Volumetric and Mass Measurements

1. Volumetric and Mass Measurements BIO 222

2. Prefixes that you will encounter: • “Small” • Deci 10-1 d (dl) • Centi 10-2 c (cm) • Milli 10-3 m (msec) • Micro 10-6m(mg) • Nano 10-9 n (nm) • Pico 10-12 p (pM) • Femto 10-15 f (fmol)

3. “Large” • Kilo 103 k (km) • Mega 106 M (Mb) • Giga 109 G (GHz)

4. 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).

5. 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).

6. 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.

7. Balances Top-loading Analytical

8. 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.

9. Balances in Plyler 139 (Instrumentation Lab) Left: MicrobalanceMechanical design (many knobs on front.) Right: “Analytical plus” weighs to 0.00001 g (10 µg)Electronic design

10. 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.

11. Tools for weighing

12. Steps in weighing(balance in chemical stockroom)

13. 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 g1 liter of water weighs 1 kg • Consider the purposes of common glassware:

14. 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

15. 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.

16. 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.)

17. 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.

18. Volumetric flask

19. 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

20. 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

21. Adjustable pipettors

22. 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 reactiveor likely to contaminate • may adsorb ions and other molecules • wettable: a film of water is left clinging to glass

23. 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”

24. 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).

25. 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.

26. TC vs. TD: questions to ponder • TD pipettes are much more common than TC.Why?

27. TC vs. TD: questions to ponder • TC volumetric flasks are much more common than TD. Why?

28. TC vs. TD: questions to ponder • Both kinds of graduated cylinders are common. In which situation would you use each?

29. TC vs. TD: questions to ponder • A plastic graduated cylinder has the inscription “TC/TD.” Why?

30. 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.