volumetric and mass measurements

BIO 222

Volumetric and Mass Measurements

Prefixes that you will encounter: “Small”

Deci 10-1 d (dl)Centi 10-2 c (cm) Milli 10-3 m (msec) Micro 10-6 g) Nano 10-9 n (nm) Pico 10-12 p (pM) Femto 10-15 f

(fmol)

“Large”Kilo 103 k (km)Mega 106 M (Mb) Giga 109 G (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 balancesTop-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.

Left: MicrobalanceMechanical design

(many knobs on front.)

Right: “Analytical plus” weighs to 0.00001 g (10 µg)Electronic design

Balances in Plyler 139 (Instrumentation Lab)

Typical weighing procedureUse 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 cc1 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 pouringmL graduations, if present, are VERY

approximate; neither accurate nor precise

Conical (Erlenmeyer) Flasks: Mixing and storingShape 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: MeasuringCylindrical 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: PipettesGlass/plastic serological pipettes

1 mL, 5 mL, 10 mL, 20 mL

Pipettors with disposable tipsMost are adjustable up to a specified max.

volume10 µL, 20 µL, 100 µL, 200 µL, 1000 µL, 5000

µL

Glass pipettesTop: 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 disposablenot very fragile can’t be heated on hotplate or burnermay be reactive or contaminate sampleoften the graduations are imprecise

GlassMore expensiveless reactive or likely to contaminatemay 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. TDWhen 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 itemsPasteur 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.