Why is cotton better than filter paper for filtering?

Question

I just did an experiment in chemistry class where we had to purify an impure substance. In one step we are told to use cotton to filter out some insoluble matter out of a solution. I used a filter paper by mistake and my prof asked me to do it all over. I don't really understand - why is cotton better than filter paper for this step? 'Cause in some later step, we need to use filter paper, not cotton. I'm just wondering - would be glad for any help. Thnx.

Answer 1

First things first, did you see any difference between using a filter paper and using cotton fibers at this step?

Understand that not every sheet of filter paper is the same.  To cut costs, many teaching laboratories use a common, inexpensive filter paper (such as grade No. 2) which has a medium flow rate and a medium pore size (about 8 µm).  This will not catch everything.

You might have been able to use a grade No. 6 paper which is slow, but has a pore size of only 2.5 µm.  This item, however is expensive in comparison.

Cotton fibers have very similar chemical properties to paper, but can be packed and stacked to give excellent filter rates while still retaining small insoluble particles.  They are also inexpensive.

In the experiment that you did, the first filtering required retaining small particles, so using cotton fibers makes sense.  The second filtration only required retaining medium size particles, so using filter paper was faster and less expensive than making another cotton filter.