Indicate which techniques are appropriate for preparing a dilute solution from a
ID: 1010046 • Letter: I
Question
Indicate which techniques are appropriate for preparing a dilute solution from a stock solution. Answer YES OR NO for each choice.
a) After draining the contents of a serological pipet into a volumetric flask, the remaining solution in the serological pipet should be gently expelled with a pipet bulb to complete the transfer.
b) A pipet should be filled with solution by over-filling the pipet with the amount of liquid needed. The bulb should then be replaced with a thumb or index finger so liquid can be slowly released to the desired level.
c) Once the stock solution is transferred, fill the volumetric flask with solvent until the top of the meniscus is touching the etched line.
d) The stock solution should be poured into a beaker, prior to being drawn up into the pipet.
e)The bulb should be very firmly secured onto the end of the pipet so that the pipet can be carried by the bulb.
f)The pipet should be used to withdraw liquid directly from the bottle of stock solution to avoid contamination.
g)Any unused stock solution should be returned back to the stock solution bottle.
h) The pipet should be conditioned before use by drawing a small volume of stock solution into the pipet, turning it sideways and rotating so that all the walls are coated with solution.
i) After draining the contents of a serological pipet into a volumetric flask, the transfer is complete. The small volume of material left in the pipet is not to be transferred
j) A pipet must be reconditioned between every use of the same stock solution.
Explanation / Answer
a) After draining the contents of a serological pipet into a volumetric flask, the remaining solution in the serological pipet should be gently expelled with a pipet bulb to complete the transfer.
Yes the serological pipette is calibrated such that it must be blown-out of the tip to deliver the full volume of the pipette. So, the volume of the solution delivered also counts the liquid present in the tip of the pipette.
b) A pipet should be filled with solution by over-filling the pipet with the amount of liquid needed. The bulb should then be replaced with a thumb or index finger so liquid can be slowly released to the desired level.
No as we fill the pipette slightly above the calibration mark and adjust it such that the lower meniscus of the liquid touches the zero level (calibration mark)
c) Once the stock solution is transferred, fill the volumetric flask with solvent until the top of the meniscus is touching the etched line.
No because the lower meniscus or the bottom of the meniscus must touch the etched line.
d) The stock solution should be poured into a beaker, prior to being drawn up into the pipet.
Yes because drawing a solution directly from the stock may contaminate the stock solution.
e)The bulb should be very firmly secured onto the end of the pipet so that the pipet can be carried by the bulb.
No the bulb must never be firmly secured to the pipette. It must be holded against the endd of the pipette just enough to seal it.
f)The pipet should be used to withdraw liquid directly from the bottle of stock solution to avoid contamination.
No because pipettting out directly from the stock may cause contamination of the stock (by introducing the liquid present if any in the pipette).
g)Any unused stock solution should be returned back to the stock solution bottle.
No. Unused stock solution is never returned back to the stock solution bottle as it may contaminate the whole stock solution.
h) The pipet should be conditioned before use by drawing a small volume of stock solution into the pipet, turning it sideways and rotating so that all the walls are coated with solution. Yes
i) After draining the contents of a serological pipet into a volumetric flask, the transfer is complete. The small volume of material left in the pipet is not to be transferred
No the serological pipette is calibrated such that it must be blown-out of the tip to deliver the full volume of the pipette.
j) A pipet must be reconditioned between every use of the same stock solution. No, it need not be reconditioned if we are transferring the same stock.