megalomania
April 25th, 2002, 01:59 PM
I always prefer to use vacuum filtration over gravemetric filtration because it goes quicker, and it dries the solid better. You have to choose you filtration operation depending on what you are going to do.
For example, when I extract bulk chemical product from low purity sources a towel or cotton cloth formerly known as a shirt over a bucket does fine. I only do this when I have a large amount of material and the material I am collecting or filtering out is course.
For removing particulate matter from solution, or for collecting a suitably large amount of material that requires some sembelence of purity, I use coffee filters. I use 2 or 3 of them at a time because they are not fine enough to do a great job. Coffee filters are made for filtering material with the consticiency of coffee. They are also very cheap, as good a reason as any to use them.
For higher purity work I use my collection of filter papers. Filter paper comes in many different varities and sizes. Some paper is ashless or low ash for quantitative determinations, which is not very useful unless you are an analyitical chemist. Some paper is porus to very fine, that is what we concern ourselves with. Your use of paper depends on the crystal size of the material you are filtering. For most people a generic grade paper for routine filtratation is fine. I have several grades but only the one package of the good stuff. The size of your paper is up to your filtering task. If you have a 1-L Buchner funnel you will need big diameter paper, if you have a Gooch filtering crucible you will need little diameter paper. There are also different strengths of paper; when it gets wet it gets weak, and when applying strong suction or holding large amounts of material it can tear. Weak paper is best for simple gravity filtration to remove particles, stronger paper is needed for vacuum filtration.
Then you have the speciality grades, I mentioned ashless and low ash, but you have fluted paper, and of course acid resistent paper. These do cost more, which is why I recommend neutralizing acids before filtering, or diluting the acid. You can't always do this however, so having a box of acid resistent paper around can be a good idea.
As for where you can get it, cloth and coffee filters should be obvious. General use filter paper can be purchased from those science related shops at malls if you happen to have one (like the Discovery Channel store). But for most you will have to search the Internet for a small science store that offers the stuff, there are no age or business requirements to buy filter paper, just make sure your money is a green as the next guys.
I can't tell you off hand what the difference is between all the code numbers that the various filter paper manufacturers use, they differ from company to company. You will have to shop around to get a good deal, and make sure you get the right kind for you needs; there is no sence buying 50 cm ashless microfine paper for collecting acetone peroxide :)
Take Whatman #1 paper for example. It can filter up to 11 microns, has low wet strength making it best for gravity filtration, and has a fast filtering speed. It comes in 3 cm to 50 cm diameters, and ranges from $4 to $130 a box of 100. The bigger the paper, the more it is. #2 offers 8 micron filtration, #3 offers 6 micron filtration, and it is high strength. #2 is only slightly more expensive than #1, and #3 is slightly more expensive than #2. There are several other Whatman grades which anyone can research for themselves. I suggest you visit the <a href="http://www.whatman.com/index2.html" target="_blank">Whatman homepage</a> for better descriptions of what is available.