Discussion in 'Chemistry' started by rohIT, Jan 27, 2013.

  1. rohIT Registered Senior Member

    they give statements in our books and dont add head or tail to make us understand why.
    so, i was doing this practical records for my exam and i did not understand why...

    1) pipette should "never" be held from the bulb.

    2) pipette must "never" be used for drawing strong acid or base.

    3) we should never blow out the last drop clinging to the nozzle of the pipette.

    4) conical flask should not be rinsed with the solution to be filled in it.

    5) base should be taken in the conical flask and acid in the burette (for titrations).

    6) volumetric flask should "never" be used for measuring out solutions.

    its annoying to be compelled to remember statements without understand the "because... " behind it.
    plz help.
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  3. leopold Valued Senior Member

    if i may:
    some chemicals are extremely toxic.
    you should NEVER EVER taste or smell ANY unknown lab chemical substance.
    just a whiff of some substances can kill you.
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  5. James R Just this guy, you know? Staff Member

    A few guesses...

    The glass bit might fall off the bulb and smash, with broken glass and possible nasty chemical spill.

    If you draw it too high, it might eat through the (rubber?) bulb. If it drips on something it might eat through that.

    If it is dirty from some previous contents, you may get an unwanted chemical reaction.

    Remind me what a burette is, again (?)
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  7. origin In a democracy you deserve the leaders you elect. Valued Senior Member

    James is correct on the reasons he gave. The last 2 are given below.
    Adding acids and bases together results in a strongly exothermic reaction. You should never add a base (or water) to an acid - it could result in the base or water instantly boiling and splashing chemicals into your face. Adding the acid to the base or water results in a much more slower reactions. "add acid to water like you otta"

    This is a bit confusing since a volumetric flask is used for precised volume measurements. I think they are saying don't fill a volumetric flask and use that to transfer chemicals to a graduated cyclinder for instance. That would make sense because of the design of the flask it is difficult to contron the flow out of the flask.

    It is vital that you understand the reasons or else you will not remember the safety rules and will end up getting injured. It is your responsibility to understand the reasons for the rules; if you don't understand the reasons you must ask the instructor. This is your safety - severe eye injury from acids and bases for instance way to common!!
  8. rohIT Registered Senior Member

    thanks guys

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    n thanks origin for that caution

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    thanks once again!
  9. arauca Banned Banned


    It very much depends on the acid or base concentration , I don't believe if it is one molar there is much danger.
    I agree don't add water to a concentrated sulfuric or hydrofluoric acid , even adding water to a 25% NaOH is not bad .

  10. origin In a democracy you deserve the leaders you elect. Valued Senior Member

    Gee, no kidding? I don't think the OP was worried putting a lemon slice in a glass of water.
  11. Trippy ALEA IACTA EST Staff Member

    Some of these statements may be as much about experimental accuracy as they are about saftey.

    For example - you should never hold a pippette by the bulb because pippettes are finely calibrated. Holding them by the bulb allows heat to transfer from your hands to the bulb causing it to expand and change volume, which in turn throws your calculations out by anu unknown amount.

    Likewise, you should never blow the last drop out because pippetes are calibrated to take into account the last little bit that is held in place by surface tension, consequently the only way to get an accurate volume is by leaving it in there.

    You also shpuldn't rinse out the conical flask with what will be going in it because that two can upset your volumes and calculations as it leaves a residue on the surface.

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