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Appendix C: Laboratory techniques
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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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