Volumetric and Mass Measurements BIO 222 Prefixes that

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) (p. M) (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. 01 g. 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 m. L = 1 cc 1 m. L 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 m. L 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 m. L Expensive – do not use for storage.

Volumetric flask

Smaller volumes: Pipettes Glass/plastic serological pipettes 1 m. L, 5 m. L, 10 m. L, 20 m. L 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 m. L of water into a glass container, when you pour it out it will dispense less than 100 m. L. 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 m. L, 50 m. L) – good for sample storage, whether you will centrifuge or not. Snap-top microcentrifuge tubes (1. 5 m. L), commonly called “Eppendorf tubes” – commonly used for storage of small samples.