Appendix C: Laboratory techniques

    Cards (17)

    • gross errors of technique
      careless errors
    • Mass is a measure of amount of matter, weight is the force of earth’s attraction on the object
    • weight = mass x acceleration of gravity
    • Weighing By Difference (to prepare multiple samples)
      Initial Weighing: weigh, clean, dry beaker on TL balance, add 3x the desired single sample mass (this is the weighing beaker)

      Analytical weighing by difference: take weighing beaker, 3 Erl. Flasks (don’t need to be dry), paper strip, data sheet to AL balance room.
      Weigh the weighing beaker, remove w/ strip
      Pour from weighing beaker into flask 1 w/o touching it
      Reweigh beaker, repeat above w/ flask 2 and 3

      Advantages: 4 weighing for 3 samples, no drying needed for flask, does not matter if mass isn’t evenly distributed
    • Quantitative Transfer:
      Transfer Solid Sample:
      Transfer sample using forceps to avoid contamination
      Place on weighing vessel (like a folded weighing paper or boat, beaker) on balance that is tared to 0
      Transfer weighed sample to receiving vessel, use scoopula to transfer all, rinse with small amount of solvent until no trace remains, (~ 3-5 rinsings)
      Transfer a Liquid Sample:
      Rinse dispensing vessel w/ solvent, transfer rinsing to the receiving vessel (~3 rinsings)
      If funnel is used, rinse that as well and add it to receiving vessel
    • Burettes can measure w/ accuracy +/- 0.01 mL
    • Rinsing burette
      • Check if dirty, if stopcock works
      • Use small plastic funnel to add solution to it
      • Rinse funnel first w/ solution you will add, rinse outside, wipe outside dry
      • Insert funnel and add 5-10 mL of soln. to burette w/ stopcock closed
      • Hold horizontally, rotate, drain rinsings, do this 2x more
    • Filling burette
      • Use burette funnel, fill to near zero w/ soln.
      • Open stopcock displace air from tip
      • Wipe outer stem of funnel dry, invert for later use
    • Initial reading of burette
      • Open stopcock until liquid level is a bit below zero
      • Eye level w/ meniscus, use backing card below meniscus, read the meniscus level to 0.01 mL
      • Touch off tip of burette in waste beaker, if volume decreased, stopcock is leaking
    • Titration
      • Burette tip extends 2-3 cm into flask
      • Flask on white background
      • Add soln, swirl continuously, when w/in 1-2 mL of endpoint rinse inner walls w. DI water and add titrant drop-by-drop
      • Very close to endpoint, do half-drop touch-offs with the tip, repeat until colour changes
    • Final reading of burette
      • same as initial
    • Gravity filtration:
      removes impurities with funnel, filter paper
    • Vacuum filtration:
      used to separate solid from liquid mixture. liquid goes through the funnel by vacuum pressure. faster than gravity filtration.
    • 25 mL volumetric pipettes used to deliver 25.00 + 0.03 mL
    • recrystallization
      used to purify compounds (naturally obtained or synthesized)
    • steps of recrystallization
      • solubility test to find solvent
      • Dissolve solute in near-boiling solvent
      • Allow solution to cool at room temp. then in ice bath (slow allows for more pure crystals, fast traps impurities)
      • Collect crystals by vac. filtration
      • Rinse crystals w/ cold solvent
      • Dry recrystallized, pure product on vac. Filter for 5 mins
    • refluxing
      solution is boiled w/o loss of solvent
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