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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: rarara on December 18, 2007, 12:57:21 AM

Title: question about pipet..
Post by: rarara on December 18, 2007, 12:57:21 AM: hai.. my name ira..
i want to know a little bit about pipeting procedure.. when we finish pipeting, there will be a small amount of liquid in the tip.. recently i just knew that most pipets are calibrated to account for this.. before this, when i pipet some liquid, i will rinse the pipet with the solvent.. is it true that i have done a wrong way of pipetting?
thanks for your attention yah...
Title: Re: question about pipet..
Post by: ATMyller on December 18, 2007, 02:45:50 AM: Classware usually has a EX/IN marking next to the error margin. If it is EX then its calibrated for extraction so that the remaining liquid should stay inside. Pipettes are mostly EX calibrated. When its calibrated for intake it has the IN marking and the remaining liquid needs to washed out, but its more common on classware that is used for making the solutions not measuring them like volumetric flasks.
Title: Re: question about pipet..
Post by: Borek on December 18, 2007, 04:02:06 AM: Quote from: rarara on December 18, 2007, 12:57:21 AM
is it true that i have done a wrong way of pipetting?

Yes. You should only touch your pipette end to the glass and wait for a few seconds. Pipettes are calibrated "to deliver", other types of glass can be calibrated "to contain". Subtle, but important difference, especially in the case of precise assays.

Edit: haven't noticed ATMyller already answered :(
Title: Re: question about pipet..
Post by: rarara on December 18, 2007, 05:03:00 AM: okay.. thanks for your info.. if not i will be making big misstake through the year.. ;D thanks88...how about a case when i pipet a high viscocity liquid such as cough releif.? seems like when i pipet the cough relief, a lot of liquid still remains inside the pipet as it too viscos..actually i prepare it for HPLC testing..
Title: Re: question about pipet..
Post by: Borek on December 18, 2007, 08:07:38 AM: Viscous liquids are completely different case - standard procedure and "to deliver" calibration are for water solutions with viscosity similar to that of water. For high viscosity liquids use either "to contain" pipette (IN as described by ATMyller) or calibrate your pipette before use (weigthing liquid delivered for - say - three samples). Note that your liquid (if it has high viscosity) may have density different from that of water.