posted by Sheryl on .
From part of an earlier question:
D. What filtration device would you use to isolate 2.0 g of crystals from about 50 mL of solution after performing a crystallization?
My answer was a Buchner funnel
It would need to be a SMALL Buchner funnel. And 2 g of crystals can get lost on a large sheet of filter paper. Using a regular piece of filter paper and funnel would concentrate the 2 g in the center of the cone. The filter-tip pipette might work ok here too. Most of these answers are personal preferances, of course, but I would not go with a Buchner, even a small one.
How about using a Hirsch funnel?
I think that is a step in the right direction. I think some Hirsch funnels are as small as 10 mm diameter at the small end, where the filter paper fits, and 2 g probably wouldn't get lost on that. However, you will lose some of the crystals because they will collect on the sides of the funnel (not so with paper because you can wash any solid down to the center of the cone). You can wash the sides of the Hirsch funnel down, too, but some will ALWAYS be lost when the filter is removed from the device. I expect a Hirsch funnel will do what you want it to do if you have a propensity to use vacuum filtration. My philosophy always was that 200 or 300 mL to be filtered required some thinking about how I wanted to do it but 20-50 mL I just let time take its course and I kept busy with other parts of the procedure. I didn't think it was worth the trouble of getting the funnels, suction flask, hoses, paper, etc. and putting all of it together. Another thing I had to contend with was this: About half-way through the filtration, someone would flush a commode in another part of the building, suddenly the pressure in my flask was too low, and water from the aspiration system filled the flask, sometimes completely full. That meant that I had to add a second vacuum flask, in the future, in series so that my material was not contaminated. Funny now but definitely not funny then.
here is a good colorful link to vacumn filtration techniques.]]http://orgchem.colorado.edu/hndbksupport/filt/filtration.html