaiman
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P.S.
if you are going to buy an Erlenmeyer flask I suggest that you spend the little extra on a ground-glass neck,
size to match the rest of your glassware, and of course the funnel.
I use the common 24mm neck size.
The special 'Bucher bung' is expensive, so
I used a c60mm od x bucher funnel id, disk of silicone rubber that I cut from a baking sheet ... works perfectly.
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NEMO-Chemistry
International Hazard
Posts: 1559
Registered: 29-5-2016
Location: UK
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most of my glass is mixed size with adapters and/or non ground glass
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