Appendix C: Laboratory techniques

• 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