Posted by ~christina~ on Sunday, September 9, 2007 at 10:16pm.
an addition to this is
Why doesn't my lab in organic chem use plastic pipettes?
Is it going to react with the organic solvent if you do use it instead of glass?
And it's kind of annoying that the liquid in the pipette has a tendency to go into the bulb while I'm transfering liquid to another container..am I holding it incorrectly?
If these are eye dropper type (a thin drawn out tapered tip with a rubber bulb attached at the top), I have faced a number of difficulties. Not much hand movement is necessary to eject liquid, it is difficult to keep a steady pressure on the bulb and even when the bulb is fully expanded (no more suction), movement still ejects the fluid. Volatile fluids are worse for they tend to partially vaporize causing bubbles in the pipet and that ejects fluid. Probably you aren't doing anything wrong. These devices are meant to be quickie type transfer devices. Although the literature says the glass ones can be calibrated and are quite accurate, I have not found that to be true unless the spattrering can be eliminated. Still, they are useful devices for separating fluid from a ppt that has been centrifuged as in qualitative analysis.
Some of the plasic pipets are soluble in some organic solvents.
darn..I like those plastic pipettes..
Well Thanks for the info Dr.Bob
Oh my...so it isn't just me.
Yes that's what I'm refering too except we have our own rubber bulbs we can attach to the disposable glass pipettes. However these don't have any numbers on them or anything they're just glass tubes.
I'm afraid of taking out the organic solvent or aqeous layer and accidentally ejecting the other layer into the top layer and having to start over again or if it was worse then the aqueous layer would be on the top and I'd be taking out the organic solvent and that would squirt somewhere else.
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I just read my lab text on this and it suggested using some cotton to plug up the tip but I don't know how wise that would be b/c if you were getting the organic solvent out instead of the aqueous layer wouldn't that catch some of the dissolved compound in that cotton and of course there is always the chance the cotton could come loose while your pipetting liquid from one container to another.
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Not an unusual problem with disposable pipettes and organic solvents, and is worse with low boiling solvents such as ether and acetone. This is largely due to the vapour pressure of the solvent at room temperature and the low surface tension. A tip is to keep the solvent cold.
These are also a pain with viscous solvents or polymer solutions because there is not enough 'suck' in the bulb because the capillary at the entrance to the pipette is too narrow. The trick here is to break off the tip to give a wider entrance to the pipette. The wider tube will give a lower pressure drop (Darcy's Law?).
I tend to draw my own disposable pipettes then I have control over the capillary at the tip.
If you want a measured volume, without going to the expense of an autopipette, then glass serological pipettes are useful. Another alternative are glass syringes.
A problem with using cotton wool is that the solvent may remove the oils in the wool and this contaminates the sample.
Some organic solvents will remove the plastizer from plastic pipettes, so while the pipette may not 'dissolve', the solvent may be contaminated with the plastizer.
In the end it depends on what you are using the solvent for.
Hope this helps.
Thanks Dr.Russ your tips really helped =D
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