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Chemistry: Proper Use of a Pipette

  1. Oct 22, 2011 #1
    1. The problem statement, all variables and given/known data

    Not much background information is needed. Lead (II) iodide is dissolved into deionized water, and the temperature of the solution is manipulated in order to explore the relationship between temperature and the solubility of the solution. To allow the solution to completely dissolve, it is placed inside an Erlenmeyer flask and heated to 100°C.

    "When the Erlenmeyer flask is brought to 100°C and the solid settles, the hot saturated solution is transferred to a test tube using a warm pipette. Why must the pipette be warm in completing the transfer?"

    2. Relevant equations

    N/A

    3. The attempt at a solution

    "The pipette is warm to ensure that the solubility of the lead (II) iodide is not affected by changes in temperatures caused during the transfer to the test tube and that the volume of solution measured is accurate. If the pipette is cool, a certain amount of precipitate may be left inside the pipette after the transfer, affecting the measured final results of the experiment. Also, using a cool pipette may pose a safety hazard, as its glass could crack due to a sudden shift in temperature when placed into a 100°C solution."

    Is this statement correct, or am I missing certain details that are common practice in a laboratory environment? Any help is greatly appreciated. :)

    Thank you,

    Eric.
     
  2. jcsd
  3. Oct 23, 2011 #2

    Borek

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    Staff: Mentor

    Looks OK to me - at the same time using warm pipette and warm solution is wrong, as glass is always calibrated for some temperature, so you should recalibrate it. See www.titrations.info/pipette-burette[/url] and [url]www.titrations.info/volumetric-glass-calibration[/URL]
     
    Last edited by a moderator: Apr 26, 2017
  4. Oct 26, 2011 #3

    epenguin

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    Homework Helper
    Gold Member

    ?But if the expansion coefficient of the pipette glass was the same as that of the flask glass you could hope errors due to glass expansion cancel each other out. :smile:

    Liquids expand more than solids so I guess that causes a larger error than the glass.
    Oh.
    Well isn't glass an honorary solid?

    Anyway what sort of error are we going to get? Something a lot smaller than the solubility effect.
     
  5. Oct 26, 2011 #4

    Ygggdrasil

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    Science Advisor

    One reason to use a pre-warmed pipette would be for reasons of reproducibility. If you pipette up the warm liquid in a cold pipette the glass will warm by some amount during the transfer, but it will not likely have enough time to reach thermal equilibrium with the liquid. Therefore, potentially every transfer will have a slightly different volume because the pipette was a different temperature during each transfer.

    If the pipette is pre-warmed to the temperature of the solution, however, you will not have this problem of the pipette changing temperature during the transfer. While the volume will not be calibrated correctly, you can at least pipette the same amount reproducibly. In other words, this methods allow you to pipette precise amounts albeit not accurate amounts.
     
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