October 04, 2022, 07:01:48 PM
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Topic: How to measure an exact volume of ethanol using single volume pipette  (Read 6817 times)

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Offline 5kov

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Hello guys

I'm trying to transfer an exact volume of ethanol (95%) from a volumetric flask into a receiving flask. I'm using 25 ml single volume pipette. As you know the volumetric pipettes should not be blown out because they are calibrated in a manner that takes into account the solution which remains at the tip. The problem is that the surface tension of the ethanol is different from the surface tension of the pure water and when the pipette is drained out there's almost no ethanol left at the tip. Do you think that the pipette should be recalibrated using ethanol/water mixture with a given concentration (95% in my case) or the error due to the surface tension difference is insignificant?

Offline enahs

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If you need to exactly 25.00000 (however accurate your pipette is), then yes, recalibrate it.

There are, however other issues with pipettes and organic solvents not properly taught, and if you were using it properly it would be more accurate.

Pipettes work off of vacuums and vapor pressures. You pull a vacuum with the bulb. The problem is, with many organic solvents, once you have the liquid in the pipette it starts to evaporate relatively fast (as compared to water). Increasing the pressure in the pipette and "pushing" more out. So you should pipette up and down 5-10x (depending on solvent vapor pressure) and try and saturate the pipette with gas form of the solvent, and then fill the pipette up to mark.

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