Volumetric and Mass Measurements
BIO 222
Prefixes that you will encounter:
 “Small”
 Deci
 Centi
 Milli
 Micro
 Nano
 Pico
 Femto
10-1
10-2
10-3
10-6
10-9
10-12
10-15
d
c
m
m
n
p
f
(dl)
(cm)
(msec)
(mg)
(nm)
(pM)
(fmol)
 “Large”
 Kilo
 Mega
 Giga
103
106
109
k
M
G
(km)
(Mb)
(GHz)
Mass Measurements
 Mass – a measure of the amount of matter an object has;
constant regardless of position
 Weight – a measure of how strongly an object is pulled by
the earth’s gravity; varies with the distance from the
earth’s center
 As long as you are working at or near the earth’s surface,
weight is an accurate indicator of mass.
 Basic unit = Gram (g).
Laboratory Balances:
Measuring Mass
 Scientists usually refer to a “balance” not “scale”
 Most are now electronic, with digital readout and few
moving parts.
 Older mechanical balances with knobs are still in use. If you need
to use one, seek instruction (wrong use can damage them).
 Maximum capacity of balance is usually shown on the
instrument. DO NOT EXCEED.
 Be sure that surface is level (check bubble).
Common laboratory balances
 Top-loading balance: Weighs to 0.01g.
Wind screen not needed.
 Analytical balance: Weighs to 0.0001 g
(i.e. 1/10 of a milligram). Has a wind screen to protect
the balance pan from breezes.
 Microbalance: Weighs to 0.000001 g (1µg) or less. For
specialized uses only.
 Intermediate sizes exist as well.
Balances
Top-loading
Analytical
Bubble level on top-loading balance
If bubble is not
centered, adjust
the feet of the
balance until it
is. Consult a
professor or lab
coordinator if in
doubt.
Balances in Plyler 139
(Instrumentation Lab)
Left: Microbalance
Mechanical design
(many knobs on front.)
Right: “Analytical plus” weighs to
0.00001 g (10 µg)
Electronic design
Typical weighing procedure
 Use weighing paper or plastic “boat”
 Place on pan. Close windscreen. Press tare (or zero) to
bring the display back to zero.
 Open windscreen. Carefully add substance. Wait for
reading to stabilize.
 As you near the desired mass, tap spatula to add a few
grains at a time. Close windscreen before final
measurement.
 When finished, clean balance.
Tools for weighing
Steps in weighing
(balance in chemical stockroom)
Volumetric measurements
 Basic units of volumetric measurement is liter
(abbreviation is letter L – upper or lower case)
 1 mL = 1 cc
 1 mL of water weighs 1 g
1 liter of water weighs 1 kg
 Consider the purposes of common glassware:
Beakers: Mixing and dispensing
 Wide mouth, good for stirring but
hard to seal for storage
 Lip, good for pouring
 mL graduations, if present, are VERY approximate; neither
accurate nor precise
Conical (Erlenmeyer) Flasks: Mixing
and storing
 Shape is good for swirling to mix solutions.
 Narrow mouth is easy to seal for storage
(using stopper or Parafilm).
 Not as easy to pour things into or
out of it, compared to a beaker.
 Like a beaker, graduations
are neither accurate nor precise.
Graduated cylinders: Measuring
 Cylindrical shape gives
good precision and accuracy.
 Graduations can measure
any volume within the size
range of the cylinder.
 Not an ideal shape for mixing,
but it can be done. (More on
this when we discuss
making solutions.)
Volumetric flasks:
Measuring and making solutions
 Narrow neck gives excellent precision.
 Individually calibrated for excellent accuracy.
 Shape good for swirling to dissolve.
 Can only be used to measure one volume
 Common sizes: 10, 25, 50, 100, 250, 500, 1000 mL
 Expensive – do not use for storage.
Volumetric flask
Smaller volumes: Pipettes
 Glass/plastic serological pipettes
 1 mL, 5 mL, 10 mL, 20 mL
 Pipettors with disposable tips
 Most are adjustable up to a specified max. volume
 10 µL, 20 µL, 100 µL, 200 µL, 1000 µL, 5000 µL
Glass pipettes
Top: volumetric
pipette - measures
a single volume up
to an engraved line.
Bottom three:
serological
pipettes - many
graduations, can
measure odd
volumes, somewhat
less accurate than
volumetric pipettes
Adjustable pipettors
Most labware can be glass or plastic
 Plastic





inexpensive and disposable
not very fragile
can’t be heated on hotplate or burner
may be reactive or contaminate sample
often the graduations are imprecise
 Glass




More expensive
less reactive or likely to contaminate
may adsorb ions and other molecules
wettable: a film of water is left clinging to glass
Wettability issues (esp. glass)
 If you put exactly 100 mL of water into a glass container,
when you pour it out it will dispense less than 100 mL. The
rest is clinging to the glass.
 Accurate instruments (graduated cylinders, volumetric
flasks, serological pipets) may be marked TC or TD.
 TC = “to contain”
 TD = “to deliver”
TC vs. TD
 When an instrument marked TC is filled to a particular
graduation, it contains that volume. When emptied, it will
deliver less than that volume, due to wettability/clinging
water.
 To compensate for this, TD instruments actually contain
slightly more than the marked volume, but they deliver the
specified amount.
 This assumes the liquid is water. Can’t use TD for other
liquids, because greater or lesser amount might cling
(e.g. maple syrup, alcohol).
TC vs. TD – which to choose?
 Use a TD if you are using water, and you care most about the
volume that comes out.
 Use a TC:
 If measuring something other than water.
 If you care most about how much is inside, e.g. if mixing
salt and water to a total volume of exactly 1 L.
TC vs. TD: questions to ponder
 TD pipettes are much more common than TC.
Why?
TC vs. TD: questions to ponder
 TC volumetric flasks are much more common than TD. Why?
TC vs. TD: questions to ponder
 Both kinds of graduated cylinders are common. In which
situation would you use each?
TC vs. TD: questions to ponder
 A plastic graduated cylinder has the inscription “TC/TD.”
Why?
Other items
 Pasteur or transfer pipet – cheap, thin disposable, “eye
dropper”
 Screw-cap centrifuge tubes (15 mL, 50 mL) – good for
sample storage, whether you will centrifuge or not.
 Snap-top microcentrifuge tubes (1.5 mL), commonly
called “Eppendorf tubes” – commonly used for storage of
small samples.